diff --git a/Cargo.lock b/Cargo.lock
index 0515319b2..a2e317c22 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3584,8 +3584,8 @@ checksum = "42703706b716c37f96a77aea830392ad231f44c9e9a67872fa5548707e11b11c"
 
 [[package]]
 name = "fsst"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-array",
  "rand 0.9.5",
@@ -5059,8 +5059,8 @@ checksum = "e037a2e1d8d5fdbd49b16a4ea09d5d6401c1f29eca5ff29d03d3824dba16256a"
 
 [[package]]
 name = "lance"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arc-swap",
  "arrow",
@@ -5133,8 +5133,8 @@ dependencies = [
 
 [[package]]
 name = "lance-arrow"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-array",
  "arrow-buffer",
@@ -5156,7 +5156,7 @@ dependencies = [
 [[package]]
 name = "lance-arrow-scalar"
 version = "58.0.0"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-array",
  "arrow-buffer",
@@ -5170,7 +5170,7 @@ dependencies = [
 [[package]]
 name = "lance-arrow-stats"
 version = "58.0.0"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-array",
  "arrow-schema",
@@ -5179,8 +5179,8 @@ dependencies = [
 
 [[package]]
 name = "lance-bitpacking"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrayref",
  "crunchy",
@@ -5190,8 +5190,8 @@ dependencies = [
 
 [[package]]
 name = "lance-core"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-array",
  "arrow-buffer",
@@ -5228,8 +5228,8 @@ dependencies = [
 
 [[package]]
 name = "lance-datafusion"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow",
  "arrow-array",
@@ -5260,8 +5260,8 @@ dependencies = [
 
 [[package]]
 name = "lance-datagen"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow",
  "arrow-array",
@@ -5278,8 +5278,8 @@ dependencies = [
 
 [[package]]
 name = "lance-derive"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -5288,8 +5288,8 @@ dependencies = [
 
 [[package]]
 name = "lance-encoding"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-arith",
  "arrow-array",
@@ -5322,8 +5322,8 @@ dependencies = [
 
 [[package]]
 name = "lance-file"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-arith",
  "arrow-array",
@@ -5355,8 +5355,8 @@ dependencies = [
 
 [[package]]
 name = "lance-geo"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "datafusion",
  "geo-traits",
@@ -5370,8 +5370,8 @@ dependencies = [
 
 [[package]]
 name = "lance-index"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arc-swap",
  "arrow",
@@ -5442,8 +5442,8 @@ dependencies = [
 
 [[package]]
 name = "lance-index-core"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-array",
  "arrow-schema",
@@ -5466,8 +5466,8 @@ dependencies = [
 
 [[package]]
 name = "lance-io"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow",
  "arrow-array",
@@ -5494,6 +5494,7 @@ dependencies = [
  "path_abs",
  "pin-project",
  "prost",
+ "quick-xml 0.41.0",
  "rand 0.9.5",
  "reqsign-core",
  "reqsign-file-read-tokio",
@@ -5509,8 +5510,8 @@ dependencies = [
 
 [[package]]
 name = "lance-jsonb"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "byteorder",
  "ethnum",
@@ -5527,8 +5528,8 @@ dependencies = [
 
 [[package]]
 name = "lance-linalg"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-array",
  "arrow-schema",
@@ -5542,8 +5543,8 @@ dependencies = [
 
 [[package]]
 name = "lance-namespace"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow",
  "async-trait",
@@ -5557,8 +5558,8 @@ dependencies = [
 
 [[package]]
 name = "lance-namespace-impls"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arc-swap",
  "arrow",
@@ -5612,8 +5613,8 @@ dependencies = [
 
 [[package]]
 name = "lance-select"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-array",
  "arrow-buffer",
@@ -5627,8 +5628,8 @@ dependencies = [
 
 [[package]]
 name = "lance-table"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow",
  "arrow-array",
@@ -5668,8 +5669,8 @@ dependencies = [
 
 [[package]]
 name = "lance-testing"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "arrow-array",
  "arrow-schema",
@@ -5682,8 +5683,8 @@ dependencies = [
 
 [[package]]
 name = "lance-tokenizer"
-version = "14.0.0-beta.15"
-source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.15#765641dc0d7100f032fdebc00d73eef49711c4d4"
+version = "14.0.0-beta.18"
+source = "git+https://github.com/lance-format/lance.git?tag=v14.0.0-beta.18#f80bf1d1e59f40d249bce2539b78e41d93dee9ec"
 dependencies = [
  "frostem",
  "icu_segmenter",
@@ -6819,8 +6820,7 @@ dependencies = [
 [[package]]
 name = "object_store"
 version = "0.14.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f1796bc93603f78c5760a69f2d58badc9618d22adade0a95385bb2adbae4eb94"
+source = "git+https://github.com/apache/arrow-rs-object-store.git?rev=c10758c207c0103355f940f0c736fb8ecb157b04#c10758c207c0103355f940f0c736fb8ecb157b04"
 dependencies = [
  "async-trait",
  "aws-lc-rs",
diff --git a/Cargo.toml b/Cargo.toml
index 5033b1529..c6b9e84fe 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,20 +13,20 @@ categories = ["database-implementations"]
 rust-version = "1.94.0"
 
 [workspace.dependencies]
-lance = { "version" = "=14.0.0-beta.15", default-features = false, "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-core = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-datagen = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-file = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-io = { "version" = "=14.0.0-beta.15", default-features = false, "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-index = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-linalg = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-namespace = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-namespace-impls = { "version" = "=14.0.0-beta.15", default-features = false, "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-table = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-testing = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-datafusion = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-encoding = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
-lance-arrow = { "version" = "=14.0.0-beta.15", "tag" = "v14.0.0-beta.15", "git" = "https://github.com/lance-format/lance.git" }
+lance = { "version" = "=14.0.0-beta.18", default-features = false, "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-core = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-datagen = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-file = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-io = { "version" = "=14.0.0-beta.18", default-features = false, "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-index = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-linalg = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-namespace = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-namespace-impls = { "version" = "=14.0.0-beta.18", default-features = false, "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-table = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-testing = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-datafusion = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-encoding = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
+lance-arrow = { "version" = "=14.0.0-beta.18", "tag" = "v14.0.0-beta.18", "git" = "https://github.com/lance-format/lance.git" }
 lancedb = { path = "rust/lancedb", default-features = false }
 ahash = "0.8"
 # Note that this one does not include pyarrow
@@ -79,6 +79,10 @@ tonic = { version = "0.14", features = ["tls-native-roots", "tls-aws-lc"] }
 uuid = { version = "1.7.0", features = ["v4", "v7"] }
 chrono = { version = "0.4", default-features = false, features = ["clock"] }
 
+[patch.crates-io]
+# Lance v14.0.0-beta.18 requires multipart write modes from this upstream revision.
+object_store = { git = "https://github.com/apache/arrow-rs-object-store.git", rev = "c10758c207c0103355f940f0c736fb8ecb157b04" }
+
 [profile.ci]
 debug = "line-tables-only"
 inherits = "dev"
diff --git a/deny.toml b/deny.toml
index d112095ec..d5f42dbff 100644
--- a/deny.toml
+++ b/deny.toml
@@ -218,4 +218,6 @@ allow-registry = ["https://github.com/rust-lang/crates.io-index"]
 # releases are cut to crates.io. Allow that specific host.
 allow-git = [
     "https://github.com/lance-format/lance",
+    # Pinned by Lance for the multipart write mode API (see [patch.crates-io] in Cargo.toml).
+    "https://github.com/apache/arrow-rs-object-store",
 ]
diff --git a/java/pom.xml b/java/pom.xml
index 33446040b..465554706 100644
--- a/java/pom.xml
+++ b/java/pom.xml
@@ -28,7 +28,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <arrow.version>15.0.0</arrow.version>
-        <lance-core.version>14.0.0-beta.15</lance-core.version>
+        <lance-core.version>14.0.0-beta.18</lance-core.version>
         <jackson.version>2.18.11</jackson.version>
         <spotless.skip>false</spotless.skip>
         <spotless.version>2.30.0</spotless.version>
diff --git a/rust/lancedb/src/query.rs b/rust/lancedb/src/query.rs
index 07db69bb3..d66932847 100644
--- a/rust/lancedb/src/query.rs
+++ b/rust/lancedb/src/query.rs
@@ -3806,8 +3806,12 @@ mod tests {
         assert_eq!(results[0].num_columns(), 1);
     }
 
+    #[rstest::rstest]
+    #[case::single_row(1)]
+    #[case::two_rows(2)]
+    #[case::three_rows(3)]
     #[tokio::test]
-    async fn test_take_offsets_preserves_duplicate_multiplicity() {
+    async fn test_take_offsets_preserves_duplicate_multiplicity(#[case] max_batch_length: u32) {
         let tmp_dir = tempdir().unwrap();
         let table = make_test_table(&tmp_dir).await;
 
@@ -3815,7 +3819,7 @@ mod tests {
             .take_offsets(vec![5, 1, 5, 17])
             .select(Select::Columns(vec!["id".to_string()]))
             .execute_with_options(QueryExecutionOptions {
-                max_batch_length: 2,
+                max_batch_length,
                 ..Default::default()
             })
             .await
@@ -3824,7 +3828,11 @@ mod tests {
             .await
             .unwrap();
 
-        assert_eq!(results.len(), 2);
+        assert!(
+            results
+                .iter()
+                .all(|batch| batch.num_rows() <= max_batch_length as usize)
+        );
         assert!(results.iter().all(|batch| batch.num_columns() == 1));
         let mut ids = results
             .iter()
diff --git a/rust/lancedb/src/table/merge.rs b/rust/lancedb/src/table/merge.rs
index fbe82a198..c74980c5e 100644
--- a/rust/lancedb/src/table/merge.rs
+++ b/rust/lancedb/src/table/merge.rs
@@ -2035,11 +2035,13 @@ mod lsm_tests {
         );
     }
 
-    /// Claims the base table's bitmap index and keeps it as a B-tree, whose
-    /// flush rows are in the shape the bitmap trainer takes.
+    /// Claims the base table's bitmap index and keeps it as a B-tree.
     #[derive(Debug)]
     struct BitmapAsBTree;
 
+    #[derive(Debug)]
+    struct BitmapAsBTreeIndex(lance::dataset::mem_wal::index::BTreeMemIndex);
+
     fn btree_plugin() -> Arc<dyn lance::dataset::mem_wal::index::MemIndexPlugin> {
         lance::dataset::mem_wal::MemIndexRegistry::default()
             .plugin_for_details_url("/lance.table.BTreeIndexDetails")
@@ -2059,7 +2061,8 @@ mod lsm_tests {
             lance_index::IndexType::Bitmap
         }
         fn training_criteria(&self) -> lance_index::scalar::registry::TrainingCriteria {
-            btree_plugin().training_criteria()
+            use lance_index::scalar::registry::{TrainingCriteria, TrainingOrdering};
+            TrainingCriteria::new(TrainingOrdering::Values).with_row_id()
         }
         fn validate(
             &self,
@@ -2071,7 +2074,57 @@ mod lsm_tests {
             &self,
             ctx: &lance::dataset::mem_wal::index::MemIndexBuildContext<'_>,
         ) -> lance_core::Result<Arc<dyn lance::dataset::mem_wal::index::MemIndex>> {
-            btree_plugin().create(ctx)
+            let (column, field_id) = ctx.single_column()?;
+            Ok(Arc::new(BitmapAsBTreeIndex(
+                lance::dataset::mem_wal::index::BTreeMemIndex::new(field_id, column.to_string()),
+            )))
+        }
+    }
+
+    #[async_trait::async_trait]
+    impl lance::dataset::mem_wal::index::MemIndex for BitmapAsBTreeIndex {
+        fn columns(&self) -> &[String] {
+            self.0.columns()
+        }
+        fn can_answer(&self, query: &dyn lance::dataset::mem_wal::index::MemQuery) -> bool {
+            self.0.can_answer(query)
+        }
+        fn insert(&self, batch: &RecordBatch, row_offset: u64) -> lance_core::Result<()> {
+            self.0.insert(batch, row_offset)
+        }
+        fn resident_bytes(&self) -> usize {
+            self.0.resident_bytes()
+        }
+        fn search(
+            &self,
+            query: &dyn lance::dataset::mem_wal::index::MemQuery,
+            ctx: &lance::dataset::mem_wal::index::SearchContext,
+        ) -> lance_core::Result<Option<lance::dataset::mem_wal::index::MemMatches>> {
+            self.0.search(query, ctx)
+        }
+        async fn flush(
+            &self,
+            ctx: &lance::dataset::mem_wal::index::FlushContext<'_>,
+        ) -> lance_core::Result<lance::dataset::mem_wal::index::FlushOutcome> {
+            // BTree now trains on row addresses, while Bitmap still takes row IDs.
+            // These coincide in the flushed generation, which uses physical IDs.
+            assert!(!ctx.generation()?.dataset.manifest().uses_stable_row_ids());
+            let batches = self
+                .0
+                .to_training_batches(ctx.batch_size)?
+                .into_iter()
+                .map(|batch| {
+                    assert_eq!(batch.schema().field(1).name(), lance_core::ROW_ADDR);
+                    let schema = Arc::new(Schema::new(vec![
+                        batch.schema().field(0).clone(),
+                        Field::new(lance_core::ROW_ID, DataType::UInt64, false),
+                    ]));
+                    RecordBatch::try_new(schema, batch.columns().to_vec())
+                })
+                .collect::<std::result::Result<Vec<_>, _>>()?;
+            Ok(lance::dataset::mem_wal::index::FlushOutcome::from_batches(
+                batches,
+            ))
         }
     }