chore: update lance dependency to v7.2.0-beta.4

Bump version: 0.30.0-beta.1 → 0.30.1-beta.0
Bump version: 0.33.0-beta.1 → 0.33.1-beta.0
2026-06-03 12:20:40 +00:00 · 2026-06-02 04:16:10 +00:00 · 2026-06-01 12:41:45 +00:00 · 2026-06-01 12:41:02 +00:00 · 2026-06-01 20:40:07 +08:00 · 2026-05-30 19:22:50 -07:00
51 changed files with 3249 additions and 520 deletions
--- a/.bumpversion.toml
+++ b/.bumpversion.toml
@@ -1,5 +1,5 @@
 [tool.bumpversion]
-current_version = "0.30.0-beta.1"
+current_version = "0.30.1-beta.0"
 parse = """(?x)
    (?P<major>0|[1-9]\\d*)\\.
    (?P<minor>0|[1-9]\\d*)\\.
--- a/.github/workflows/build_windows_wheel/action.yml
+++ b/.github/workflows/build_windows_wheel/action.yml
@@ -29,7 +29,3 @@ runs:
        args: ${{ inputs.args }}
        docker-options: "-e PIP_EXTRA_INDEX_URL='https://pypi.fury.io/lance-format/ https://pypi.fury.io/lancedb/'"
        working-directory: python
-    - uses: actions/upload-artifact@v4
-      with:
-        name: windows-wheels
-        path: python\target\wheels
--- a/.github/workflows/pypi-publish.yml
+++ b/.github/workflows/pypi-publish.yml
@@ -8,6 +8,9 @@ on:
    # This should trigger a dry run (we skip the final publish step)
    paths:
      - .github/workflows/pypi-publish.yml
+      - .github/workflows/build_linux_wheel/action.yml
+      - .github/workflows/build_mac_wheel/action.yml
+      - .github/workflows/build_windows_wheel/action.yml
      - Cargo.toml # Change in dependency frequently breaks builds
      - Cargo.lock

@@ -21,32 +24,21 @@ jobs:
  linux:
    name: Python ${{ matrix.config.platform }} manylinux${{ matrix.config.manylinux }}
    timeout-minutes: 60
-    permissions:
-      id-token: write
-      contents: read
    strategy:
      matrix:
        config:
-          - platform: x86_64
-            manylinux: "2_17"
-            extra_args: ""
-            runner: ubuntu-22.04
          - platform: x86_64
            manylinux: "2_28"
            extra_args: "--features fp16kernels"
            runner: ubuntu-22.04
-          - platform: aarch64
-            manylinux: "2_17"
-            extra_args: ""
-            # For successful fat LTO builds, we need a large runner to avoid OOM errors.
-            runner: ubuntu-2404-8x-arm64
+          # For successful fat LTO builds, we need a large runner to avoid OOM errors.
          - platform: aarch64
            manylinux: "2_28"
            extra_args: "--features fp16kernels"
            runner: ubuntu-2404-8x-arm64
    runs-on: ${{ matrix.config.runner }}
    steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
        with:
          fetch-depth: 0
          lfs: true
@@ -60,15 +52,14 @@ jobs:
          args: "--release --strip ${{ matrix.config.extra_args }}"
          arm-build: ${{ matrix.config.platform == 'aarch64' }}
          manylinux: ${{ matrix.config.manylinux }}
-      - uses: ./.github/workflows/upload_wheel
+      - uses: actions/upload-artifact@v7
        if: startsWith(github.ref, 'refs/tags/python-v')
        with:
-          fury_token: ${{ secrets.FURY_TOKEN }}
+          name: wheels-linux-${{ matrix.config.platform }}-${{ matrix.config.manylinux }}
+          path: target/wheels/lancedb-*.whl
+          if-no-files-found: error
  mac:
    timeout-minutes: 90
-    permissions:
-      id-token: write
-      contents: read
    runs-on: ${{ matrix.config.runner }}
    strategy:
      matrix:
@@ -78,7 +69,7 @@ jobs:
    env:
      MACOSX_DEPLOYMENT_TARGET: 10.15
    steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
        with:
          fetch-depth: 0
          lfs: true
@@ -90,18 +81,21 @@ jobs:
        with:
          python-minor-version: 10
          args: "--release --strip --target ${{ matrix.config.target }} --features fp16kernels"
-      - uses: ./.github/workflows/upload_wheel
+      - uses: actions/upload-artifact@v7
        if: startsWith(github.ref, 'refs/tags/python-v')
        with:
-          fury_token: ${{ secrets.FURY_TOKEN }}
+          name: wheels-mac-${{ matrix.config.target }}
+          path: target/wheels/lancedb-*.whl
+          if-no-files-found: error
  windows:
-    timeout-minutes: 60
-    permissions:
-      id-token: write
-      contents: read
+    timeout-minutes: 90
    runs-on: windows-latest
+    env:
+      # link.exe is single-threaded and the long pole on Windows builds. Use
+      # rustc's bundled lld-link instead.
+      CARGO_TARGET_X86_64_PC_WINDOWS_MSVC_LINKER: rust-lld
    steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
        with:
          fetch-depth: 0
          lfs: true
@@ -113,18 +107,70 @@ jobs:
        with:
          python-minor-version: 10
          args: "--release --strip"
-          vcpkg_token: ${{ secrets.VCPKG_GITHUB_PACKAGES }}
-      - uses: ./.github/workflows/upload_wheel
+      - uses: actions/upload-artifact@v7
        if: startsWith(github.ref, 'refs/tags/python-v')
        with:
-          fury_token: ${{ secrets.FURY_TOKEN }}
+          name: wheels-windows
+          path: target/wheels/lancedb-*.whl
+          if-no-files-found: error
+  publish:
+    name: Publish wheels
+    if: startsWith(github.ref, 'refs/tags/python-v')
+    needs: [linux, mac, windows]
+    runs-on: ubuntu-latest
+    permissions:
+      id-token: write
+      contents: read
+    steps:
+      - uses: actions/checkout@v6
+      - name: Download wheel artifacts
+        uses: actions/download-artifact@v8
+        with:
+          pattern: wheels-*
+          path: target/wheels
+          merge-multiple: true
+      - name: List wheels
+        run: ls -la target/wheels
+      - name: Choose repo
+        id: choose_repo
+        run: |
+          if [[ ${{ github.ref }} == *beta* ]]; then
+            echo "repo=fury" >> $GITHUB_OUTPUT
+          else
+            echo "repo=pypi" >> $GITHUB_OUTPUT
+          fi
+      - name: Publish to Fury
+        if: steps.choose_repo.outputs.repo == 'fury'
+        env:
+          FURY_TOKEN: ${{ secrets.FURY_TOKEN }}
+        run: |
+          shopt -s nullglob
+          WHEELS=(target/wheels/lancedb-*.whl)
+          if [[ ${#WHEELS[@]} -eq 0 ]]; then
+            echo "No wheels found in target/wheels/" >&2
+            exit 1
+          fi
+          for WHEEL in "${WHEELS[@]}"; do
+            echo "Uploading $WHEEL to Fury"
+            curl -f -F package=@"$WHEEL" "https://$FURY_TOKEN@push.fury.io/lancedb/"
+          done
+      # NOTE: pypa/gh-action-pypi-publish must be invoked directly from a
+      # workflow file, not from inside a composite action. When called from a
+      # composite, `github.action_repository` is empty (actions/runner#2473)
+      # and the action falls back to `github.repository`, producing a bogus
+      # `docker://ghcr.io/<repo>:<ref>` image reference that GHA tries to pull.
+      - name: Publish to PyPI
+        if: steps.choose_repo.outputs.repo == 'pypi'
+        uses: pypa/gh-action-pypi-publish@release/v1
+        with:
+          packages-dir: target/wheels/
  gh-release:
    if: startsWith(github.ref, 'refs/tags/python-v')
    runs-on: ubuntu-latest
    permissions:
      contents: write
    steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
        with:
          fetch-depth: 0
          lfs: true
@@ -187,13 +233,13 @@ jobs:
  report-failure:
    name: Report Workflow Failure
    runs-on: ubuntu-latest
-    needs: [linux, mac, windows]
+    needs: [linux, mac, windows, publish]
    permissions:
      contents: read
      issues: write
    if: always() && failure() && startsWith(github.ref, 'refs/tags/python-v')
    steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
      - uses: ./.github/actions/create-failure-issue
        with:
          job-results: ${{ toJSON(needs) }}
--- a/.github/workflows/upload_wheel/action.yml
+++ b/.github/workflows/upload_wheel/action.yml
@@ -1,34 +0,0 @@
-name: upload-wheel
-
-description: "Upload wheels to Pypi"
-inputs:
-  fury_token:
-    required: true
-    description: "release token for the fury repo"
-
-runs:
-  using: "composite"
-  steps:
-  - name: Choose repo
-    shell: bash
-    id: choose_repo
-    run: |
-      if [[ ${{ github.ref }} == *beta* ]]; then
-        echo "repo=fury" >> $GITHUB_OUTPUT
-      else
-        echo "repo=pypi" >> $GITHUB_OUTPUT
-      fi
-  - name: Publish to Fury
-    if: steps.choose_repo.outputs.repo == 'fury'
-    shell: bash
-    env:
-      FURY_TOKEN: ${{ inputs.fury_token }}
-    run: |
-      WHEEL=$(ls target/wheels/lancedb-*.whl 2> /dev/null | head -n 1)
-      echo "Uploading $WHEEL to Fury"
-      curl -f -F package=@$WHEEL https://$FURY_TOKEN@push.fury.io/lancedb/
-  - name: Publish to PyPI
-    if: steps.choose_repo.outputs.repo == 'pypi'
-    uses: pypa/gh-action-pypi-publish@release/v1
-    with:
-      packages-dir: target/wheels/
--- a/Cargo.lock
+++ b/Cargo.lock
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,20 +13,20 @@ categories = ["database-implementations"]
 rust-version = "1.91.0"

 [workspace.dependencies]
-lance = { "version" = "=7.0.0-rc.1", default-features = false, "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-core = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-datagen = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-file = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-io = { "version" = "=7.0.0-rc.1", default-features = false, "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-index = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-linalg = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-namespace = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-namespace-impls = { "version" = "=7.0.0-rc.1", default-features = false, "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-table = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-testing = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-datafusion = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-encoding = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
-lance-arrow = { "version" = "=7.0.0-rc.1", "tag" = "v7.0.0-rc.1", "git" = "https://github.com/lance-format/lance.git" }
+lance = { "version" = "=7.2.0-beta.4", default-features = false, "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-core = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-datagen = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-file = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-io = { "version" = "=7.2.0-beta.4", default-features = false, "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-index = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-linalg = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-namespace = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-namespace-impls = { "version" = "=7.2.0-beta.4", default-features = false, "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-table = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-testing = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-datafusion = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-encoding = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
+lance-arrow = { "version" = "=7.2.0-beta.4", "tag" = "v7.2.0-beta.4", "git" = "https://github.com/lance-format/lance.git" }
 ahash = "0.8"
 # Note that this one does not include pyarrow
 arrow = { version = "58.0.0", optional = false }
--- a/docs/src/java/java.md
+++ b/docs/src/java/java.md
@@ -14,7 +14,7 @@ Add the following dependency to your `pom.xml`:
 <dependency>
    <groupId>com.lancedb</groupId>
    <artifactId>lancedb-core</artifactId>
-    <version>0.30.0-beta.1</version>
+    <version>0.30.1-beta.0</version>
 </dependency>
 ```

--- a/docs/src/js/classes/MergeInsertBuilder.md
+++ b/docs/src/js/classes/MergeInsertBuilder.md
@@ -76,6 +76,57 @@ the query optimizer chooses a suboptimal path.

 ***

+### useLsmWrite()
+
+```ts
+useLsmWrite(useLsmWrite): MergeInsertBuilder
+```
+
+Controls whether the merge uses the MemWAL LSM write path.
+
+By default (unset), a `mergeInsert` on a table with an LSM write spec is
+routed through Lance's MemWAL shard writer, and a table without one uses
+the standard path. Pass `false` to force the standard path even when a
+spec is set. Pass `true` to require a spec — `mergeInsert` rejects if none
+is installed.
+
+#### Parameters
+
+* **useLsmWrite**: `boolean`
+    Whether to use the LSM write path.
+
+#### Returns
+
+[`MergeInsertBuilder`](MergeInsertBuilder.md)
+
+***
+
+### validateSingleShard()
+
+```ts
+validateSingleShard(validateSingleShard): MergeInsertBuilder
+```
+
+Controls how an LSM merge checks that its input targets a single shard.
+
+When a table has an LSM write spec, every row in a `mergeInsert` call must
+route to the same shard. When `true` (the default), every row is inspected
+to verify this. When `false`, only the first row is inspected and the
+shard it routes to is used for the whole input — a faster path for callers
+that have already pre-sharded their input. Has no effect on tables without
+an LSM write spec.
+
+#### Parameters
+
+* **validateSingleShard**: `boolean`
+    Whether to check every row routes to one shard. Defaults to `true`.
+
+#### Returns
+
+[`MergeInsertBuilder`](MergeInsertBuilder.md)
+
+***
+
 ### whenMatchedUpdateAll()

 ```ts
--- a/docs/src/js/classes/Table.md
+++ b/docs/src/js/classes/Table.md
@@ -187,6 +187,25 @@ Any attempt to use the table after it is closed will result in an error.

 ***

+### closeLsmWriters()
+
+```ts
+abstract closeLsmWriters(): Promise<void>
+```
+
+Drain and close any cached MemWAL shard writers held for this table.
+
+When an [LsmWriteSpec](../interfaces/LsmWriteSpec.md) is installed, `mergeInsert` opens MemWAL
+shard writers and caches them for reuse across calls. This closes them,
+flushing pending data; writers reopen lazily on the next `mergeInsert`.
+It is a no-op when no writers are cached.
+
+#### Returns
+
+`Promise`&lt;`void`&gt;
+
+***
+
 ### countRows()

 ```ts
--- a/docs/src/js/interfaces/LsmWriteSpec.md
+++ b/docs/src/js/interfaces/LsmWriteSpec.md
@@ -11,7 +11,10 @@ Specification selecting Lance's MemWAL LSM-style write path for

 `specType` is `"bucket"`, `"identity"`, or `"unsharded"`. For `"bucket"`,
 `column` and `numBuckets` are required; for `"identity"`, `column` is
-required.
+required and must be a deterministic function of the unenforced primary
+key (every row with a given primary key must always produce the same
+`column` value, or upserts of that key can land in different shards and a
+stale version can win).

 ## Properties

--- a/docs/src/js/interfaces/MergeResult.md
+++ b/docs/src/js/interfaces/MergeResult.md
@@ -32,6 +32,14 @@ numInsertedRows: number;

 ***

+### numRows
+
+```ts
+numRows: number;
+```
+
+***
+
 ### numUpdatedRows

 ```ts
--- a/java/lancedb-core/pom.xml
+++ b/java/lancedb-core/pom.xml
@@ -8,7 +8,7 @@
    <parent>
      <groupId>com.lancedb</groupId>
      <artifactId>lancedb-parent</artifactId>
-      <version>0.30.0-beta.1</version>
+      <version>0.30.1-beta.0</version>
      <relativePath>../pom.xml</relativePath>
    </parent>

--- a/java/pom.xml
+++ b/java/pom.xml
@@ -6,7 +6,7 @@

    <groupId>com.lancedb</groupId>
    <artifactId>lancedb-parent</artifactId>
-    <version>0.30.0-beta.1</version>
+    <version>0.30.1-beta.0</version>
    <packaging>pom</packaging>
    <name>${project.artifactId}</name>
    <description>LanceDB Java SDK Parent POM</description>
@@ -28,7 +28,7 @@
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <arrow.version>15.0.0</arrow.version>
-        <lance-core.version>7.0.0-beta.13</lance-core.version>
+        <lance-core.version>7.2.0-beta.4</lance-core.version>
        <spotless.skip>false</spotless.skip>
        <spotless.version>2.30.0</spotless.version>
        <spotless.java.googlejavaformat.version>1.7</spotless.java.googlejavaformat.version>
--- a/nodejs/Cargo.toml
+++ b/nodejs/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "lancedb-nodejs"
 edition.workspace = true
-version = "0.30.0-beta.1"
+version = "0.30.1-beta.0"
 publish = false
 license.workspace = true
 description.workspace = true
--- a/nodejs/test/table.test.ts
+++ b/nodejs/test/table.test.ts
@@ -2625,3 +2625,97 @@ describe("setLsmWriteSpec / unsetLsmWriteSpec", () => {
    ).rejects.toThrow();
  });
 });
+
+describe("LSM merge insert", () => {
+  let tmpDir: tmp.DirResult;
+
+  beforeEach(() => {
+    tmpDir = tmp.dirSync({ unsafeCleanup: true });
+  });
+  afterEach(() => tmpDir.removeCallback());
+
+  async function bucketTable(conn: Connection): Promise<Table> {
+    // The primary key column must be non-nullable.
+    const table = await conn.createEmptyTable(
+      "t",
+      new arrow.Schema([
+        new arrow.Field("id", new arrow.Utf8(), false),
+        new arrow.Field("value", new arrow.Float64(), true),
+      ]),
+    );
+    await table.add([
+      { id: "a", value: 1 },
+      { id: "b", value: 2 },
+    ]);
+    await table.setUnenforcedPrimaryKey("id");
+    // numBuckets = 1: every row routes to the single bucket.
+    await table.setLsmWriteSpec({
+      specType: "bucket",
+      column: "id",
+      numBuckets: 1,
+    });
+    return table;
+  }
+
+  it("routes merge_insert through the shard writer", async () => {
+    const conn = await connect(tmpDir.name);
+    const table = await bucketTable(conn);
+
+    const res = await table
+      .mergeInsert("id")
+      .whenMatchedUpdateAll()
+      .whenNotMatchedInsertAll()
+      .execute([
+        { id: "c", value: 3 },
+        { id: "d", value: 4 },
+      ]);
+    // LSM path: rows go to the MemWAL, so only numRows is populated.
+    expect(res.numRows).toBe(2);
+    expect(res.version).toBe(0);
+    expect(res.numInsertedRows).toBe(0);
+
+    await table.closeLsmWriters();
+  });
+
+  it("falls back to the standard path with useLsmWrite(false)", async () => {
+    const conn = await connect(tmpDir.name);
+    const table = await bucketTable(conn);
+
+    const res = await table
+      .mergeInsert("id")
+      .whenNotMatchedInsertAll()
+      .useLsmWrite(false)
+      .execute([
+        { id: "b", value: 9 },
+        { id: "e", value: 5 },
+      ]);
+    // Standard path commits: id="e" inserted ("b" already exists).
+    expect(res.numInsertedRows).toBe(1);
+    expect(await table.countRows()).toBe(3);
+  });
+
+  it("supports validateSingleShard(false)", async () => {
+    const conn = await connect(tmpDir.name);
+    const table = await bucketTable(conn);
+
+    const res = await table
+      .mergeInsert("id")
+      .whenMatchedUpdateAll()
+      .whenNotMatchedInsertAll()
+      .validateSingleShard(false)
+      .execute([{ id: "f", value: 6 }]);
+    expect(res.numRows).toBe(1);
+  });
+
+  it("rejects a non-upsert merge under an LSM spec", async () => {
+    const conn = await connect(tmpDir.name);
+    const table = await bucketTable(conn);
+
+    await expect(
+      table
+        .mergeInsert("id")
+        .whenNotMatchedInsertAll()
+        .execute([{ id: "g", value: 7 }]),
+    ).rejects.toThrow();
+  });
+});
--- a/nodejs/lancedb/merge.ts
+++ b/nodejs/lancedb/merge.ts
@@ -87,6 +87,41 @@ export class MergeInsertBuilder {
      this.#schema,
    );
  }
+  /**
+   * Controls whether the merge uses the MemWAL LSM write path.
+   *
+   * By default (unset), a `mergeInsert` on a table with an LSM write spec is
+   * routed through Lance's MemWAL shard writer, and a table without one uses
+   * the standard path. Pass `false` to force the standard path even when a
+   * spec is set. Pass `true` to require a spec — `mergeInsert` rejects if none
+   * is installed.
+   *
+   * @param useLsmWrite - Whether to use the LSM write path.
+   */
+  useLsmWrite(useLsmWrite: boolean): MergeInsertBuilder {
+    return new MergeInsertBuilder(
+      this.#native.useLsmWrite(useLsmWrite),
+      this.#schema,
+    );
+  }
+  /**
+   * Controls how an LSM merge checks that its input targets a single shard.
+   *
+   * When a table has an LSM write spec, every row in a `mergeInsert` call must
+   * route to the same shard. When `true` (the default), every row is inspected
+   * to verify this. When `false`, only the first row is inspected and the
+   * shard it routes to is used for the whole input — a faster path for callers
+   * that have already pre-sharded their input. Has no effect on tables without
+   * an LSM write spec.
+   *
+   * @param validateSingleShard - Whether to check every row routes to one shard. Defaults to `true`.
+   */
+  validateSingleShard(validateSingleShard: boolean): MergeInsertBuilder {
+    return new MergeInsertBuilder(
+      this.#native.validateSingleShard(validateSingleShard),
+      this.#schema,
+    );
+  }
  /**
   * Executes the merge insert operation
   *
--- a/nodejs/lancedb/table.ts
+++ b/nodejs/lancedb/table.ts
@@ -161,7 +161,10 @@ export interface Version {
 *
 * `specType` is `"bucket"`, `"identity"`, or `"unsharded"`. For `"bucket"`,
 * `column` and `numBuckets` are required; for `"identity"`, `column` is
- * required.
+ * required and must be a deterministic function of the unenforced primary
+ * key (every row with a given primary key must always produce the same
+ * `column` value, or upserts of that key can land in different shards and a
+ * stale version can win).
 */
 export interface LsmWriteSpec {
  /** One of `"bucket"`, `"identity"`, or `"unsharded"`. */
@@ -567,6 +570,16 @@ export abstract class Table {
   * @returns {Promise<void>}
   */
  abstract unsetLsmWriteSpec(): Promise<void>;
+  /**
+   * Drain and close any cached MemWAL shard writers held for this table.
+   *
+   * When an {@link LsmWriteSpec} is installed, `mergeInsert` opens MemWAL
+   * shard writers and caches them for reuse across calls. This closes them,
+   * flushing pending data; writers reopen lazily on the next `mergeInsert`.
+   * It is a no-op when no writers are cached.
+   * @returns {Promise<void>}
+   */
+  abstract closeLsmWriters(): Promise<void>;
  /** Retrieve the version of the table */

  abstract version(): Promise<number>;
@@ -1041,6 +1054,10 @@ export class LocalTable extends Table {
    return await this.inner.unsetLsmWriteSpec();
  }

+  async closeLsmWriters(): Promise<void> {
+    return await this.inner.closeLsmWriters();
+  }
+
  async version(): Promise<number> {
    return await this.inner.version();
  }
--- a/nodejs/npm/darwin-arm64/package.json
+++ b/nodejs/npm/darwin-arm64/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "@lancedb/lancedb-darwin-arm64",
-	"version": "0.30.0-beta.1",
+	"version": "0.30.1-beta.0",
 	"os": ["darwin"],
 	"cpu": ["arm64"],
 	"main": "lancedb.darwin-arm64.node",
--- a/nodejs/npm/linux-arm64-gnu/package.json
+++ b/nodejs/npm/linux-arm64-gnu/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "@lancedb/lancedb-linux-arm64-gnu",
-	"version": "0.30.0-beta.1",
+	"version": "0.30.1-beta.0",
 	"os": ["linux"],
 	"cpu": ["arm64"],
 	"main": "lancedb.linux-arm64-gnu.node",
--- a/nodejs/npm/linux-arm64-musl/package.json
+++ b/nodejs/npm/linux-arm64-musl/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "@lancedb/lancedb-linux-arm64-musl",
-	"version": "0.30.0-beta.1",
+	"version": "0.30.1-beta.0",
 	"os": ["linux"],
 	"cpu": ["arm64"],
 	"main": "lancedb.linux-arm64-musl.node",
--- a/nodejs/npm/linux-x64-gnu/package.json
+++ b/nodejs/npm/linux-x64-gnu/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "@lancedb/lancedb-linux-x64-gnu",
-	"version": "0.30.0-beta.1",
+	"version": "0.30.1-beta.0",
 	"os": ["linux"],
 	"cpu": ["x64"],
 	"main": "lancedb.linux-x64-gnu.node",
--- a/nodejs/npm/linux-x64-musl/package.json
+++ b/nodejs/npm/linux-x64-musl/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "@lancedb/lancedb-linux-x64-musl",
-	"version": "0.30.0-beta.1",
+	"version": "0.30.1-beta.0",
 	"os": ["linux"],
 	"cpu": ["x64"],
 	"main": "lancedb.linux-x64-musl.node",
--- a/nodejs/npm/win32-arm64-msvc/package.json
+++ b/nodejs/npm/win32-arm64-msvc/package.json
@@ -1,6 +1,6 @@
 {
  "name": "@lancedb/lancedb-win32-arm64-msvc",
-  "version": "0.30.0-beta.1",
+  "version": "0.30.1-beta.0",
  "os": [
    "win32"
  ],
--- a/nodejs/npm/win32-x64-msvc/package.json
+++ b/nodejs/npm/win32-x64-msvc/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "@lancedb/lancedb-win32-x64-msvc",
-	"version": "0.30.0-beta.1",
+	"version": "0.30.1-beta.0",
 	"os": ["win32"],
 	"cpu": ["x64"],
 	"main": "lancedb.win32-x64-msvc.node",
--- a/nodejs/package-lock.json
+++ b/nodejs/package-lock.json
@@ -1,12 +1,12 @@
 {
  "name": "@lancedb/lancedb",
-  "version": "0.30.0-beta.1",
+  "version": "0.30.1-beta.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "@lancedb/lancedb",
-      "version": "0.30.0-beta.1",
+      "version": "0.30.1-beta.0",
      "cpu": [
        "x64",
        "arm64"
--- a/nodejs/package.json
+++ b/nodejs/package.json
@@ -11,7 +11,7 @@
    "ann"
  ],
  "private": false,
-  "version": "0.30.0-beta.1",
+  "version": "0.30.1-beta.0",
  "main": "dist/index.js",
  "exports": {
    ".": "./dist/index.js",
--- a/nodejs/src/merge.rs
+++ b/nodejs/src/merge.rs
@@ -50,6 +50,20 @@ impl NativeMergeInsertBuilder {
        this
    }

+    #[napi]
+    pub fn use_lsm_write(&self, use_lsm_write: bool) -> Self {
+        let mut this = self.clone();
+        this.inner.use_lsm_write(use_lsm_write);
+        this
+    }
+
+    #[napi]
+    pub fn validate_single_shard(&self, validate_single_shard: bool) -> Self {
+        let mut this = self.clone();
+        this.inner.validate_single_shard(validate_single_shard);
+        this
+    }
+
    #[napi(catch_unwind)]
    pub async fn execute(&self, buf: Buffer) -> napi::Result<MergeResult> {
        let data = ipc_file_to_batches(buf.to_vec())
--- a/nodejs/src/table.rs
+++ b/nodejs/src/table.rs
@@ -391,6 +391,11 @@ impl Table {
            .default_error()
    }

+    #[napi(catch_unwind)]
+    pub async fn close_lsm_writers(&self) -> napi::Result<()> {
+        self.inner_ref()?.close_lsm_writers().await.default_error()
+    }
+
    #[napi(catch_unwind)]
    pub async fn version(&self) -> napi::Result<i64> {
        self.inner_ref()?
@@ -940,6 +945,7 @@ pub struct MergeResult {
    pub num_updated_rows: i64,
    pub num_deleted_rows: i64,
    pub num_attempts: i64,
+    pub num_rows: i64,
 }

 impl From<lancedb::table::MergeResult> for MergeResult {
@@ -950,6 +956,7 @@ impl From<lancedb::table::MergeResult> for MergeResult {
            num_updated_rows: value.num_updated_rows as i64,
            num_deleted_rows: value.num_deleted_rows as i64,
            num_attempts: value.num_attempts as i64,
+            num_rows: value.num_rows as i64,
        }
    }
 }
--- a/python/.bumpversion.toml
+++ b/python/.bumpversion.toml
@@ -1,5 +1,5 @@
 [tool.bumpversion]
-current_version = "0.33.0-beta.1"
+current_version = "0.33.1-beta.0"
 parse = """(?x)
    (?P<major>0|[1-9]\\d*)\\.
    (?P<minor>0|[1-9]\\d*)\\.
--- a/python/Cargo.toml
+++ b/python/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "lancedb-python"
-version = "0.33.0-beta.1"
+version = "0.33.1-beta.0"
 publish = false
 edition.workspace = true
 description = "Python bindings for LanceDB"
--- a/python/python/lancedb/_lancedb.pyi
+++ b/python/python/lancedb/_lancedb.pyi
@@ -220,6 +220,7 @@ class Table:
    async def set_unenforced_primary_key(self, columns: List[str]) -> None: ...
    async def set_lsm_write_spec(self, spec: LsmWriteSpec) -> None: ...
    async def unset_lsm_write_spec(self) -> None: ...
+    async def close_lsm_writers(self) -> None: ...
    @property
    def tags(self) -> Tags: ...
    def query(self) -> Query: ...
@@ -420,6 +421,7 @@ class MergeResult:
    num_inserted_rows: int
    num_deleted_rows: int
    num_attempts: int
+    num_rows: int

 class LsmWriteSpec:
    """Specification selecting Lance's MemWAL LSM-style write path for
--- a/python/python/lancedb/index.py
+++ b/python/python/lancedb/index.py
@@ -281,6 +281,9 @@ class HnswPq:
    m: int = 20
    ef_construction: int = 300
    target_partition_size: Optional[int] = None
+    # Name of the accelerator (e.g. "cuda") to use for IVF training. When set,
+    # create_index() dispatches to pylance to build the index on the accelerator.
+    accelerator: Optional[str] = None


@dataclass
@@ -386,6 +389,9 @@ class HnswSq:
    m: int = 20
    ef_construction: int = 300
    target_partition_size: Optional[int] = None
+    # Name of the accelerator (e.g. "cuda") to use for IVF training. When set,
+    # create_index() dispatches to pylance to build the index on the accelerator.
+    accelerator: Optional[str] = None


@dataclass
@@ -579,6 +585,9 @@ class IvfFlat:
    max_iterations: int = 50
    sample_rate: int = 256
    target_partition_size: Optional[int] = None
+    # Name of the accelerator (e.g. "cuda") to use for IVF training. When set,
+    # create_index() dispatches to pylance to build the index on the accelerator.
+    accelerator: Optional[str] = None


@dataclass
@@ -609,6 +618,9 @@ class IvfSq:
    max_iterations: int = 50
    sample_rate: int = 256
    target_partition_size: Optional[int] = None
+    # Name of the accelerator (e.g. "cuda") to use for IVF training. When set,
+    # create_index() dispatches to pylance to build the index on the accelerator.
+    accelerator: Optional[str] = None


@dataclass
@@ -739,6 +751,9 @@ class IvfPq:
    max_iterations: int = 50
    sample_rate: int = 256
    target_partition_size: Optional[int] = None
+    # Name of the accelerator (e.g. "cuda") to use for IVF training. When set,
+    # create_index() dispatches to pylance to build the index on the accelerator.
+    accelerator: Optional[str] = None


@dataclass
@@ -792,6 +807,9 @@ class IvfRq:
    max_iterations: int = 50
    sample_rate: int = 256
    target_partition_size: Optional[int] = None
+    # Name of the accelerator (e.g. "cuda") to use for IVF training. When set,
+    # create_index() dispatches to pylance to build the index on the accelerator.
+    accelerator: Optional[str] = None


 __all__ = [
--- a/python/python/lancedb/merge.py
+++ b/python/python/lancedb/merge.py
@@ -34,6 +34,8 @@ class LanceMergeInsertBuilder(object):
        self._when_not_matched_by_source_condition = None
        self._timeout = None
        self._use_index = True
+        self._use_lsm_write = None
+        self._validate_single_shard = None

    def when_matched_update_all(
        self, *, where: Optional[str] = None
@@ -96,6 +98,46 @@ class LanceMergeInsertBuilder(object):
        self._use_index = use_index
        return self

+    def use_lsm_write(self, use_lsm_write: bool) -> LanceMergeInsertBuilder:
+        """
+        Controls whether the merge uses the MemWAL LSM write path.
+
+        By default (unset), a `merge_insert` on a table with an LSM write spec
+        is routed through Lance's MemWAL shard writer, and a table without one
+        uses the standard path. Pass `False` to force the standard path even
+        when a spec is set. Pass `True` to require a spec — `merge_insert`
+        raises an error if none is installed.
+
+        Parameters
+        ----------
+        use_lsm_write: bool
+            Whether to use the LSM write path.
+        """
+        self._use_lsm_write = use_lsm_write
+        return self
+
+    def validate_single_shard(
+        self, validate_single_shard: bool
+    ) -> LanceMergeInsertBuilder:
+        """
+        Controls how an LSM merge checks that its input targets a single shard.
+
+        When a table has an LSM write spec, every row in a `merge_insert` call
+        must route to the same shard. When `True` (the default), every row is
+        inspected to verify this. When `False`, only the first row is inspected
+        and the shard it routes to is used for the whole input — a faster path
+        for callers that have already pre-sharded their input.
+
+        Has no effect on tables without an LSM write spec.
+
+        Parameters
+        ----------
+        validate_single_shard: bool
+            Whether to check every row routes to one shard. Defaults to `True`.
+        """
+        self._validate_single_shard = validate_single_shard
+        return self
+
    def execute(
        self,
        new_data: DATA,
--- a/python/python/lancedb/remote/table.py
+++ b/python/python/lancedb/remote/table.py
@@ -2,11 +2,24 @@
 # SPDX-FileCopyrightText: Copyright The LanceDB Authors

 from datetime import timedelta
+import deprecation
 import logging
 from functools import cached_property
-from typing import Any, Callable, Dict, Iterable, List, Optional, Union, Literal
+from typing import (
+    Any,
+    Callable,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    Union,
+    Literal,
+    overload,
+)
 import warnings

+from lancedb import __version__
+
 from lancedb._lancedb import (
    AddColumnsResult,
    AddResult,
@@ -32,6 +45,7 @@ from lancedb.index import (
    LabelList,
 )
 from lancedb.remote.db import LOOP
+from lancedb.table import IndexConfigType, KNOWN_METRICS
 import pyarrow as pa

 from lancedb.common import DATA, VEC, VECTOR_COLUMN_NAME
@@ -122,6 +136,11 @@ class RemoteTable(Table):
        """List all the stats of a specified index"""
        return LOOP.run(self._table.index_stats(index_uuid))

+    @deprecation.deprecated(
+        deprecated_in="0.25.0",
+        current_version=__version__,
+        details="Use create_index() with config=BTree()/Bitmap()/LabelList() instead.",
+    )
    def create_scalar_index(
        self,
        column: str,
@@ -131,7 +150,12 @@ class RemoteTable(Table):
        wait_timeout: Optional[timedelta] = None,
        name: Optional[str] = None,
    ):
-        """Creates a scalar index
+        """Creates a scalar index.
+
+        .. deprecated:: 0.25.0
+            Use :meth:`create_index` with a BTree, Bitmap, or LabelList config instead.
+            Example: ``table.create_index("column", config=BTree())``
+
        Parameters
        ----------
        column : str
@@ -162,6 +186,11 @@ class RemoteTable(Table):
            )
        )

+    @deprecation.deprecated(
+        deprecated_in="0.25.0",
+        current_version=__version__,
+        details="Use create_index() with config=FTS() instead.",
+    )
    def create_fts_index(
        self,
        column: str,
@@ -182,6 +211,12 @@ class RemoteTable(Table):
        prefix_only: bool = False,
        name: Optional[str] = None,
    ):
+        """Create a full-text search index on a column.
+
+        .. deprecated:: 0.25.0
+            Use :meth:`create_index` with an FTS config instead.
+            Example: ``table.create_index("text_column", config=FTS())``
+        """
        config = FTS(
            with_position=with_position,
            base_tokenizer=base_tokenizer,
@@ -205,9 +240,43 @@ class RemoteTable(Table):
            )
        )

+    # New unified API overload
+    @overload
    def create_index(
        self,
-        metric="l2",
+        column: str,
+        /,
+        *,
+        config: IndexConfigType,
+        wait_timeout: Optional[timedelta] = ...,
+        name: Optional[str] = ...,
+        train: bool = ...,
+    ) -> None: ...
+
+    # Legacy API overload (deprecated)
+    @overload
+    def create_index(
+        self,
+        metric: Literal["l2", "cosine", "dot", "hamming"] = ...,
+        vector_column_name: str = ...,
+        index_cache_size: Optional[int] = ...,
+        num_partitions: Optional[int] = ...,
+        num_sub_vectors: Optional[int] = ...,
+        replace: Optional[bool] = ...,
+        accelerator: Optional[str] = ...,
+        index_type: Literal[
+            "VECTOR", "IVF_FLAT", "IVF_SQ", "IVF_PQ", "IVF_HNSW_SQ", "IVF_HNSW_PQ"
+        ] = ...,
+        wait_timeout: Optional[timedelta] = ...,
+        *,
+        num_bits: int = ...,
+        name: Optional[str] = ...,
+        train: bool = ...,
+    ) -> None: ...
+
+    def create_index(
+        self,
+        metric: str = "l2",
        vector_column_name: str = VECTOR_COLUMN_NAME,
        index_cache_size: Optional[int] = None,
        num_partitions: Optional[int] = None,
@@ -218,89 +287,113 @@ class RemoteTable(Table):
        wait_timeout: Optional[timedelta] = None,
        *,
        num_bits: int = 8,
+        config: Optional[IndexConfigType] = None,
        name: Optional[str] = None,
        train: bool = True,
    ):
-        """Create an index on the table.
+        """Create an index on a column.

-        Parameters
-        ----------
-        metric : str
-            The metric to use for the index. Default is "l2".
-        vector_column_name : str
-            The name of the vector column. Default is "vector".
+        This method supports both the new unified API and the legacy API
+        for backwards compatibility. The new API takes the column name as the
+        first positional argument and an index configuration object via
+        ``config``; the legacy API takes the distance metric as the first
+        argument plus separate ``vector_column_name`` / ``num_partitions`` /
+        etc. parameters, and emits a ``DeprecationWarning``.

        Examples
        --------
-        >>> import lancedb
-        >>> import uuid
-        >>> from lancedb.schema import vector
-        >>> db = lancedb.connect("db://...", api_key="...", # doctest: +SKIP
-        ...                      region="...") # doctest: +SKIP
-        >>> table_name = uuid.uuid4().hex
-        >>> schema = pa.schema(
-        ...     [
-        ...             pa.field("id", pa.uint32(), False),
-        ...            pa.field("vector", vector(128), False),
-        ...             pa.field("s", pa.string(), False),
-        ...     ]
+        New API (recommended):
+
+        >>> table.create_index(  # doctest: +SKIP
+        ...     "vector", config=IvfPq(distance_type="l2")
        ... )
-        >>> table = db.create_table( # doctest: +SKIP
-        ...     table_name, # doctest: +SKIP
-        ...     schema=schema, # doctest: +SKIP
+        >>> table.create_index("category", config=BTree())  # doctest: +SKIP
+        >>> table.create_index("content", config=FTS())  # doctest: +SKIP
+
+        Legacy API (deprecated):
+
+        >>> table.create_index(  # doctest: +SKIP
+        ...     "l2", vector_column_name="vector"
        ... )
-        >>> table.create_index("l2", "vector") # doctest: +SKIP
        """
+        # Detect whether this is a legacy API call
+        is_legacy = self._is_legacy_create_index_call(
+            metric,
+            config,
+            num_partitions,
+            num_sub_vectors,
+            vector_column_name,
+            accelerator,
+            index_cache_size,
+            replace,
+        )

-        if accelerator is not None:
-            logging.warning(
-                "GPU accelerator is not yet supported on LanceDB cloud."
-                "If you have 100M+ vectors to index,"
-                "please contact us at contact@lancedb.com"
-            )
-        if replace is not None:
-            logging.warning(
-                "replace is not supported on LanceDB cloud."
-                "Existing indexes will always be replaced."
+        if is_legacy:
+            warnings.warn(
+                "The create_index() API with metric/num_partitions parameters is "
+                "deprecated and will be removed in a future version. "
+                "Please migrate to the new unified API:\n"
+                "  # Old (deprecated):\n"
+                "  table.create_index('l2', vector_column_name='my_vector')\n"
+                "  # New (recommended):\n"
+                "  table.create_index('my_vector', config=IvfPq(distance_type='l2'))",
+                DeprecationWarning,
+                stacklevel=2,
            )

-        index_type = index_type.upper()
-        if index_type == "VECTOR" or index_type == "IVF_PQ":
-            config = IvfPq(
-                distance_type=metric,
-                num_partitions=num_partitions,
-                num_sub_vectors=num_sub_vectors,
-                num_bits=num_bits,
-            )
-        elif index_type == "IVF_RQ":
-            config = IvfRq(
-                distance_type=metric,
-                num_partitions=num_partitions,
-                num_bits=num_bits,
-            )
-        elif index_type == "IVF_SQ":
-            config = IvfSq(distance_type=metric, num_partitions=num_partitions)
-        elif index_type == "IVF_HNSW_PQ":
-            raise ValueError(
-                "IVF_HNSW_PQ is not supported on LanceDB cloud."
-                "Please use IVF_HNSW_SQ instead."
-            )
-        elif index_type == "IVF_HNSW_SQ":
-            config = HnswSq(distance_type=metric, num_partitions=num_partitions)
-        elif index_type == "IVF_HNSW_FLAT":
-            config = HnswFlat(distance_type=metric, num_partitions=num_partitions)
-        elif index_type == "IVF_FLAT":
-            config = IvfFlat(distance_type=metric, num_partitions=num_partitions)
+            column = vector_column_name
+
+            if accelerator is not None:
+                logging.warning(
+                    "GPU accelerator is not yet supported on LanceDB cloud."
+                    "If you have 100M+ vectors to index,"
+                    "please contact us at contact@lancedb.com"
+                )
+            if replace is not None:
+                logging.warning(
+                    "replace is not supported on LanceDB cloud."
+                    "Existing indexes will always be replaced."
+                )
+
+            idx_type = index_type.upper()
+            if idx_type == "VECTOR" or idx_type == "IVF_PQ":
+                config = IvfPq(
+                    distance_type=metric,
+                    num_partitions=num_partitions,
+                    num_sub_vectors=num_sub_vectors,
+                    num_bits=num_bits,
+                )
+            elif idx_type == "IVF_RQ":
+                config = IvfRq(
+                    distance_type=metric,
+                    num_partitions=num_partitions,
+                    num_bits=num_bits,
+                )
+            elif idx_type == "IVF_SQ":
+                config = IvfSq(distance_type=metric, num_partitions=num_partitions)
+            elif idx_type == "IVF_HNSW_PQ":
+                raise ValueError(
+                    "IVF_HNSW_PQ is not supported on LanceDB cloud."
+                    "Please use IVF_HNSW_SQ instead."
+                )
+            elif idx_type == "IVF_HNSW_SQ":
+                config = HnswSq(distance_type=metric, num_partitions=num_partitions)
+            elif idx_type == "IVF_HNSW_FLAT":
+                config = HnswFlat(distance_type=metric, num_partitions=num_partitions)
+            elif idx_type == "IVF_FLAT":
+                config = IvfFlat(distance_type=metric, num_partitions=num_partitions)
+            else:
+                raise ValueError(
+                    f"Unknown vector index type: {idx_type}. Valid options are"
+                    " 'IVF_FLAT', 'IVF_PQ', 'IVF_RQ', 'IVF_SQ',"
+                    " 'IVF_HNSW_PQ', 'IVF_HNSW_SQ', 'IVF_HNSW_FLAT'"
+                )
        else:
-            raise ValueError(
-                f"Unknown vector index type: {index_type}. Valid options are"
-                " 'IVF_FLAT', 'IVF_PQ', 'IVF_RQ', 'IVF_SQ',"
-                " 'IVF_HNSW_PQ', 'IVF_HNSW_SQ', 'IVF_HNSW_FLAT'"
-            )
+            column = metric

        LOOP.run(
            self._table.create_index(
-                vector_column_name,
+                column,
                config=config,
                wait_timeout=wait_timeout,
                name=name,
@@ -308,6 +401,37 @@ class RemoteTable(Table):
            )
        )

+    def _is_legacy_create_index_call(
+        self,
+        first_arg: str,
+        config: Optional[IndexConfigType],
+        num_partitions: Optional[int],
+        num_sub_vectors: Optional[int],
+        vector_column_name: str,
+        accelerator: Optional[str],
+        index_cache_size: Optional[int],
+        replace: Optional[bool],
+    ) -> bool:
+        """Detect if this is a legacy create_index call."""
+        if config is not None:
+            return False
+        if any(
+            x is not None
+            for x in (
+                num_partitions,
+                num_sub_vectors,
+                accelerator,
+                index_cache_size,
+                replace,
+            )
+        ):
+            return True
+        if vector_column_name != VECTOR_COLUMN_NAME:
+            return True
+        if first_arg.lower() in KNOWN_METRICS:
+            return True
+        return False
+
    def add(
        self,
        data: DATA,
@@ -668,6 +792,10 @@ class RemoteTable(Table):
        """Not supported on LanceDB Cloud."""
        return LOOP.run(self._table.unset_lsm_write_spec())

+    def close_lsm_writers(self) -> None:
+        """No-op on LanceDB Cloud (no local shard writers)."""
+        return LOOP.run(self._table.close_lsm_writers())
+
    def drop_index(self, index_name: str):
        return LOOP.run(self._table.drop_index(index_name))

--- a/python/python/lancedb/table.py
+++ b/python/python/lancedb/table.py
@@ -174,6 +174,24 @@ if TYPE_CHECKING:
        DistanceType,
    )

+# Type alias for index configuration objects
+IndexConfigType = Union[
+    IvfFlat,
+    IvfPq,
+    IvfSq,
+    IvfRq,
+    HnswFlat,
+    HnswPq,
+    HnswSq,
+    BTree,
+    Bitmap,
+    LabelList,
+    FTS,
+]
+
+# Known distance metrics for legacy API detection
+KNOWN_METRICS = {"l2", "cosine", "dot", "hamming"}
+

 def _into_pyarrow_reader(
    data, schema: Optional[pa.Schema] = None
@@ -807,11 +825,49 @@ class Table(ABC):
        """
        raise NotImplementedError

+    # New unified API overload
+    @overload
    def create_index(
        self,
-        metric="l2",
-        num_partitions=256,
-        num_sub_vectors=96,
+        column: str,
+        /,
+        *,
+        config: IndexConfigType,
+        replace: bool = ...,
+        wait_timeout: Optional[timedelta] = ...,
+        name: Optional[str] = ...,
+        train: bool = ...,
+    ) -> None: ...
+
+    # Legacy API overload (deprecated)
+    @overload
+    def create_index(
+        self,
+        metric: Literal["l2", "cosine", "dot", "hamming"] = ...,
+        num_partitions: Optional[int] = ...,
+        num_sub_vectors: Optional[int] = ...,
+        vector_column_name: str = ...,
+        replace: bool = ...,
+        accelerator: Optional[str] = ...,
+        index_cache_size: Optional[int] = ...,
+        *,
+        index_type: VectorIndexType = ...,
+        wait_timeout: Optional[timedelta] = ...,
+        num_bits: int = ...,
+        max_iterations: int = ...,
+        sample_rate: int = ...,
+        m: int = ...,
+        ef_construction: int = ...,
+        name: Optional[str] = ...,
+        train: bool = ...,
+        target_partition_size: Optional[int] = ...,
+    ) -> None: ...
+
+    def create_index(
+        self,
+        metric: DistanceType = "l2",
+        num_partitions: Optional[int] = None,
+        num_sub_vectors: Optional[int] = None,
        vector_column_name: str = VECTOR_COLUMN_NAME,
        replace: bool = True,
        accelerator: Optional[str] = None,
@@ -824,46 +880,53 @@ class Table(ABC):
        sample_rate: int = 256,
        m: int = 20,
        ef_construction: int = 300,
+        config: Optional[IndexConfigType] = None,
        name: Optional[str] = None,
        train: bool = True,
        target_partition_size: Optional[int] = None,
    ):
-        """Create an index on the table.
+        """Create an index on a column.
+
+        This method supports both the new unified API and the legacy API
+        for backwards compatibility. The new API takes the column name as the
+        first positional argument and an index configuration object via
+        ``config``; the legacy API takes the distance metric as the first
+        argument plus separate ``vector_column_name`` / ``num_partitions`` /
+        etc. parameters, and emits a ``DeprecationWarning``.

        Parameters
        ----------
-        metric: str, default "l2"
-            The distance metric to use when creating the index.
-            Valid values are "l2", "cosine", "dot", or "hamming".
-            l2 is euclidean distance.
-            Hamming is available only for binary vectors.
-        num_partitions: int, default 256
-            The number of IVF partitions to use when creating the index.
-            Default is 256.
-        num_sub_vectors: int, default 96
-            The number of PQ sub-vectors to use when creating the index.
-            Default is 96.
-        vector_column_name: str, default "vector"
-            The vector column name to create the index.
-        replace: bool, default True
-            - If True, replace the existing index if it exists.
+        metric : str
+            For new API: the column name to index.
+            For legacy API: the distance metric ("l2", "cosine", "dot", "hamming").
+        config : IndexConfigType, optional
+            The index configuration object. If provided, uses the new unified API.
+            Can be one of: IvfFlat, IvfPq, IvfSq, IvfRq, HnswPq, HnswSq,
+            BTree, Bitmap, LabelList, FTS.
+        replace : bool, default True
+            Whether to replace an existing index on this column.
+        wait_timeout : timedelta, optional
+            Timeout to wait for async indexing to complete.
+        name : str, optional
+            Custom name for the index.
+        train : bool, default True
+            Whether to train the index with existing data.

-            - If False, raise an error if duplicate index exists.
-        accelerator: str, default None
-            If set, use the given accelerator to create the index.
-            Only support "cuda" for now.
-        index_cache_size : int, optional
-            The size of the index cache in number of entries. Default value is 256.
-        num_bits: int
-            The number of bits to encode sub-vectors. Only used with the IVF_PQ index.
-            Only 4 and 8 are supported.
-        wait_timeout: timedelta, optional
-            The timeout to wait if indexing is asynchronous.
-        name: str, optional
-            The name of the index. If not provided, a default name will be generated.
-        train: bool, default True
-            Whether to train the index with existing data. Vector indices always train
-            with existing data.
+        Examples
+        --------
+        New API (recommended):
+
+        >>> table.create_index(  # doctest: +SKIP
+        ...     "vector", config=IvfPq(distance_type="l2")
+        ... )
+        >>> table.create_index("category", config=BTree())  # doctest: +SKIP
+        >>> table.create_index("content", config=FTS())  # doctest: +SKIP
+
+        Legacy API (deprecated):
+
+        >>> table.create_index(  # doctest: +SKIP
+        ...     "l2", vector_column_name="vector"
+        ... )
        """
        raise NotImplementedError

@@ -1188,7 +1251,7 @@ class Table(ABC):
        ...      .when_not_matched_insert_all() \\
        ...      .execute(new_data)
        >>> res
-        MergeResult(version=2, num_updated_rows=2, num_inserted_rows=1, num_deleted_rows=0, num_attempts=1)
+        MergeResult(version=2, num_updated_rows=2, num_inserted_rows=1, num_deleted_rows=0, num_attempts=1, num_rows=3)
        >>> # The order of new rows is non-deterministic since we use
        >>> # a hash-join as part of this operation and so we sort here
        >>> table.to_arrow().sort_by("a").to_pandas()
@@ -2250,11 +2313,51 @@ class LanceTable(Table):
            dataset, allow_pyarrow_filter=False, batch_size=batch_size
        )

+    # New unified API overload
+    @overload
    def create_index(
        self,
-        metric: DistanceType = "l2",
-        num_partitions=None,
-        num_sub_vectors=None,
+        column: str,
+        /,
+        *,
+        config: IndexConfigType,
+        replace: bool = ...,
+        wait_timeout: Optional[timedelta] = ...,
+        name: Optional[str] = ...,
+        train: bool = ...,
+    ) -> None: ...
+
+    # Legacy API overload (deprecated)
+    @overload
+    def create_index(
+        self,
+        metric: Literal["l2", "cosine", "dot", "hamming"] = ...,
+        num_partitions: Optional[int] = ...,
+        num_sub_vectors: Optional[int] = ...,
+        vector_column_name: str = ...,
+        replace: bool = ...,
+        accelerator: Optional[str] = ...,
+        index_cache_size: Optional[int] = ...,
+        num_bits: int = ...,
+        index_type: Literal[
+            "IVF_FLAT", "IVF_SQ", "IVF_PQ", "IVF_RQ", "IVF_HNSW_SQ", "IVF_HNSW_PQ"
+        ] = ...,
+        max_iterations: int = ...,
+        sample_rate: int = ...,
+        m: int = ...,
+        ef_construction: int = ...,
+        *,
+        wait_timeout: Optional[timedelta] = ...,
+        name: Optional[str] = ...,
+        train: bool = ...,
+        target_partition_size: Optional[int] = ...,
+    ) -> None: ...
+
+    def create_index(
+        self,
+        metric: str = "l2",
+        num_partitions: Optional[int] = None,
+        num_sub_vectors: Optional[int] = None,
        vector_column_name: str = VECTOR_COLUMN_NAME,
        replace: bool = True,
        accelerator: Optional[str] = None,
@@ -2274,47 +2377,232 @@ class LanceTable(Table):
        m: int = 20,
        ef_construction: int = 300,
        *,
+        config: Optional[IndexConfigType] = None,
+        wait_timeout: Optional[timedelta] = None,
        name: Optional[str] = None,
        train: bool = True,
        target_partition_size: Optional[int] = None,
    ):
-        """Create an index on the table."""
-        if accelerator is not None:
-            # accelerator is only supported through pylance.
-            self.to_lance().create_index(
-                column=vector_column_name,
-                index_type=index_type,
+        """Create an index on a column.
+
+        This method supports both the new unified API and the legacy API
+        for backwards compatibility. The new API takes the column name as the
+        first positional argument and an index configuration object via
+        ``config``; the legacy API takes the distance metric as the first
+        argument plus separate ``vector_column_name`` / ``num_partitions`` /
+        etc. parameters, and emits a ``DeprecationWarning``.
+
+        Parameters
+        ----------
+        metric : str
+            For new API: the column name to index.
+            For legacy API: the distance metric ("l2", "cosine", "dot", "hamming").
+        config : IndexConfigType, optional
+            The index configuration object. If provided, uses the new unified API.
+            Can be one of: IvfFlat, IvfPq, IvfSq, IvfRq, HnswPq, HnswSq,
+            BTree, Bitmap, LabelList, FTS.
+        replace : bool, default True
+            Whether to replace an existing index on this column.
+        wait_timeout : timedelta, optional
+            Timeout to wait for async indexing to complete.
+        name : str, optional
+            Custom name for the index.
+        train : bool, default True
+            Whether to train the index with existing data.
+
+        Examples
+        --------
+        New API (recommended):
+
+        >>> table.create_index(  # doctest: +SKIP
+        ...     "vector", config=IvfPq(distance_type="l2")
+        ... )
+        >>> table.create_index("category", config=BTree())  # doctest: +SKIP
+        >>> table.create_index("content", config=FTS())  # doctest: +SKIP
+
+        Legacy API (deprecated):
+
+        >>> table.create_index(  # doctest: +SKIP
+        ...     "l2", vector_column_name="vector"
+        ... )
+        """
+        # Detect whether this is a legacy API call
+        is_legacy = self._is_legacy_create_index_call(
+            metric,
+            config,
+            num_partitions,
+            num_sub_vectors,
+            vector_column_name,
+            accelerator,
+            index_cache_size,
+        )
+
+        if is_legacy:
+            warnings.warn(
+                "The create_index() API with metric/num_partitions parameters is "
+                "deprecated and will be removed in a future version. "
+                "Please migrate to the new unified API:\n"
+                "  # Old (deprecated):\n"
+                "  table.create_index('l2', vector_column_name='my_vector')\n"
+                "  # New (recommended):\n"
+                "  table.create_index('my_vector', config=IvfPq(distance_type='l2'))",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+
+            # Legacy API: first arg is the distance metric
+            column = vector_column_name
+
+            # Build config from legacy parameters
+            config = self._build_vector_config_from_legacy_params(
                metric=metric,
+                index_type=index_type,
                num_partitions=num_partitions,
                num_sub_vectors=num_sub_vectors,
-                replace=replace,
-                accelerator=accelerator,
-                index_cache_size=index_cache_size,
                num_bits=num_bits,
+                max_iterations=max_iterations,
+                sample_rate=sample_rate,
                m=m,
                ef_construction=ef_construction,
                target_partition_size=target_partition_size,
+                accelerator=accelerator,
            )
-            self.checkout_latest()
-            return
-        elif index_type == "IVF_FLAT":
-            config = IvfFlat(
+
+            # Handle accelerator through pylance
+            if accelerator is not None:
+                self.to_lance().create_index(
+                    column=column,
+                    index_type=index_type,
+                    metric=metric,
+                    num_partitions=num_partitions,
+                    num_sub_vectors=num_sub_vectors,
+                    replace=replace,
+                    accelerator=accelerator,
+                    index_cache_size=index_cache_size,
+                    num_bits=num_bits,
+                    m=m,
+                    ef_construction=ef_construction,
+                    target_partition_size=target_partition_size,
+                )
+                self.checkout_latest()
+                return
+        else:
+            # New API: metric is the column name
+            column = metric
+
+            # Check if config has accelerator set and dispatch to pylance
+            if config is not None and hasattr(config, "accelerator"):
+                acc = getattr(config, "accelerator", None)
+                if acc is not None:
+                    # Dispatch to pylance for GPU acceleration
+                    index_type_map = {
+                        "IvfFlat": "IVF_FLAT",
+                        "IvfSq": "IVF_SQ",
+                        "IvfPq": "IVF_PQ",
+                        "IvfRq": "IVF_RQ",
+                        "HnswPq": "IVF_HNSW_PQ",
+                        "HnswSq": "IVF_HNSW_SQ",
+                    }
+                    cfg_type = type(config).__name__
+                    lance_index_type = index_type_map.get(cfg_type, "IVF_PQ")
+
+                    self.to_lance().create_index(
+                        column=column,
+                        index_type=lance_index_type,
+                        metric=getattr(config, "distance_type", "l2"),
+                        num_partitions=getattr(config, "num_partitions", None),
+                        num_sub_vectors=getattr(config, "num_sub_vectors", None),
+                        replace=replace,
+                        accelerator=acc,
+                        num_bits=getattr(config, "num_bits", 8),
+                        m=getattr(config, "m", 20),
+                        ef_construction=getattr(config, "ef_construction", 300),
+                        target_partition_size=getattr(
+                            config, "target_partition_size", None
+                        ),
+                    )
+                    self.checkout_latest()
+                    return
+
+        return LOOP.run(
+            self._table.create_index(
+                column,
+                replace=replace,
+                config=config,
+                wait_timeout=wait_timeout,
+                name=name,
+                train=train,
+            )
+        )
+
+    def _is_legacy_create_index_call(
+        self,
+        first_arg: str,
+        config: Optional[IndexConfigType],
+        num_partitions: Optional[int],
+        num_sub_vectors: Optional[int],
+        vector_column_name: str,
+        accelerator: Optional[str],
+        index_cache_size: Optional[int],
+    ) -> bool:
+        """Detect if this is a legacy create_index call."""
+        # If config is provided, it's definitely the new API
+        if config is not None:
+            return False
+
+        # If old-style parameters were explicitly set, it's legacy
+        if any(
+            x is not None
+            for x in (num_partitions, num_sub_vectors, accelerator, index_cache_size)
+        ):
+            return True
+
+        # If vector_column_name differs from default, it's legacy
+        if vector_column_name != VECTOR_COLUMN_NAME:
+            return True
+
+        # If first arg is a known metric, assume legacy
+        if first_arg.lower() in KNOWN_METRICS:
+            return True
+
+        # Otherwise assume new API
+        return False
+
+    def _build_vector_config_from_legacy_params(
+        self,
+        metric: str,
+        index_type: str,
+        num_partitions: Optional[int],
+        num_sub_vectors: Optional[int],
+        num_bits: int,
+        max_iterations: int,
+        sample_rate: int,
+        m: int,
+        ef_construction: int,
+        target_partition_size: Optional[int],
+        accelerator: Optional[str],
+    ) -> IndexConfigType:
+        """Build an index config object from legacy parameters."""
+        if index_type == "IVF_FLAT":
+            return IvfFlat(
                distance_type=metric,
                num_partitions=num_partitions,
                max_iterations=max_iterations,
                sample_rate=sample_rate,
                target_partition_size=target_partition_size,
+                accelerator=accelerator,
            )
        elif index_type == "IVF_SQ":
-            config = IvfSq(
+            return IvfSq(
                distance_type=metric,
                num_partitions=num_partitions,
                max_iterations=max_iterations,
                sample_rate=sample_rate,
                target_partition_size=target_partition_size,
+                accelerator=accelerator,
            )
        elif index_type == "IVF_PQ":
-            config = IvfPq(
+            return IvfPq(
                distance_type=metric,
                num_partitions=num_partitions,
                num_sub_vectors=num_sub_vectors,
@@ -2322,18 +2610,20 @@ class LanceTable(Table):
                max_iterations=max_iterations,
                sample_rate=sample_rate,
                target_partition_size=target_partition_size,
+                accelerator=accelerator,
            )
        elif index_type == "IVF_RQ":
-            config = IvfRq(
+            return IvfRq(
                distance_type=metric,
                num_partitions=num_partitions,
                num_bits=num_bits,
                max_iterations=max_iterations,
                sample_rate=sample_rate,
                target_partition_size=target_partition_size,
+                accelerator=accelerator,
            )
        elif index_type == "IVF_HNSW_PQ":
-            config = HnswPq(
+            return HnswPq(
                distance_type=metric,
                num_partitions=num_partitions,
                num_sub_vectors=num_sub_vectors,
@@ -2343,9 +2633,10 @@ class LanceTable(Table):
                m=m,
                ef_construction=ef_construction,
                target_partition_size=target_partition_size,
+                accelerator=accelerator,
            )
        elif index_type == "IVF_HNSW_SQ":
-            config = HnswSq(
+            return HnswSq(
                distance_type=metric,
                num_partitions=num_partitions,
                max_iterations=max_iterations,
@@ -2353,9 +2644,10 @@ class LanceTable(Table):
                m=m,
                ef_construction=ef_construction,
                target_partition_size=target_partition_size,
+                accelerator=accelerator,
            )
        elif index_type == "IVF_HNSW_FLAT":
-            config = HnswFlat(
+            return HnswFlat(
                distance_type=metric,
                num_partitions=num_partitions,
                max_iterations=max_iterations,
@@ -2367,16 +2659,6 @@ class LanceTable(Table):
        else:
            raise ValueError(f"Unknown index type {index_type}")

-        return LOOP.run(
-            self._table.create_index(
-                vector_column_name,
-                replace=replace,
-                config=config,
-                name=name,
-                train=train,
-            )
-        )
-
    def drop_index(self, name: str) -> None:
        """
        Drops an index from the table
@@ -2476,6 +2758,11 @@ class LanceTable(Table):
        """
        return LOOP.run(self._table.latest_storage_options())

+    @deprecation.deprecated(
+        deprecated_in="0.25.0",
+        current_version=__version__,
+        details="Use create_index() with config=BTree()/Bitmap()/LabelList() instead.",
+    )
    def create_scalar_index(
        self,
        column: str,
@@ -2484,6 +2771,12 @@ class LanceTable(Table):
        index_type: ScalarIndexType = "BTREE",
        name: Optional[str] = None,
    ):
+        """Create a scalar index on a column.
+
+        .. deprecated:: 0.25.0
+            Use :meth:`create_index` with a BTree, Bitmap, or LabelList config instead.
+            Example: ``table.create_index("column", config=BTree())``
+        """
        if index_type == "BTREE":
            config = BTree()
        elif index_type == "BITMAP":
@@ -2496,6 +2789,11 @@ class LanceTable(Table):
            self._table.create_index(column, replace=replace, config=config, name=name)
        )

+    @deprecation.deprecated(
+        deprecated_in="0.25.0",
+        current_version=__version__,
+        details="Use create_index() with config=FTS() instead.",
+    )
    def create_fts_index(
        self,
        field_names: Union[str, List[str]],
@@ -2519,6 +2817,12 @@ class LanceTable(Table):
        prefix_only: bool = False,
        name: Optional[str] = None,
    ):
+        """Create a full-text search index on a column.
+
+        .. deprecated:: 0.25.0
+            Use :meth:`create_index` with an FTS config instead.
+            Example: ``table.create_index("text_column", config=FTS())``
+        """
        self._ensure_no_legacy_fts_index()

        if use_tantivy:
@@ -3297,6 +3601,11 @@ class LanceTable(Table):
        [`AsyncTable.unset_lsm_write_spec`][lancedb.AsyncTable.unset_lsm_write_spec]."""
        return LOOP.run(self._table.unset_lsm_write_spec())

+    def close_lsm_writers(self) -> None:
+        """Close cached MemWAL shard writers. See
+        [`AsyncTable.close_lsm_writers`][lancedb.AsyncTable.close_lsm_writers]."""
+        return LOOP.run(self._table.close_lsm_writers())
+
    def uses_v2_manifest_paths(self) -> bool:
        """
        Check if the table is using the new v2 manifest paths.
@@ -3905,6 +4214,16 @@ class AsyncTable:
        """
        await self._inner.unset_lsm_write_spec()

+    async def close_lsm_writers(self) -> None:
+        """Drain and close any cached MemWAL shard writers for this table.
+
+        When an LSM write spec is installed, `merge_insert` opens MemWAL shard
+        writers and caches them for reuse across calls. This closes them,
+        flushing pending data; writers reopen lazily on the next
+        `merge_insert`. It is a no-op when no writers are cached.
+        """
+        await self._inner.close_lsm_writers()
+
    @property
    def name(self) -> str:
        """The name of the table."""
@@ -4355,7 +4674,7 @@ class AsyncTable:
        ...      .when_not_matched_insert_all() \\
        ...      .execute(new_data)
        >>> res
-        MergeResult(version=2, num_updated_rows=2, num_inserted_rows=1, num_deleted_rows=0, num_attempts=1)
+        MergeResult(version=2, num_updated_rows=2, num_inserted_rows=1, num_deleted_rows=0, num_attempts=1, num_rows=3)
        >>> # The order of new rows is non-deterministic since we use
        >>> # a hash-join as part of this operation and so we sort here
        >>> table.to_arrow().sort_by("a").to_pandas()
@@ -4735,6 +5054,8 @@ class AsyncTable:
                when_not_matched_by_source_condition=merge._when_not_matched_by_source_condition,
                timeout=merge._timeout,
                use_index=merge._use_index,
+                use_lsm_write=merge._use_lsm_write,
+                validate_single_shard=merge._validate_single_shard,
            ),
        )

--- a/python/python/tests/docs/test_merge_insert.py
+++ b/python/python/tests/docs/test_merge_insert.py
@@ -57,7 +57,7 @@ async def test_upsert_async(mem_db_async):
    await table.count_rows()  # 3
    res
    # MergeResult(version=2, num_updated_rows=1,
-    # num_inserted_rows=1, num_deleted_rows=0)
+    # num_inserted_rows=1, num_deleted_rows=0, num_rows=2)
    # --8<-- [end:upsert_basic_async]
    assert await table.count_rows() == 3
    assert res.version == 2
@@ -86,7 +86,7 @@ def test_insert_if_not_exists(mem_db):
    table.count_rows()  # 3
    res
    # MergeResult(version=2, num_updated_rows=0,
-    # num_inserted_rows=1, num_deleted_rows=0)
+    # num_inserted_rows=1, num_deleted_rows=0, num_rows=1)
    # --8<-- [end:insert_if_not_exists]
    assert table.count_rows() == 3
    assert res.version == 2
@@ -116,7 +116,7 @@ async def test_insert_if_not_exists_async(mem_db_async):
    await table.count_rows()  # 3
    res
    # MergeResult(version=2, num_updated_rows=0,
-    # num_inserted_rows=1, num_deleted_rows=0)
+    # num_inserted_rows=1, num_deleted_rows=0, num_rows=1)
    # --8<-- [end:insert_if_not_exists]
    assert await table.count_rows() == 3
    assert res.version == 2
@@ -150,7 +150,7 @@ def test_replace_range(mem_db):
    table.count_rows("doc_id = 1")  # 1
    res
    # MergeResult(version=2, num_updated_rows=1,
-    # num_inserted_rows=0, num_deleted_rows=1)
+    # num_inserted_rows=0, num_deleted_rows=1, num_rows=1)
    # --8<-- [end:insert_if_not_exists]
    assert table.count_rows("doc_id = 1") == 1
    assert res.version == 2
@@ -185,7 +185,7 @@ async def test_replace_range_async(mem_db_async):
    await table.count_rows("doc_id = 1")  # 1
    res
    # MergeResult(version=2, num_updated_rows=1,
-    # num_inserted_rows=0, num_deleted_rows=1)
+    # num_inserted_rows=0, num_deleted_rows=1, num_rows=1)
    # --8<-- [end:insert_if_not_exists]
    assert await table.count_rows("doc_id = 1") == 1
    assert res.version == 2
--- a/python/python/tests/test_fts.py
+++ b/python/python/tests/test_fts.py
@@ -215,11 +215,12 @@ def test_reject_legacy_tantivy_index(table):

@pytest.mark.parametrize("with_position", [True, False])
 def test_create_inverted_index(table, with_position):
-    table.create_fts_index(
-        "text",
-        with_position=with_position,
-        name="custom_fts_index",
-    )
+    with pytest.warns(DeprecationWarning, match="create_fts_index"):
+        table.create_fts_index(
+            "text",
+            with_position=with_position,
+            name="custom_fts_index",
+        )
    indices = table.list_indices()
    fts_indices = [i for i in indices if i.index_type == "FTS"]
    assert any(i.name == "custom_fts_index" for i in fts_indices)
--- a/python/python/tests/test_index.py
+++ b/python/python/tests/test_index.py
@@ -162,12 +162,13 @@ async def test_create_bitmap_index(some_table: AsyncTable):
    await some_table.create_index("data", config=Bitmap())
    indices = await some_table.list_indices()
    assert len(indices) == 3
+    # list_indices returns indices in alphabetical order by name
    assert indices[0].index_type == "Bitmap"
-    assert indices[0].columns == ["id"]
+    assert indices[0].columns == ["data"]
    assert indices[1].index_type == "Bitmap"
-    assert indices[1].columns == ["is_active"]
+    assert indices[1].columns == ["id"]
    assert indices[2].index_type == "Bitmap"
-    assert indices[2].columns == ["data"]
+    assert indices[2].columns == ["is_active"]

    index_name = indices[0].name
    stats = await some_table.index_stats(index_name)
--- a/python/python/tests/test_merge_insert_lsm.py
+++ b/python/python/tests/test_merge_insert_lsm.py
@@ -0,0 +1,196 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+"""Tests for the MemWAL LSM ``merge_insert`` dispatch."""
+
+from datetime import timedelta
+
+import lancedb
+import pyarrow as pa
+import pytest
+from lancedb._lancedb import LsmWriteSpec
+
+SCHEMA = pa.schema(
+    [
+        pa.field("id", pa.int64(), nullable=False),
+        pa.field("value", pa.int64(), nullable=False),
+    ]
+)
+
+REGION_SCHEMA = pa.schema(
+    [
+        pa.field("id", pa.int64(), nullable=False),
+        pa.field("region", pa.utf8(), nullable=False),
+    ]
+)
+
+
+def _reader(ids):
+    batch = pa.RecordBatch.from_arrays(
+        [
+            pa.array(ids, type=pa.int64()),
+            pa.array(list(range(len(ids))), type=pa.int64()),
+        ],
+        schema=SCHEMA,
+    )
+    return pa.RecordBatchReader.from_batches(SCHEMA, [batch])
+
+
+def _region_reader(rows):
+    batch = pa.RecordBatch.from_arrays(
+        [
+            pa.array([row[0] for row in rows], type=pa.int64()),
+            pa.array([row[1] for row in rows], type=pa.utf8()),
+        ],
+        schema=REGION_SCHEMA,
+    )
+    return pa.RecordBatchReader.from_batches(REGION_SCHEMA, [batch])
+
+
+def _bucket_table(tmp_path):
+    """A table with ``id`` as the primary key and a single-bucket LSM spec."""
+    db = lancedb.connect(tmp_path, read_consistency_interval=timedelta(seconds=0))
+    table = db.create_table("t", _reader([1, 2, 3]))
+    table.set_unenforced_primary_key("id")
+    # num_buckets = 1: every row routes to the single bucket.
+    table.set_lsm_write_spec(LsmWriteSpec.bucket("id", 1))
+    return table
+
+
+def test_lsm_merge_insert_bucket(tmp_path):
+    table = _bucket_table(tmp_path)
+    # Empty `on` defaults to the primary key.
+    result = (
+        table.merge_insert([])
+        .when_matched_update_all()
+        .when_not_matched_insert_all()
+        .execute(_reader([3, 4, 5]))
+    )
+    # LSM path: rows go to the MemWAL, so only num_rows is populated.
+    assert result.num_rows == 3
+    assert result.version == 0
+    assert result.num_inserted_rows == 0
+    assert result.num_updated_rows == 0
+
+
+def test_lsm_merge_insert_unsharded(tmp_path):
+    db = lancedb.connect(tmp_path, read_consistency_interval=timedelta(seconds=0))
+    table = db.create_table("t", _reader([1, 2, 3]))
+    table.set_unenforced_primary_key("id")
+    table.set_lsm_write_spec(LsmWriteSpec.unsharded())
+    result = (
+        table.merge_insert("id")
+        .when_matched_update_all()
+        .when_not_matched_insert_all()
+        .execute(_reader([10, 11, 12, 13]))
+    )
+    assert result.num_rows == 4
+
+
+def test_lsm_merge_insert_identity(tmp_path):
+    db = lancedb.connect(tmp_path, read_consistency_interval=timedelta(seconds=0))
+    table = db.create_table("t", _region_reader([(1, "us"), (2, "us")]))
+    table.set_unenforced_primary_key("id")
+    table.set_lsm_write_spec(LsmWriteSpec.identity("region"))
+    # All rows share one identity value, so they route to one shard.
+    result = (
+        table.merge_insert([])
+        .when_matched_update_all()
+        .when_not_matched_insert_all()
+        .execute(_region_reader([(3, "us"), (4, "us")]))
+    )
+    assert result.num_rows == 2
+
+
+def test_lsm_merge_insert_use_lsm_write_false(tmp_path):
+    table = _bucket_table(tmp_path)  # rows id = 1, 2, 3
+    # use_lsm_write(False) opts out: the standard path runs and commits.
+    result = (
+        table.merge_insert("id")
+        .when_not_matched_insert_all()
+        .use_lsm_write(False)
+        .execute(_reader([3, 4, 5]))
+    )
+    assert result.num_inserted_rows == 2
+    assert table.count_rows() == 5
+
+
+def test_lsm_merge_insert_validate_single_shard_off(tmp_path):
+    table = _bucket_table(tmp_path)
+    result = (
+        table.merge_insert([])
+        .when_matched_update_all()
+        .when_not_matched_insert_all()
+        .validate_single_shard(False)
+        .execute(_reader([6, 7, 8]))
+    )
+    assert result.num_rows == 3
+
+
+def test_lsm_merge_insert_use_lsm_write_true_requires_spec(tmp_path):
+    # A table with a primary key but no LSM write spec installed.
+    db = lancedb.connect(tmp_path, read_consistency_interval=timedelta(seconds=0))
+    table = db.create_table("t", _reader([1, 2, 3]))
+    table.set_unenforced_primary_key("id")
+    with pytest.raises(Exception, match="use_lsm_write"):
+        (
+            table.merge_insert("id")
+            .when_matched_update_all()
+            .when_not_matched_insert_all()
+            .use_lsm_write(True)
+            .execute(_reader([4]))
+        )
+
+
+def test_lsm_merge_insert_rejects_on_not_primary_key(tmp_path):
+    table = _bucket_table(tmp_path)
+    with pytest.raises(Exception, match="primary key"):
+        (
+            table.merge_insert("value")
+            .when_matched_update_all()
+            .when_not_matched_insert_all()
+            .execute(_reader([1]))
+        )
+
+
+def test_lsm_merge_insert_rejects_non_upsert(tmp_path):
+    table = _bucket_table(tmp_path)
+    # Insert-only (no when_matched_update_all) is not the upsert shape.
+    with pytest.raises(Exception, match="upsert"):
+        table.merge_insert([]).when_not_matched_insert_all().execute(_reader([4]))
+
+
+def test_lsm_close_writers(tmp_path):
+    table = _bucket_table(tmp_path)
+    (
+        table.merge_insert([])
+        .when_matched_update_all()
+        .when_not_matched_insert_all()
+        .execute(_reader([7, 8]))
+    )
+    table.close_lsm_writers()
+    # The writer reopens lazily on the next merge_insert.
+    result = (
+        table.merge_insert([])
+        .when_matched_update_all()
+        .when_not_matched_insert_all()
+        .execute(_reader([9]))
+    )
+    assert result.num_rows == 1
+
+
+@pytest.mark.asyncio
+async def test_async_lsm_merge_insert(tmp_path):
+    db = await lancedb.connect_async(
+        tmp_path, read_consistency_interval=timedelta(seconds=0)
+    )
+    table = await db.create_table("t", _reader([1, 2, 3]))
+    await table.set_unenforced_primary_key("id")
+    await table.set_lsm_write_spec(LsmWriteSpec.bucket("id", 1))
+
+    builder = (
+        table.merge_insert([]).when_matched_update_all().when_not_matched_insert_all()
+    )
+    result = await builder.execute(_reader([3, 4, 5]))
+    assert result.num_rows == 3
+    await table.close_lsm_writers()
--- a/python/python/tests/test_remote_db.py
+++ b/python/python/tests/test_remote_db.py
@@ -436,22 +436,25 @@ def test_table_create_indices():
        # This is a smoke-test.
        table = db.create_table("test", [{"id": 1}])

-        # Test create_scalar_index with custom name
-        table.create_scalar_index(
-            "id", wait_timeout=timedelta(seconds=2), name="custom_scalar_idx"
-        )
+        # Test create_scalar_index with custom name (legacy method)
+        with pytest.warns(DeprecationWarning, match="create_scalar_index"):
+            table.create_scalar_index(
+                "id", wait_timeout=timedelta(seconds=2), name="custom_scalar_idx"
+            )

-        # Test create_fts_index with custom name
-        table.create_fts_index(
-            "text", wait_timeout=timedelta(seconds=2), name="custom_fts_idx"
-        )
+        # Test create_fts_index with custom name (legacy method)
+        with pytest.warns(DeprecationWarning, match="create_fts_index"):
+            table.create_fts_index(
+                "text", wait_timeout=timedelta(seconds=2), name="custom_fts_idx"
+            )

-        # Test create_index with custom name
-        table.create_index(
-            vector_column_name="vector",
-            wait_timeout=timedelta(seconds=10),
-            name="custom_vector_idx",
-        )
+        # Test create_index with custom name (legacy form: vector_column_name kwarg)
+        with pytest.warns(DeprecationWarning, match="create_index"):
+            table.create_index(
+                vector_column_name="vector",
+                wait_timeout=timedelta(seconds=10),
+                name="custom_vector_idx",
+            )

        # Validate that the name parameter was passed correctly in requests
        assert len(received_requests) == 3
@@ -480,6 +483,98 @@ def test_table_create_indices():
        table.drop_index("custom_fts_idx")


+def test_remote_create_index_new_api():
+    received_requests = []
+
+    def handler(request):
+        if request.path == "/v1/table/test/create_index/":
+            content_len = int(request.headers.get("Content-Length", 0))
+            body = request.rfile.read(content_len) if content_len > 0 else b""
+            received_requests.append(json.loads(body) if body else {})
+            request.send_response(200)
+            request.end_headers()
+        elif request.path == "/v1/table/test/create/?mode=create":
+            request.send_response(200)
+            request.send_header("Content-Type", "application/json")
+            request.end_headers()
+            request.wfile.write(b"{}")
+        elif request.path == "/v1/table/test/describe/":
+            request.send_response(200)
+            request.send_header("Content-Type", "application/json")
+            request.end_headers()
+            request.wfile.write(
+                json.dumps(
+                    dict(
+                        version=1,
+                        schema=dict(
+                            fields=[
+                                dict(name="id", type={"type": "int64"}, nullable=False),
+                                dict(
+                                    name="category",
+                                    type={"type": "string"},
+                                    nullable=False,
+                                ),
+                                dict(
+                                    name="text", type={"type": "string"}, nullable=False
+                                ),
+                                dict(
+                                    name="vector",
+                                    type={
+                                        "type": "fixed_size_list",
+                                        "fields": [
+                                            dict(
+                                                name="item",
+                                                type={"type": "float"},
+                                                nullable=True,
+                                            )
+                                        ],
+                                        "length": 2,
+                                    },
+                                    nullable=False,
+                                ),
+                            ]
+                        ),
+                    )
+                ).encode()
+            )
+        else:
+            request.send_response(404)
+            request.end_headers()
+
+    from lancedb.index import BTree, FTS, IvfPq, IvfRq
+
+    with mock_lancedb_connection(handler) as db:
+        table = db.create_table("test", [{"id": 1}])
+
+        # New API: column-first, config= kwarg. Should NOT emit DeprecationWarning.
+        import warnings as _warnings
+
+        with _warnings.catch_warnings():
+            _warnings.simplefilter("error", DeprecationWarning)
+            table.create_index("vector", config=IvfPq(distance_type="l2"))
+            table.create_index("category", config=BTree())
+            table.create_index("text", config=FTS())
+            # IvfRq via new API
+            table.create_index("vector", config=IvfRq(distance_type="l2"))
+
+        # Legacy index_type="IVF_RQ" routes to IvfRq config under the hood.
+        with pytest.warns(DeprecationWarning, match="create_index"):
+            table.create_index(
+                vector_column_name="vector",
+                index_type="IVF_RQ",
+                num_partitions=8,
+            )
+
+        assert len(received_requests) == 5
+        assert [req["column"] for req in received_requests] == [
+            "vector",
+            "category",
+            "text",
+            "vector",
+            "vector",
+        ]
+
+
 def test_table_wait_for_index_timeout():
    def handler(request):
        index_stats = dict(
--- a/python/python/tests/test_table.py
+++ b/python/python/tests/test_table.py
@@ -4,6 +4,7 @@

 import os
 import sys
+import warnings
 from datetime import date, datetime, timedelta
 from time import sleep
 from typing import List
@@ -11,7 +12,7 @@ from unittest.mock import patch

 import lancedb
 from lancedb.dependencies import _PANDAS_AVAILABLE
-from lancedb.index import HnswFlat, HnswPq, HnswSq, IvfPq
+from lancedb.index import BTree, FTS, HnswFlat, HnswPq, HnswSq, IvfPq
 import numpy as np
 import polars as pl
 import pyarrow as pa
@@ -928,7 +929,12 @@ def test_create_index_method(mock_create_index, mem_db: DBConnection):
        num_bits=4,
    )
    mock_create_index.assert_called_with(
-        "vector", replace=True, config=expected_config, name=None, train=True
+        "vector",
+        replace=True,
+        config=expected_config,
+        wait_timeout=None,
+        name=None,
+        train=True,
    )

    # Test with target_partition_size
@@ -948,7 +954,12 @@ def test_create_index_method(mock_create_index, mem_db: DBConnection):
        target_partition_size=8192,
    )
    mock_create_index.assert_called_with(
-        "vector", replace=True, config=expected_config, name=None, train=True
+        "vector",
+        replace=True,
+        config=expected_config,
+        wait_timeout=None,
+        name=None,
+        train=True,
    )

    # target_partition_size has a default value,
@@ -967,7 +978,12 @@ def test_create_index_method(mock_create_index, mem_db: DBConnection):
        num_bits=4,
    )
    mock_create_index.assert_called_with(
-        "vector", replace=True, config=expected_config, name=None, train=True
+        "vector",
+        replace=True,
+        config=expected_config,
+        wait_timeout=None,
+        name=None,
+        train=True,
    )

    table.create_index(
@@ -978,7 +994,12 @@ def test_create_index_method(mock_create_index, mem_db: DBConnection):
    )
    expected_config = HnswPq(distance_type="dot")
    mock_create_index.assert_called_with(
-        "my_vector", replace=False, config=expected_config, name=None, train=True
+        "my_vector",
+        replace=False,
+        config=expected_config,
+        wait_timeout=None,
+        name=None,
+        train=True,
    )

    table.create_index(
@@ -993,7 +1014,12 @@ def test_create_index_method(mock_create_index, mem_db: DBConnection):
        distance_type="cosine", sample_rate=0.1, m=29, ef_construction=10
    )
    mock_create_index.assert_called_with(
-        "my_vector", replace=True, config=expected_config, name=None, train=True
+        "my_vector",
+        replace=True,
+        config=expected_config,
+        wait_timeout=None,
+        name=None,
+        train=True,
    )

    table.create_index(
@@ -1008,7 +1034,12 @@ def test_create_index_method(mock_create_index, mem_db: DBConnection):
        distance_type="cosine", sample_rate=0.1, m=29, ef_construction=10
    )
    mock_create_index.assert_called_with(
-        "my_vector", replace=True, config=expected_config, name=None, train=True
+        "my_vector",
+        replace=True,
+        config=expected_config,
+        wait_timeout=None,
+        name=None,
+        train=True,
    )


@@ -1032,6 +1063,7 @@ def test_create_index_name_and_train_parameters(
        "vector",
        replace=True,
        config=expected_config,
+        wait_timeout=None,
        name="my_custom_index",
        train=True,
    )
@@ -1039,13 +1071,82 @@ def test_create_index_name_and_train_parameters(
    # Test with train=False
    table.create_index(vector_column_name="vector", train=False)
    mock_create_index.assert_called_with(
-        "vector", replace=True, config=expected_config, name=None, train=False
+        "vector",
+        replace=True,
+        config=expected_config,
+        wait_timeout=None,
+        name=None,
+        train=False,
    )

    # Test with both name and train
    table.create_index(vector_column_name="vector", name="my_index_name", train=True)
    mock_create_index.assert_called_with(
-        "vector", replace=True, config=expected_config, name="my_index_name", train=True
+        "vector",
+        replace=True,
+        config=expected_config,
+        wait_timeout=None,
+        name="my_index_name",
+        train=True,
+    )
+
+
+@patch("lancedb.table.AsyncTable.create_index")
+def test_create_index_legacy_emits_deprecation_warning(
+    mock_create_index, mem_db: DBConnection
+):
+    table = mem_db.create_table(
+        "test",
+        data=[{"vector": [3.1, 4.1]}, {"vector": [5.9, 26.5]}],
+    )
+
+    with pytest.warns(DeprecationWarning, match="create_index"):
+        table.create_index(metric="l2", num_partitions=8, vector_column_name="vector")
+
+
+@patch("lancedb.table.AsyncTable.create_index")
+def test_create_index_new_api(mock_create_index, mem_db: DBConnection):
+    table = mem_db.create_table(
+        "test",
+        data=[
+            {"vector": [3.1, 4.1], "category": "a", "text": "hello world"},
+            {"vector": [5.9, 26.5], "category": "b", "text": "goodbye"},
+        ],
+    )
+
+    # Vector index via new API should not warn
+    with warnings.catch_warnings():
+        warnings.simplefilter("error", DeprecationWarning)
+        table.create_index("vector", config=IvfPq(distance_type="l2"))
+    mock_create_index.assert_called_with(
+        "vector",
+        replace=True,
+        config=IvfPq(distance_type="l2"),
+        wait_timeout=None,
+        name=None,
+        train=True,
+    )
+
+    # Scalar index via new API
+    table.create_index("category", config=BTree())
+    mock_create_index.assert_called_with(
+        "category",
+        replace=True,
+        config=BTree(),
+        wait_timeout=None,
+        name=None,
+        train=True,
+    )
+
+    # FTS index via new API
+    table.create_index("text", config=FTS(with_position=True))
+    mock_create_index.assert_called_with(
+        "text",
+        replace=True,
+        config=FTS(with_position=True),
+        wait_timeout=None,
+        name=None,
+        train=True,
    )


@@ -1861,8 +1962,9 @@ def test_create_scalar_index(mem_db: DBConnection):
        "my_table",
        data=test_data,
    )
-    # Test with default name
-    table.create_scalar_index("x")
+    # Test with default name; confirm DeprecationWarning fires
+    with pytest.warns(DeprecationWarning, match="create_scalar_index"):
+        table.create_scalar_index("x")
    indices = table.list_indices()
    assert len(indices) == 1
    scalar_index = indices[0]
--- a/python/src/table.rs
+++ b/python/src/table.rs
@@ -143,18 +143,20 @@ pub struct MergeResult {
    pub num_inserted_rows: u64,
    pub num_deleted_rows: u64,
    pub num_attempts: u32,
+    pub num_rows: u64,
 }

 #[pymethods]
 impl MergeResult {
    pub fn __repr__(&self) -> String {
        format!(
-            "MergeResult(version={}, num_updated_rows={}, num_inserted_rows={}, num_deleted_rows={}, num_attempts={})",
+            "MergeResult(version={}, num_updated_rows={}, num_inserted_rows={}, num_deleted_rows={}, num_attempts={}, num_rows={})",
            self.version,
            self.num_updated_rows,
            self.num_inserted_rows,
            self.num_deleted_rows,
-            self.num_attempts
+            self.num_attempts,
+            self.num_rows
        )
    }
 }
@@ -167,6 +169,7 @@ impl From<lancedb::table::MergeResult> for MergeResult {
            num_inserted_rows: result.num_inserted_rows,
            num_deleted_rows: result.num_deleted_rows,
            num_attempts: result.num_attempts,
+            num_rows: result.num_rows,
        }
    }
 }
@@ -194,6 +197,12 @@ impl LsmWriteSpec {
    }

    /// Identity sharding — shard by the raw value of `column`.
+    ///
+    /// `column` must be a deterministic function of the unenforced primary
+    /// key: every row with a given primary key must always produce the same
+    /// `column` value, or upserts of that key can land in different shards
+    /// and a stale version can win. Typically `column` is the primary key
+    /// itself or a stable attribute of it.
    #[staticmethod]
    pub fn identity(column: String) -> Self {
        Self {
@@ -933,6 +942,12 @@ impl Table {
        if let Some(use_index) = parameters.use_index {
            builder.use_index(use_index);
        }
+        if let Some(use_lsm_write) = parameters.use_lsm_write {
+            builder.use_lsm_write(use_lsm_write);
+        }
+        if let Some(validate_single_shard) = parameters.validate_single_shard {
+            builder.validate_single_shard(validate_single_shard);
+        }

        future_into_py(self_.py(), async move {
            let res = builder.execute(Box::new(batches)).await.infer_error()?;
@@ -971,6 +986,13 @@ impl Table {
        })
    }

+    pub fn close_lsm_writers(self_: PyRef<'_, Self>) -> PyResult<Bound<'_, PyAny>> {
+        let inner = self_.inner_ref()?.clone();
+        future_into_py(self_.py(), async move {
+            inner.close_lsm_writers().await.infer_error()
+        })
+    }
+
    pub fn uses_v2_manifest_paths(self_: PyRef<'_, Self>) -> PyResult<Bound<'_, PyAny>> {
        let inner = self_.inner_ref()?.clone();
        future_into_py(self_.py(), async move {
@@ -1124,6 +1146,8 @@ pub struct MergeInsertParams {
    when_not_matched_by_source_condition: Option<String>,
    timeout: Option<std::time::Duration>,
    use_index: Option<bool>,
+    use_lsm_write: Option<bool>,
+    validate_single_shard: Option<bool>,
 }

 #[pyclass]
--- a/rust-toolchain.toml
+++ b/rust-toolchain.toml
@@ -1,2 +1,2 @@
 [toolchain]
-channel = "1.94.0"
+channel = "1.95.0"
--- a/rust/lancedb/Cargo.toml
+++ b/rust/lancedb/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "lancedb"
-version = "0.30.0-beta.1"
+version = "0.30.1-beta.0"
 edition.workspace = true
 description = "LanceDB: A serverless, low-latency vector database for AI applications"
 license.workspace = true
@@ -75,7 +75,7 @@ reqwest = { version = "0.12.0", default-features = false, features = [
    "stream",
 ], optional = true }
 http = { version = "1", optional = true } # Matching what is in reqwest
-uuid = { version = "1.7.0", features = ["v4"] }
+uuid = { version = "1.7.0", features = ["v4", "v5"] }
 polars-arrow = { version = ">=0.37,<0.40.0", optional = true }
 polars = { version = ">=0.37,<0.40.0", optional = true }
 hf-hub = { version = "0.4.1", optional = true, default-features = false, features = [
--- a/rust/lancedb/src/dataloader/permutation/shuffle.rs
+++ b/rust/lancedb/src/dataloader/permutation/shuffle.rs
@@ -464,11 +464,9 @@ mod tests {
        let mut iter = ids.into_iter().map(|o| o.unwrap());
        while let Some(first) = iter.next() {
            let rows_left_in_clump = if first == 4470 { 19 } else { 29 };
-            let mut expected_next = first + 1;
-            for _ in 0..rows_left_in_clump {
+            for expected_next in (first + 1)..=(first + rows_left_in_clump) {
                let next = iter.next().unwrap();
                assert_eq!(next, expected_next);
-                expected_next += 1;
            }
        }
    }
--- a/rust/lancedb/src/remote/client.rs
+++ b/rust/lancedb/src/remote/client.rs
@@ -908,6 +908,15 @@ mod tests {
    use serial_test::serial;
    use std::time::Duration;

+    // Serializes the env-var-mutating tests below: cargo test runs tests in
+    // parallel, but several of these tests read and write the same process-
+    // global env vars (`LANCEDB_USER_ID*`), so they would race without this.
+    static ENV_MUTEX: std::sync::Mutex<()> = std::sync::Mutex::new(());
+
+    fn lock_env() -> std::sync::MutexGuard<'static, ()> {
+        ENV_MUTEX.lock().unwrap_or_else(|e| e.into_inner())
+    }
+
    #[test]
    fn test_timeout_config_default() {
        let config = TimeoutConfig::default();
@@ -1166,6 +1175,7 @@ mod tests {
    #[test]
    #[serial(user_id_env)]
    fn test_resolve_user_id_none() {
+        let _guard = lock_env();
        let config = ClientConfig::default();
        // Clear env vars that might be set from other tests
        // SAFETY: This is only called in tests
@@ -1179,6 +1189,7 @@ mod tests {
    #[test]
    #[serial(user_id_env)]
    fn test_resolve_user_id_from_env() {
+        let _guard = lock_env();
        // SAFETY: This is only called in tests
        unsafe {
            std::env::set_var("LANCEDB_USER_ID", "env-user-id");
@@ -1194,6 +1205,7 @@ mod tests {
    #[test]
    #[serial(user_id_env)]
    fn test_resolve_user_id_from_env_key() {
+        let _guard = lock_env();
        // SAFETY: This is only called in tests
        unsafe {
            std::env::remove_var("LANCEDB_USER_ID");
@@ -1215,6 +1227,7 @@ mod tests {
    #[test]
    #[serial(user_id_env)]
    fn test_resolve_user_id_direct_takes_precedence() {
+        let _guard = lock_env();
        // SAFETY: This is only called in tests
        unsafe {
            std::env::set_var("LANCEDB_USER_ID", "env-user-id");
@@ -1233,6 +1246,7 @@ mod tests {
    #[test]
    #[serial(user_id_env)]
    fn test_resolve_user_id_empty_env_ignored() {
+        let _guard = lock_env();
        // SAFETY: This is only called in tests
        unsafe {
            std::env::set_var("LANCEDB_USER_ID", "");
--- a/rust/lancedb/src/remote/table.rs
+++ b/rust/lancedb/src/remote/table.rs
@@ -1805,6 +1805,7 @@ impl<S: HttpSend> BaseTable for RemoteTable<S> {
                num_inserted_rows: 0,
                num_updated_rows: 0,
                num_attempts: 0,
+                num_rows: 0,
            });
        }

--- a/rust/lancedb/src/table.rs
+++ b/rust/lancedb/src/table.rs
@@ -89,7 +89,6 @@ use futures::future::join_all;
 pub use lance::dataset::refs::{TagContents, Tags as LanceTags};
 pub use lance::dataset::scanner::DatasetRecordBatchStream;
 use lance::dataset::statistics::DatasetStatisticsExt;
-use lance_index::frag_reuse::FRAG_REUSE_INDEX_NAME;
 pub use lance_index::optimize::OptimizeOptions;
 pub use optimize::{CompactionOptions, OptimizeAction, OptimizeStats};
 pub use schema_evolution::{AddColumnsResult, AlterColumnsResult, DropColumnsResult};
@@ -367,6 +366,14 @@ impl LsmWriteSpec {

    /// Construct an identity-sharding spec (shard by the raw value of
    /// `column`) with no maintained indexes.
+    ///
+    /// `column` must be a deterministic function of the unenforced primary
+    /// key: every row with a given primary key must always produce the same
+    /// `column` value. MemWAL dedups upserts by primary key but tracks
+    /// generations per shard, so if the same key is written with two
+    /// different `column` values its versions land in different shards and a
+    /// stale value can win. Typically `column` is the primary key itself, or
+    /// a stable attribute of it (e.g. a tenant id).
    pub fn identity(column: impl Into<String>) -> Self {
        Self::Identity {
            column: column.into(),
@@ -581,6 +588,13 @@ pub trait BaseTable: std::fmt::Display + std::fmt::Debug + Send + Sync {
            message: "unset_lsm_write_spec is not supported on this table type".into(),
        })
    }
+    /// Drain and close any cached MemWAL shard writers for this table.
+    ///
+    /// The default implementation is a no-op; table types that maintain
+    /// MemWAL shard writers override it.
+    async fn close_lsm_writers(&self) -> Result<()> {
+        Ok(())
+    }
    /// Gets the table tag manager.
    async fn tags(&self) -> Result<Box<dyn Tags + '_>>;
    /// Optimize the dataset.
@@ -1387,6 +1401,16 @@ impl Table {
        self.inner.unset_lsm_write_spec().await
    }

+    /// Drain and close any cached MemWAL shard writers held for this table.
+    ///
+    /// When an [`LsmWriteSpec`] is installed, `merge_insert` opens MemWAL shard
+    /// writers and caches them for reuse across calls. This closes them,
+    /// flushing pending data; writers reopen lazily on the next `merge_insert`.
+    /// It is a no-op when no writers are cached.
+    pub async fn close_lsm_writers(&self) -> Result<()> {
+        self.inner.close_lsm_writers().await
+    }
+
    /// Retrieve the version of the table
    ///
    /// LanceDb supports versioning.  Every operation that modifies the table increases
@@ -2830,6 +2854,10 @@ impl BaseTable for NativeTable {
        merge::lsm::unset_lsm_write_spec(self).await
    }

+    async fn close_lsm_writers(&self) -> Result<()> {
+        merge::lsm::close_lsm_writers(self).await
+    }
+
    /// Delete rows from the table
    async fn delete(&self, predicate: Predicate<'_>) -> Result<DeleteResult> {
        delete::execute_delete(self, predicate).await
@@ -2864,71 +2892,32 @@ impl BaseTable for NativeTable {

    async fn list_indices(&self) -> Result<Vec<IndexConfig>> {
        let dataset = self.dataset.get().await?;
-        let indices = dataset.load_indices().await?;
-        let results = futures::stream::iter(indices.as_slice())
-            .then(|idx| async {
-                // skip Lance internal indexes
-                if idx.name == FRAG_REUSE_INDEX_NAME {
-                    return None;
-                }
-
-                let stats = match dataset.index_statistics(idx.name.as_str()).await {
-                    Ok(stats) => stats,
-                    Err(e) => {
-                        log::warn!(
-                            "Failed to get statistics for index {} ({}): {}",
-                            idx.name,
-                            idx.uuid,
-                            e
-                        );
-                        return None;
-                    }
-                };
-
-                let stats: serde_json::Value = match serde_json::from_str(&stats) {
-                    Ok(stats) => stats,
-                    Err(e) => {
-                        log::warn!(
-                            "Failed to deserialize index statistics for index {} ({}): {}",
-                            idx.name,
-                            idx.uuid,
-                            e
-                        );
-                        return None;
-                    }
-                };
-
-                let Some(index_type) = stats.get("index_type").and_then(|v| v.as_str()) else {
-                    log::warn!(
-                        "Index statistics was missing 'index_type' field for index {} ({})",
-                        idx.name,
-                        idx.uuid
-                    );
-                    return None;
-                };
-
-                let index_type: crate::index::IndexType = match index_type.parse() {
+        let indices = dataset
+            .describe_indices(None)
+            .await?
+            .into_iter()
+            .filter_map(|idx_desc| {
+                let index_type: crate::index::IndexType = match idx_desc.index_type().parse() {
                    Ok(index_type) => index_type,
                    Err(e) => {
                        log::warn!(
-                            "Failed to parse index type for index {} ({}): {}",
-                            idx.name,
-                            idx.uuid,
+                            "Failed to parse index type for index {}: {}",
+                            idx_desc.name(),
                            e
                        );
                        return None;
                    }
                };

-                let mut columns = Vec::with_capacity(idx.fields.len());
-                for field_id in &idx.fields {
-                    let field_path = match dataset.schema().field_path(*field_id) {
+                let field_ids = idx_desc.field_ids();
+                let mut columns = Vec::with_capacity(field_ids.len());
+                for field_id in field_ids {
+                    let field_path = match dataset.schema().field_path(*field_id as i32) {
                        Ok(field_path) => field_path,
                        Err(e) => {
                            log::warn!(
-                                "Failed to resolve field path for index {} ({}) field id {}: {}",
-                                idx.name,
-                                idx.uuid,
+                                "Failed to resolve field path for index {} field id {}: {}",
+                                idx_desc.name(),
                                field_id,
                                e
                            );
@@ -2938,17 +2927,14 @@ impl BaseTable for NativeTable {
                    columns.push(field_path);
                }

-                let name = idx.name.clone();
                Some(IndexConfig {
+                    name: idx_desc.name().to_string(),
                    index_type,
                    columns,
-                    name,
                })
            })
-            .collect::<Vec<_>>()
-            .await;
-
-        Ok(results.into_iter().flatten().collect())
+            .collect();
+        Ok(indices)
    }

    async fn uri(&self) -> Result<String> {
@@ -3058,11 +3044,12 @@ impl BaseTable for NativeTable {
        let p99 = *sorted_sizes.get(num_fragments * 99 / 100).unwrap_or(&0);
        let min = sorted_sizes.first().copied().unwrap_or(0);
        let max = sorted_sizes.last().copied().unwrap_or(0);
-        let mean = if num_fragments == 0 {
-            0
-        } else {
-            sorted_sizes.iter().copied().sum::<usize>() / num_fragments
-        };
+        let mean = sorted_sizes
+            .iter()
+            .copied()
+            .sum::<usize>()
+            .checked_div(num_fragments)
+            .unwrap_or(0);

        let frag_stats = FragmentStatistics {
            num_fragments,
@@ -4062,26 +4049,27 @@ mod tests {
        let index_configs = table.list_indices().await.unwrap();
        assert_eq!(index_configs.len(), 5);

+        // list_indices returns indices in alphabetical order by name
        let mut configs_iter = index_configs.into_iter();
        let index = configs_iter.next().unwrap();
        assert_eq!(index.index_type, crate::index::IndexType::Bitmap);
        assert_eq!(index.columns, vec!["category".to_string()]);

-        let index = configs_iter.next().unwrap();
-        assert_eq!(index.index_type, crate::index::IndexType::Bitmap);
-        assert_eq!(index.columns, vec!["is_active".to_string()]);
-
        let index = configs_iter.next().unwrap();
        assert_eq!(index.index_type, crate::index::IndexType::Bitmap);
        assert_eq!(index.columns, vec!["data".to_string()]);

        let index = configs_iter.next().unwrap();
        assert_eq!(index.index_type, crate::index::IndexType::Bitmap);
-        assert_eq!(index.columns, vec!["large_data".to_string()]);
+        assert_eq!(index.columns, vec!["is_active".to_string()]);

        let index = configs_iter.next().unwrap();
        assert_eq!(index.index_type, crate::index::IndexType::Bitmap);
        assert_eq!(index.columns, vec!["large_category".to_string()]);
+
+        let index = configs_iter.next().unwrap();
+        assert_eq!(index.index_type, crate::index::IndexType::Bitmap);
+        assert_eq!(index.columns, vec!["large_data".to_string()]);
    }

    #[tokio::test]
--- a/rust/lancedb/src/table/dataset.rs
+++ b/rust/lancedb/src/table/dataset.rs
@@ -8,6 +8,7 @@ use std::{

 use lance::{Dataset, dataset::refs};

+use crate::table::merge::lsm::ShardWriterCache;
 use crate::{Error, error::Result, utils::background_cache::BackgroundCache};

 /// A wrapper around a [Dataset] that provides consistency checks.
@@ -18,6 +19,10 @@ use crate::{Error, error::Result, utils::background_cache::BackgroundCache};
 pub struct DatasetConsistencyWrapper {
    state: Arc<Mutex<DatasetState>>,
    consistency: ConsistencyMode,
+    /// The single MemWAL `ShardWriter` for this dataset, co-located so it is
+    /// cached for the session and shares the dataset's lifecycle. A dataset
+    /// writes to one shard at a time. Shared by `Arc` across clones.
+    shard_writer: Arc<ShardWriterCache>,
 }

 /// The current dataset and whether it is pinned to a specific version.
@@ -67,9 +72,15 @@ impl DatasetConsistencyWrapper {
                pinned_version: None,
            })),
            consistency,
+            shard_writer: Arc::new(ShardWriterCache::default()),
        }
    }

+    /// The MemWAL `ShardWriter` cache co-located with this dataset.
+    pub(crate) fn shard_writer(&self) -> &Arc<ShardWriterCache> {
+        &self.shard_writer
+    }
+
    /// Get the current dataset.
    ///
    /// Behavior depends on the consistency mode:
--- a/rust/lancedb/src/table/merge.rs
+++ b/rust/lancedb/src/table/merge.rs
@@ -41,6 +41,16 @@ pub struct MergeResult {
    /// A value of 1 means the operation succeeded on the first try.
    #[serde(default)]
    pub num_attempts: u32,
+    /// Total number of rows written.
+    ///
+    /// On the standard `merge_insert` path this equals
+    /// `num_inserted_rows + num_updated_rows`. On the MemWAL LSM write path the
+    /// insert/update breakdown is not known until compaction; in that mode
+    /// `num_inserted_rows`, `num_updated_rows`, `num_deleted_rows`, `version`
+    /// and `num_attempts` are all `0` and this field holds the total number of
+    /// rows written through the shard writer.
+    #[serde(default)]
+    pub num_rows: u64,
 }

 /// A builder used to create and run a merge insert operation
@@ -57,6 +67,8 @@ pub struct MergeInsertBuilder {
    pub(crate) when_not_matched_by_source_delete_filt: Option<String>,
    pub(crate) timeout: Option<Duration>,
    pub(crate) use_index: bool,
+    pub(crate) use_lsm_write: Option<bool>,
+    pub(crate) validate_single_shard: bool,
 }

 impl MergeInsertBuilder {
@@ -71,6 +83,8 @@ impl MergeInsertBuilder {
            when_not_matched_by_source_delete_filt: None,
            timeout: None,
            use_index: true,
+            use_lsm_write: None,
+            validate_single_shard: true,
        }
    }

@@ -150,6 +164,34 @@ impl MergeInsertBuilder {
        self
    }

+    /// Controls whether `merge_insert` uses the MemWAL LSM write path.
+    ///
+    /// By default (unset), a `merge_insert` on a table with an
+    /// [`LsmWriteSpec`](super::LsmWriteSpec) installed is routed through
+    /// Lance's MemWAL shard writer, and a table without one uses the standard
+    /// path. Calling this with `false` forces the standard path even when a
+    /// spec is set. Calling it with `true` requires a spec — `merge_insert`
+    /// errors if none is installed.
+    pub fn use_lsm_write(&mut self, use_lsm_write: bool) -> &mut Self {
+        self.use_lsm_write = Some(use_lsm_write);
+        self
+    }
+
+    /// Controls how an LSM `merge_insert` checks that its input targets a
+    /// single shard.
+    ///
+    /// When a table has an LSM write spec, every row in a `merge_insert` call
+    /// must route to the same shard. When `true` (the default), every row is
+    /// inspected to verify this. When `false`, only the first row is inspected
+    /// and the shard it routes to is used for the whole input — a faster path
+    /// for callers that have already pre-sharded their input.
+    ///
+    /// Has no effect on tables without an LSM write spec.
+    pub fn validate_single_shard(&mut self, validate_single_shard: bool) -> &mut Self {
+        self.validate_single_shard = validate_single_shard;
+        self
+    }
+
    /// Executes the merge insert operation
    ///
    /// Returns version and statistics about the merge operation including the number of rows
@@ -167,6 +209,23 @@ pub(crate) async fn execute_merge_insert(
    params: MergeInsertBuilder,
    new_data: Box<dyn RecordBatchReader + Send>,
 ) -> Result<MergeResult> {
+    match lsm::lsm_dispatch_decision(table, &params).await? {
+        lsm::LsmDispatch::Lsm(plan) => {
+            let future =
+                lsm::execute_lsm_merge_insert(table, plan, params.validate_single_shard, new_data);
+            return match params.timeout {
+                Some(timeout) => match tokio::time::timeout(timeout, future).await {
+                    Ok(result) => result,
+                    Err(_) => Err(Error::Runtime {
+                        message: "merge insert timed out".to_string(),
+                    }),
+                },
+                None => future.await,
+            };
+        }
+        lsm::LsmDispatch::Standard => {}
+    }
+
    let dataset = table.dataset.get().await?;
    let mut builder = LanceMergeInsertBuilder::try_new(dataset.clone(), params.on)?;
    match (
@@ -219,6 +278,7 @@ pub(crate) async fn execute_merge_insert(
        num_inserted_rows: stats.num_inserted_rows,
        num_deleted_rows: stats.num_deleted_rows,
        num_attempts: stats.num_attempts,
+        num_rows: stats.num_inserted_rows + stats.num_updated_rows,
    })
 }

@@ -327,3 +387,366 @@ mod tests {
        assert_eq!(table.count_rows(None).await.unwrap(), 25);
    }
 }
+
+#[cfg(test)]
+mod lsm_tests {
+    use std::sync::Arc;
+
+    use arrow_array::{
+        Int64Array, RecordBatch, RecordBatchIterator, RecordBatchReader, StringArray,
+    };
+    use arrow_schema::{DataType, Field, Schema};
+    use tempfile::{TempDir, tempdir};
+
+    use crate::connect;
+    use crate::error::Error;
+    use crate::table::{LsmWriteSpec, Table};
+
+    /// A reader of `[id: Int64, value: Int64]` rows; `value` is `0..n`.
+    fn id_value_reader(ids: Vec<i64>) -> Box<dyn RecordBatchReader + Send> {
+        let schema = Arc::new(Schema::new(vec![
+            Field::new("id", DataType::Int64, false),
+            Field::new("value", DataType::Int64, false),
+        ]));
+        let n = ids.len() as i64;
+        let batch = RecordBatch::try_new(
+            schema.clone(),
+            vec![
+                Arc::new(Int64Array::from(ids)),
+                Arc::new(Int64Array::from_iter_values(0..n)),
+            ],
+        )
+        .unwrap();
+        Box::new(RecordBatchIterator::new(vec![Ok(batch)], schema))
+    }
+
+    /// A reader of `[id: Int64, region: Utf8]` rows.
+    fn id_region_reader(rows: Vec<(i64, &str)>) -> Box<dyn RecordBatchReader + Send> {
+        let schema = Arc::new(Schema::new(vec![
+            Field::new("id", DataType::Int64, false),
+            Field::new("region", DataType::Utf8, false),
+        ]));
+        let ids: Vec<i64> = rows.iter().map(|(id, _)| *id).collect();
+        let regions: Vec<&str> = rows.iter().map(|(_, region)| *region).collect();
+        let batch = RecordBatch::try_new(
+            schema.clone(),
+            vec![
+                Arc::new(Int64Array::from(ids)),
+                Arc::new(StringArray::from(regions)),
+            ],
+        )
+        .unwrap();
+        Box::new(RecordBatchIterator::new(vec![Ok(batch)], schema))
+    }
+
+    /// A multi-batch reader of `[id: Int64, region: Utf8]` rows.
+    fn id_region_multi_reader(batches: Vec<Vec<(i64, &str)>>) -> Box<dyn RecordBatchReader + Send> {
+        let schema = Arc::new(Schema::new(vec![
+            Field::new("id", DataType::Int64, false),
+            Field::new("region", DataType::Utf8, false),
+        ]));
+        let records: Vec<_> = batches
+            .into_iter()
+            .map(|rows| {
+                let ids: Vec<i64> = rows.iter().map(|(id, _)| *id).collect();
+                let regions: Vec<&str> = rows.iter().map(|(_, region)| *region).collect();
+                Ok(RecordBatch::try_new(
+                    schema.clone(),
+                    vec![
+                        Arc::new(Int64Array::from(ids)),
+                        Arc::new(StringArray::from(regions)),
+                    ],
+                )
+                .unwrap())
+            })
+            .collect();
+        Box::new(RecordBatchIterator::new(records, schema))
+    }
+
+    /// Create an `[id, value]` table with `id` as the unenforced primary key.
+    async fn id_value_table(dir: &TempDir) -> Table {
+        let conn = connect(dir.path().to_str().unwrap())
+            .execute()
+            .await
+            .unwrap();
+        let table = conn
+            .create_table("t", id_value_reader(vec![1, 2, 3]))
+            .execute()
+            .await
+            .unwrap();
+        table.set_unenforced_primary_key(["id"]).await.unwrap();
+        table
+    }
+
+    #[tokio::test]
+    async fn lsm_merge_insert_bucket() {
+        let dir = tempdir().unwrap();
+        let table = id_value_table(&dir).await;
+        // num_buckets = 1: every row routes to the single bucket.
+        table
+            .set_lsm_write_spec(LsmWriteSpec::bucket("id", 1))
+            .await
+            .unwrap();
+
+        // Empty `on` defaults to the primary key.
+        let mut builder = table.merge_insert(&[]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        let result = builder
+            .execute(id_value_reader(vec![3, 4, 5]))
+            .await
+            .unwrap();
+
+        // LSM path: rows go to the MemWAL, the breakdown is unknown until
+        // compaction, so only `num_rows` is populated.
+        assert_eq!(result.num_rows, 3);
+        assert_eq!(result.version, 0);
+        assert_eq!(result.num_inserted_rows, 0);
+        assert_eq!(result.num_updated_rows, 0);
+    }
+
+    #[tokio::test]
+    async fn lsm_merge_insert_unsharded() {
+        let dir = tempdir().unwrap();
+        let table = id_value_table(&dir).await;
+        table
+            .set_lsm_write_spec(LsmWriteSpec::unsharded())
+            .await
+            .unwrap();
+
+        let mut builder = table.merge_insert(&["id"]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        let result = builder
+            .execute(id_value_reader(vec![10, 11, 12, 13]))
+            .await
+            .unwrap();
+        assert_eq!(result.num_rows, 4);
+    }
+
+    #[tokio::test]
+    async fn lsm_merge_insert_identity() {
+        let dir = tempdir().unwrap();
+        let conn = connect(dir.path().to_str().unwrap())
+            .execute()
+            .await
+            .unwrap();
+        let table = conn
+            .create_table("t", id_region_reader(vec![(1, "us"), (2, "us")]))
+            .execute()
+            .await
+            .unwrap();
+        table.set_unenforced_primary_key(["id"]).await.unwrap();
+        table
+            .set_lsm_write_spec(LsmWriteSpec::identity("region"))
+            .await
+            .unwrap();
+
+        // All rows share one identity value, so they route to one shard.
+        let mut builder = table.merge_insert(&[]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        let result = builder
+            .execute(id_region_reader(vec![(3, "us"), (4, "us")]))
+            .await
+            .unwrap();
+        assert_eq!(result.num_rows, 2);
+    }
+
+    #[tokio::test]
+    async fn lsm_merge_insert_use_lsm_write_false_falls_back() {
+        let dir = tempdir().unwrap();
+        let table = id_value_table(&dir).await;
+        table
+            .set_lsm_write_spec(LsmWriteSpec::bucket("id", 1))
+            .await
+            .unwrap();
+
+        // use_lsm_write(false) opts out: the standard path runs and commits.
+        let mut builder = table.merge_insert(&["id"]);
+        builder.when_not_matched_insert_all().use_lsm_write(false);
+        let result = builder
+            .execute(id_value_reader(vec![3, 4, 5]))
+            .await
+            .unwrap();
+
+        assert_eq!(result.num_inserted_rows, 2);
+        assert_eq!(table.count_rows(None).await.unwrap(), 5);
+    }
+
+    #[tokio::test]
+    async fn lsm_merge_insert_rejects_on_not_primary_key() {
+        let dir = tempdir().unwrap();
+        let table = id_value_table(&dir).await;
+        table
+            .set_lsm_write_spec(LsmWriteSpec::bucket("id", 1))
+            .await
+            .unwrap();
+
+        let mut builder = table.merge_insert(&["value"]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        let err = builder.execute(id_value_reader(vec![1])).await.unwrap_err();
+        assert!(matches!(err, Error::InvalidInput { .. }), "got {err:?}");
+    }
+
+    #[tokio::test]
+    async fn lsm_merge_insert_rejects_non_upsert() {
+        let dir = tempdir().unwrap();
+        let table = id_value_table(&dir).await;
+        table
+            .set_lsm_write_spec(LsmWriteSpec::bucket("id", 1))
+            .await
+            .unwrap();
+
+        // Insert-only (no when_matched_update_all) is not the upsert shape.
+        let mut builder = table.merge_insert(&[]);
+        builder.when_not_matched_insert_all();
+        let err = builder.execute(id_value_reader(vec![4])).await.unwrap_err();
+        assert!(matches!(err, Error::InvalidInput { .. }), "got {err:?}");
+    }
+
+    #[tokio::test]
+    async fn lsm_close_writers_then_reopen() {
+        let dir = tempdir().unwrap();
+        let table = id_value_table(&dir).await;
+        table
+            .set_lsm_write_spec(LsmWriteSpec::bucket("id", 1))
+            .await
+            .unwrap();
+
+        let mut builder = table.merge_insert(&[]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        builder.execute(id_value_reader(vec![7, 8])).await.unwrap();
+
+        table.close_lsm_writers().await.unwrap();
+
+        // The writer reopens lazily on the next merge_insert.
+        let mut builder = table.merge_insert(&[]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        let result = builder.execute(id_value_reader(vec![9])).await.unwrap();
+        assert_eq!(result.num_rows, 1);
+    }
+
+    #[tokio::test]
+    async fn lsm_merge_insert_multi_batch() {
+        let dir = tempdir().unwrap();
+        let conn = connect(dir.path().to_str().unwrap())
+            .execute()
+            .await
+            .unwrap();
+        let table = conn
+            .create_table("t", id_region_reader(vec![(1, "us")]))
+            .execute()
+            .await
+            .unwrap();
+        table.set_unenforced_primary_key(["id"]).await.unwrap();
+        table
+            .set_lsm_write_spec(LsmWriteSpec::identity("region"))
+            .await
+            .unwrap();
+
+        // Multiple batches that all route to one shard are written together.
+        let mut builder = table.merge_insert(&[]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        let result = builder
+            .execute(id_region_multi_reader(vec![
+                vec![(2, "us"), (3, "us")],
+                vec![(4, "us")],
+            ]))
+            .await
+            .unwrap();
+        assert_eq!(result.num_rows, 3);
+
+        // Batches that route to different shards are rejected; the validation
+        // runs before any write, so no partial write is left behind.
+        let mut builder = table.merge_insert(&[]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        let err = builder
+            .execute(id_region_multi_reader(vec![
+                vec![(5, "us")],
+                vec![(6, "eu")],
+            ]))
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::InvalidInput { .. }), "got {err:?}");
+    }
+
+    #[tokio::test]
+    async fn lsm_merge_insert_use_lsm_write_true_requires_spec() {
+        let dir = tempdir().unwrap();
+        // id_value_table sets a primary key but no LSM write spec.
+        let table = id_value_table(&dir).await;
+
+        let mut builder = table.merge_insert(&["id"]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all()
+            .use_lsm_write(true);
+        let err = builder.execute(id_value_reader(vec![4])).await.unwrap_err();
+        assert!(matches!(err, Error::InvalidInput { .. }), "got {err:?}");
+    }
+
+    #[tokio::test]
+    async fn lsm_merge_insert_rejects_second_shard() {
+        let dir = tempdir().unwrap();
+        let conn = connect(dir.path().to_str().unwrap())
+            .execute()
+            .await
+            .unwrap();
+        let table = conn
+            .create_table("t", id_region_reader(vec![(1, "us")]))
+            .execute()
+            .await
+            .unwrap();
+        table.set_unenforced_primary_key(["id"]).await.unwrap();
+        table
+            .set_lsm_write_spec(LsmWriteSpec::identity("region"))
+            .await
+            .unwrap();
+
+        // The first merge_insert opens the single writer for shard "us".
+        let mut builder = table.merge_insert(&[]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        builder
+            .execute(id_region_reader(vec![(2, "us")]))
+            .await
+            .unwrap();
+
+        // A merge_insert routing to a different shard is rejected.
+        let mut builder = table.merge_insert(&[]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        let err = builder
+            .execute(id_region_reader(vec![(3, "eu")]))
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::InvalidInput { .. }), "got {err:?}");
+
+        // After closing the writer, a different shard can be written.
+        table.close_lsm_writers().await.unwrap();
+        let mut builder = table.merge_insert(&[]);
+        builder
+            .when_matched_update_all(None)
+            .when_not_matched_insert_all();
+        builder
+            .execute(id_region_reader(vec![(4, "eu")]))
+            .await
+            .unwrap();
+    }
+}
--- a/rust/lancedb/src/table/merge/lsm.rs
+++ b/rust/lancedb/src/table/merge/lsm.rs