docs: state what a computed column promises

The declaration API shipped without a runnable example, and none of the three binding docs said when values appear, what happens to them when an input changes, which schema operations a declaration blocks, or that the feature is local-only. Those are the questions a caller has to answer before using it. Adds Rust doctests on both entry points and the same semantics to the Python and TypeScript parameter docs, plus a worked Python example. Also adds the abstract refresh_column that both concrete Python tables already implemented, so the surface is declared in one place and the cross-references resolve.
fix: make computed columns explicitly local-only
2026-08-26 07:58:31 +00:00 · 2026-08-12 21:01:11 -07:00 · 2026-08-12 20:00:18 -07:00 · 2026-08-12 20:00:18 -07:00 · 2026-08-12 20:00:18 -07:00 · 2026-08-12 20:00:18 -07:00
22 changed files with 1962 additions and 87 deletions
@@ -3455,8 +3455,8 @@ checksum = "42703706b716c37f96a77aea830392ad231f44c9e9a67872fa5548707e11b11c"

 [[package]]
 name = "fsst"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-array",
 "rand 0.9.5",
@@ -4815,8 +4815,8 @@ checksum = "e037a2e1d8d5fdbd49b16a4ea09d5d6401c1f29eca5ff29d03d3824dba16256a"

 [[package]]
 name = "lance"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arc-swap",
 "arrow",
@@ -4890,8 +4890,8 @@ dependencies = [

 [[package]]
 name = "lance-arrow"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-array",
 "arrow-buffer",
@@ -4913,7 +4913,7 @@ dependencies = [
 [[package]]
 name = "lance-arrow-scalar"
 version = "58.0.0"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-array",
 "arrow-buffer",
@@ -4927,7 +4927,7 @@ dependencies = [
 [[package]]
 name = "lance-arrow-stats"
 version = "58.0.0"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-array",
 "arrow-schema",
@@ -4936,8 +4936,8 @@ dependencies = [

 [[package]]
 name = "lance-bitpacking"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrayref",
 "crunchy",
@@ -4947,8 +4947,8 @@ dependencies = [

 [[package]]
 name = "lance-core"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-array",
 "arrow-buffer",
@@ -4988,8 +4988,8 @@ dependencies = [

 [[package]]
 name = "lance-datafusion"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow",
 "arrow-array",
@@ -5019,8 +5019,8 @@ dependencies = [

 [[package]]
 name = "lance-datagen"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow",
 "arrow-array",
@@ -5037,8 +5037,8 @@ dependencies = [

 [[package]]
 name = "lance-derive"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "proc-macro2",
 "quote",
@@ -5047,8 +5047,8 @@ dependencies = [

 [[package]]
 name = "lance-encoding"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-arith",
 "arrow-array",
@@ -5082,8 +5082,8 @@ dependencies = [

 [[package]]
 name = "lance-file"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-arith",
 "arrow-array",
@@ -5114,8 +5114,8 @@ dependencies = [

 [[package]]
 name = "lance-index"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arc-swap",
 "arrow",
@@ -5182,8 +5182,8 @@ dependencies = [

 [[package]]
 name = "lance-index-core"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-array",
 "arrow-schema",
@@ -5205,8 +5205,8 @@ dependencies = [

 [[package]]
 name = "lance-io"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow",
 "arrow-array",
@@ -5242,8 +5242,8 @@ dependencies = [

 [[package]]
 name = "lance-linalg"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-array",
 "arrow-buffer",
@@ -5259,8 +5259,8 @@ dependencies = [

 [[package]]
 name = "lance-namespace"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow",
 "async-trait",
@@ -5272,8 +5272,8 @@ dependencies = [

 [[package]]
 name = "lance-namespace-impls"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow",
 "arrow-ipc",
@@ -5326,8 +5326,8 @@ dependencies = [

 [[package]]
 name = "lance-select"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-array",
 "arrow-buffer",
@@ -5342,8 +5342,8 @@ dependencies = [

 [[package]]
 name = "lance-table"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow",
 "arrow-array",
@@ -5383,8 +5383,8 @@ dependencies = [

 [[package]]
 name = "lance-testing"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "arrow-array",
 "arrow-schema",
@@ -5397,8 +5397,8 @@ dependencies = [

 [[package]]
 name = "lance-tokenizer"
-version = "11.0.0-beta.6"
-source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.6#5ab688688c411111d94d279bacd037d7c319dc10"
+version = "11.0.0-beta.7"
+source = "git+https://github.com/lance-format/lance.git?tag=v11.0.0-beta.7#e581c49338bc83baf1ea50c5e235bd702f3fbeea"
 dependencies = [
 "frostem",
 "icu_segmenter",
@@ -13,20 +13,20 @@ categories = ["database-implementations"]
 rust-version = "1.91.0"

 [workspace.dependencies]
-lance = { "version" = "=11.0.0-beta.6", default-features = false, "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-core = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-datagen = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-file = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-io = { "version" = "=11.0.0-beta.6", default-features = false, "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-index = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-linalg = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-namespace = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-namespace-impls = { "version" = "=11.0.0-beta.6", default-features = false, "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-table = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-testing = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-datafusion = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-encoding = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
-lance-arrow = { "version" = "=11.0.0-beta.6", "tag" = "v11.0.0-beta.6", "git" = "https://github.com/lance-format/lance.git" }
+lance = { "version" = "=11.0.0-beta.7", default-features = false, "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-core = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-datagen = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-file = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-io = { "version" = "=11.0.0-beta.7", default-features = false, "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-index = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-linalg = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-namespace = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-namespace-impls = { "version" = "=11.0.0-beta.7", default-features = false, "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-table = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-testing = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-datafusion = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-encoding = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "git" = "https://github.com/lance-format/lance.git" }
+lance-arrow = { "version" = "=11.0.0-beta.7", "tag" = "v11.0.0-beta.7", "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 }
@@ -69,14 +69,33 @@ abstract addColumns(newColumnTransforms): Promise<AddColumnsResult>

 Add new columns with defined values.

+The `{ computed }` form stores the expression rather than evaluating it
+now: the column is committed with no values, and rows get them from
+[Table#refreshColumn](Table.md#refreshcolumn). Declaring one therefore costs the same on a
+large table as on an empty one.
+
+A refresh does not revisit rows it has already filled, so mutating an
+input leaves the value computed at fill time; recomputing means dropping
+the column and declaring it again. While a declaration reads a column,
+that column cannot be renamed, retyped or dropped.
+
+Computed columns are local-only: LanceDB Cloud and Enterprise reject a
+declaration.
+
 #### Parameters

-* **newColumnTransforms**: `Field`&lt;`any`&gt; \| `Field`&lt;`any`&gt;[] \| `Schema`&lt;`any`&gt; \| [`AddColumnsSql`](../interfaces/AddColumnsSql.md)[]
+* **newColumnTransforms**:
+    \| `Field`&lt;`any`&gt;
+    \| `Field`&lt;`any`&gt;[]
+    \| `Schema`&lt;`any`&gt;
+    \| [`AddColumnsSql`](../interfaces/AddColumnsSql.md)[]
+    \| `object`
    Either:
    - An array of objects with column names and SQL expressions to calculate values
    - A single Arrow Field defining one column with its data type (column will be initialized with null values)
    - An array of Arrow Fields defining columns with their data types (columns will be initialized with null values)
    - An Arrow Schema defining columns with their data types (columns will be initialized with null values)
+    - `{ computed }`, declaring columns defined by a SQL expression whose type and inputs are derived from it

 #### Returns

@@ -85,6 +104,13 @@ Add new columns with defined values.
 A promise that resolves to an object
 containing the new version number of the table after adding the columns.

+#### Example
+
+```ts
+await table.addColumns({ computed: [{ name: "doubled", valueSql: "x * 2" }] });
+const { rowsFilled } = await table.refreshColumn("doubled");
+```
+
 ***

 ### alterColumns()
@@ -718,6 +744,32 @@ for await (const batch of table.query()) {

 ***

+### refreshColumn()
+
+```ts
+abstract refreshColumn(column): Promise<RefreshColumnResult>
+```
+
+Fill the rows of a computed column that hold no value yet.
+
+Rows appended since the last refresh are filled by the next one; rows
+already filled are left as they are, so the call is idempotent and does
+not observe a mutated input. Local tables only.
+
+#### Parameters
+
+* **column**: `string`
+    The name of the computed column to fill.
+
+#### Returns
+
+`Promise`&lt;[`RefreshColumnResult`](../interfaces/RefreshColumnResult.md)&gt;
+
+A promise that resolves to the
+number of rows filled and the new version number of the table.
+
+***
+
 ### restore()

 ```ts
@@ -105,6 +105,7 @@
 - [OptimizeOptions](interfaces/OptimizeOptions.md)
 - [OptimizeStats](interfaces/OptimizeStats.md)
 - [QueryExecutionOptions](interfaces/QueryExecutionOptions.md)
+- [RefreshColumnResult](interfaces/RefreshColumnResult.md)
 - [RemovalStats](interfaces/RemovalStats.md)
 - [RenameTableOptions](interfaces/RenameTableOptions.md)
 - [RestNamespaceConfig](interfaces/RestNamespaceConfig.md)
@@ -0,0 +1,23 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / RefreshColumnResult
+
+# Interface: RefreshColumnResult
+
+## Properties
+
+### rowsFilled
+
+```ts
+rowsFilled: number;
+```
+
+***
+
+### version
+
+```ts
+version: number;
+```
@@ -3340,3 +3340,45 @@ describe("LSM merge insert", () => {
    await expect(table.query().useLsm(true).toArray()).rejects.toThrow();
  });
 });
+
+describe("computed columns", () => {
+  let tmpDir: tmp.DirResult;
+  beforeEach(() => {
+    tmpDir = tmp.dirSync({ unsafeCleanup: true });
+  });
+  afterEach(() => tmpDir.removeCallback());
+
+  it("declares a column and fills it on refresh", async () => {
+    const db = await connect(tmpDir.name);
+    const table = await db.createTable("computed", [{ x: 1 }, { x: 2 }]);
+
+    await table.addColumns({
+      computed: [{ name: "doubled", valueSql: "x * 2" }],
+    });
+    let rows = await table.query().toArray();
+    expect(rows.map((r) => r.doubled)).toEqual([null, null]);
+
+    const result = await table.refreshColumn("doubled");
+    expect(result.rowsFilled).toBe(2);
+
+    rows = await table.query().toArray();
+    expect(rows.map((r) => r.doubled).sort()).toEqual([2, 4]);
+  });
+
+  it("fills rows added since the last refresh", async () => {
+    const db = await connect(tmpDir.name);
+    const table = await db.createTable("computed_append", [{ x: 1 }]);
+
+    await table.addColumns({
+      computed: [{ name: "doubled", valueSql: "x * 2" }],
+    });
+    await table.refreshColumn("doubled");
+    await table.add([{ x: 5 }]);
+
+    const result = await table.refreshColumn("doubled");
+    expect(result.rowsFilled).toBe(1);
+
+    const rows = await table.query().toArray();
+    expect(rows.map((r) => r.doubled).sort()).toEqual([10, 2]);
+  });
+});
@@ -50,6 +50,7 @@ export {
  MergeResult,
  AddResult,
  AddColumnsResult,
+  RefreshColumnResult,
  AlterColumnsResult,
  UpdateFieldMetadataResult,
  DeleteResult,
@@ -33,6 +33,7 @@ import {
  Job,
  Branches as NativeBranches,
  OptimizeStats,
+  RefreshColumnResult,
  TableStatistics,
  Tags,
  UpdateFieldMetadataResult,
@@ -525,18 +526,54 @@ export abstract class Table {
  abstract vectorSearch(vector: IntoVector | MultiVector): VectorQuery;
  /**
   * Add new columns with defined values.
+   *
+   * The `{ computed }` form stores the expression rather than evaluating it
+   * now: the column is committed with no values, and rows get them from
+   * {@link Table#refreshColumn}. Declaring one therefore costs the same on a
+   * large table as on an empty one.
+   *
+   * A refresh does not revisit rows it has already filled, so mutating an
+   * input leaves the value computed at fill time; recomputing means dropping
+   * the column and declaring it again. While a declaration reads a column,
+   * that column cannot be renamed, retyped or dropped.
+   *
+   * Computed columns are local-only: LanceDB Cloud and Enterprise reject a
+   * declaration.
   * @param {AddColumnsSql[] | Field | Field[] | Schema} newColumnTransforms Either:
   *   - An array of objects with column names and SQL expressions to calculate values
   *   - A single Arrow Field defining one column with its data type (column will be initialized with null values)
   *   - An array of Arrow Fields defining columns with their data types (columns will be initialized with null values)
   *   - An Arrow Schema defining columns with their data types (columns will be initialized with null values)
+   *   - `{ computed }`, declaring columns defined by a SQL expression whose type and inputs are derived from it
   * @returns {Promise<AddColumnsResult>} A promise that resolves to an object
   * containing the new version number of the table after adding the columns.
+   * @example
+   * ```ts
+   * await table.addColumns({ computed: [{ name: "doubled", valueSql: "x * 2" }] });
+   * const { rowsFilled } = await table.refreshColumn("doubled");
+   * ```
   */
  abstract addColumns(
-    newColumnTransforms: AddColumnsSql[] | Field | Field[] | Schema,
+    newColumnTransforms:
+      | AddColumnsSql[]
+      | Field
+      | Field[]
+      | Schema
+      | { computed: AddColumnsSql[] },
  ): Promise<AddColumnsResult>;

+  /**
+   * Fill the rows of a computed column that hold no value yet.
+   *
+   * Rows appended since the last refresh are filled by the next one; rows
+   * already filled are left as they are, so the call is idempotent and does
+   * not observe a mutated input. Local tables only.
+   * @param {string} column The name of the computed column to fill.
+   * @returns {Promise<RefreshColumnResult>} A promise that resolves to the
+   * number of rows filled and the new version number of the table.
+   */
+  abstract refreshColumn(column: string): Promise<RefreshColumnResult>;
+
  /**
   * Alter the name or nullability of columns.
   * @param {ColumnAlteration[]} columnAlterations One or more alterations to
@@ -1088,8 +1125,22 @@ export class LocalTable extends Table {
  // TODO: Support BatchUDF

  async addColumns(
-    newColumnTransforms: AddColumnsSql[] | Field | Field[] | Schema,
+    newColumnTransforms:
+      | AddColumnsSql[]
+      | Field
+      | Field[]
+      | Schema
+      | { computed: AddColumnsSql[] },
  ): Promise<AddColumnsResult> {
+    // Columns defined by an expression are declared, not materialized here.
+    if (
+      typeof newColumnTransforms === "object" &&
+      !Array.isArray(newColumnTransforms) &&
+      "computed" in newColumnTransforms
+    ) {
+      return await this.inner.addComputedColumns(newColumnTransforms.computed);
+    }
+
    // Handle single Field -> convert to array of Fields
    if (newColumnTransforms instanceof Field) {
      newColumnTransforms = [newColumnTransforms];
@@ -1124,6 +1175,10 @@ export class LocalTable extends Table {
    throw new Error("Invalid input type for addColumns");
  }

+  async refreshColumn(column: string): Promise<RefreshColumnResult> {
+    return await this.inner.refreshColumn(column);
+  }
+
  async alterColumns(
    columnAlterations: ColumnAlteration[],
  ): Promise<AlterColumnsResult> {
@@ -347,6 +347,30 @@ impl Table {
        Ok(res.into())
    }

+    #[napi(catch_unwind)]
+    pub async fn add_computed_columns(
+        &self,
+        columns: Vec<AddColumnsSql>,
+    ) -> napi::Result<AddColumnsResult> {
+        let table = self.inner_ref()?;
+        let mut builder = table.add_columns();
+        for column in columns {
+            builder = builder.computed(column.name, column.value_sql);
+        }
+        let res = builder.execute().await.default_error()?;
+        Ok(res.into())
+    }
+
+    #[napi(catch_unwind)]
+    pub async fn refresh_column(&self, column: String) -> napi::Result<RefreshColumnResult> {
+        let res = self
+            .inner_ref()?
+            .refresh_column(column)
+            .await
+            .default_error()?;
+        Ok(res.into())
+    }
+
    #[napi(catch_unwind)]
    pub async fn add_columns_with_schema(
        &self,
@@ -1196,6 +1220,21 @@ pub struct AddColumnsResult {
    pub version: i64,
 }

+#[napi(object)]
+pub struct RefreshColumnResult {
+    pub rows_filled: i64,
+    pub version: i64,
+}
+
+impl From<lancedb::table::RefreshColumnResult> for RefreshColumnResult {
+    fn from(value: lancedb::table::RefreshColumnResult) -> Self {
+        Self {
+            rows_filled: value.rows_filled as i64,
+            version: value.version as i64,
+        }
+    }
+}
+
 impl From<lancedb::table::AddColumnsResult> for AddColumnsResult {
    fn from(value: lancedb::table::AddColumnsResult) -> Self {
        Self {
@@ -335,6 +335,10 @@ class Table:
    ) -> list[FtsToken]: ...
    async def delete(self, filter: Union[str, PyExpr]) -> DeleteResult: ...
    async def add_columns(self, columns: list[tuple[str, str]]) -> AddColumnsResult: ...
+    async def add_computed_columns(
+        self, columns: list[tuple[str, str]]
+    ) -> AddColumnsResult: ...
+    async def refresh_column(self, column: str) -> RefreshColumnResult: ...
    async def add_columns_with_schema(self, schema: pa.Schema) -> AddColumnsResult: ...
    async def alter_columns(
        self, columns: list[dict[str, Any]]
@@ -680,6 +684,10 @@ class LsmWriteSpec:
 class AddColumnsResult:
    version: int

+class RefreshColumnResult:
+    rows_filled: int
+    version: int
+
 class AlterColumnsResult:
    version: int

@@ -958,9 +958,21 @@ class RemoteTable(Table):
    def count_rows(self, filter: Optional[str] = None) -> int:
        return LOOP.run(self._table.count_rows(filter))

-    def add_columns(self, transforms: Dict[str, str]) -> AddColumnsResult:
+    def add_columns(
+        self,
+        transforms: Dict[str, str] | None = None,
+        *,
+        computed: Dict[str, str] | None = None,
+    ) -> AddColumnsResult:
+        if computed:
+            raise NotImplementedError(
+                "computed columns are supported only on local tables"
+            )
        return LOOP.run(self._table.add_columns(transforms))

+    def refresh_column(self, column: str):
+        raise NotImplementedError("computed columns are supported only on local tables")
+
    def alter_columns(
        self, *alterations: Iterable[Dict[str, str]]
    ) -> AlterColumnsResult:
@@ -176,6 +176,7 @@ if TYPE_CHECKING:
        CompactionStats,
        Tag,
        AddColumnsResult,
+        RefreshColumnResult,
        AddResult,
        AlterColumnsResult,
        UpdateFieldMetadataResult,
@@ -1916,7 +1917,14 @@ class Table(ABC):

    @abstractmethod
    def add_columns(
-        self, transforms: Dict[str, str] | pa.Field | List[pa.Field] | pa.Schema
+        self,
+        transforms: Dict[str, str]
+        | pa.Field
+        | List[pa.Field]
+        | pa.Schema
+        | None = None,
+        *,
+        computed: Dict[str, str] | None = None,
    ):
        """
        Add new columns with defined values.
@@ -1930,11 +1938,68 @@ class Table(ABC):
            Alternatively, a pyarrow Field or Schema can be provided to add
            new columns with the specified data types. The new columns will
            be initialized with null values.
+        computed: Dict[str, str], optional
+            A map of column name to a SQL expression defining the column. The
+            column's type and inputs are derived from the expression, so no
+            data type is supplied.
+
+            Unlike ``transforms``, the expression is stored rather than
+            evaluated now: the column is committed with no values, and rows get
+            them from [`refresh_column`][lancedb.table.Table.refresh_column].
+            Declaring one therefore costs the same on a large table as on an
+            empty one.
+
+            A refresh does not revisit rows it has already filled, so mutating
+            an input leaves the value computed at fill time; recomputing means
+            dropping the column and declaring it again. While a declaration
+            reads a column, that column cannot be renamed, retyped or dropped.
+
+            Local tables only; LanceDB Cloud and Enterprise raise
+            ``NotImplementedError``. Cannot be combined with ``transforms``.

        Returns
        -------
        AddColumnsResult
            version: the new version number of the table after adding columns.
+
+        Examples
+        --------
+        >>> import lancedb
+        >>> db = lancedb.connect("./.lancedb")
+        >>> table = db.create_table("computed_demo", [{"x": 1}, {"x": 2}])
+        >>> table.add_columns(computed={"doubled": "x * 2"})
+        AddColumnsResult(version=2)
+        >>> table.refresh_column("doubled")
+        RefreshColumnResult(rows_filled=2, version=3)
+        >>> table.to_arrow().sort_by("x").to_pandas()
+           x  doubled
+        0  1        2
+        1  2        4
+        """
+
+    @abstractmethod
+    def refresh_column(self, column: str) -> "RefreshColumnResult":
+        """
+        Fill the rows of a computed column that hold no value yet.
+
+        Declared with ``add_columns(computed=...)``, a column starts empty and
+        gets its values here. Rows appended since the last refresh are filled
+        by the next one; rows already filled are left as they are, so the call
+        is idempotent and does not observe a mutated input.
+
+        Local tables only; LanceDB Cloud and Enterprise raise
+        ``NotImplementedError``.
+
+        Parameters
+        ----------
+        column: str
+            The name of the computed column to fill.
+
+        Returns
+        -------
+        RefreshColumnResult
+            rows_filled: the number of rows given a value.
+            version: the new version number of the table.
        """

    @abstractmethod
@@ -3939,9 +4004,21 @@ class LanceTable(Table):
        return LOOP.run(self._table.index_stats(index_name))

    def add_columns(
-        self, transforms: Dict[str, str] | pa.field | List[pa.field] | pa.Schema
+        self,
+        transforms: Dict[str, str]
+        | pa.field
+        | List[pa.field]
+        | pa.Schema
+        | None = None,
+        *,
+        computed: Dict[str, str] | None = None,
    ) -> AddColumnsResult:
-        return LOOP.run(self._table.add_columns(transforms))
+        return LOOP.run(self._table.add_columns(transforms, computed=computed))
+
+    def refresh_column(self, column: str) -> "RefreshColumnResult":
+        """Fill a computed column's unfilled rows. See
+        [`AsyncTable.refresh_column`][lancedb.AsyncTable.refresh_column]."""
+        return LOOP.run(self._table.refresh_column(column))

    def alter_columns(
        self, *alterations: Iterable[Dict[str, str]]
@@ -5856,7 +5933,14 @@ class AsyncTable:
        return await self._inner.update(updates_sql, where)

    async def add_columns(
-        self, transforms: dict[str, str] | pa.field | List[pa.field] | pa.Schema
+        self,
+        transforms: dict[str, str]
+        | pa.field
+        | List[pa.field]
+        | pa.Schema
+        | None = None,
+        *,
+        computed: dict[str, str] | None = None,
    ) -> AddColumnsResult:
        """
        Add new columns with defined values.
@@ -5869,6 +5953,21 @@ class AsyncTable:
            each row in the table, and can reference existing columns.
            Alternatively, you can pass a pyarrow field or schema to add
            new columns with NULLs.
+        computed: Dict[str, str], optional
+            A map of column name to a SQL expression defining the column. The
+            column's type and inputs are derived from the expression.
+
+            Unlike ``transforms``, the expression is stored rather than
+            evaluated now: the column is committed with no values, and rows get
+            them from
+            [`refresh_column`][lancedb.table.AsyncTable.refresh_column].
+
+            A refresh does not revisit rows it has already filled, so mutating
+            an input leaves the value computed at fill time. While a
+            declaration reads a column, that column cannot be renamed, retyped
+            or dropped.
+
+            Local tables only. Cannot be combined with ``transforms``.

        Returns
        -------
@@ -5882,11 +5981,43 @@ class AsyncTable:
            {isinstance(f, pa.Field) for f in transforms}
        ):
            transforms = pa.schema(transforms)
+        if computed:
+            if transforms:
+                raise ValueError(
+                    "add_columns cannot take both transforms and computed columns"
+                )
+            return await self._inner.add_computed_columns(list(computed.items()))
+        if transforms is None:
+            raise ValueError("add_columns requires transforms or computed columns")
        if isinstance(transforms, pa.Schema):
            return await self._inner.add_columns_with_schema(transforms)
        else:
            return await self._inner.add_columns(list(transforms.items()))

+    async def refresh_column(self, column: str) -> RefreshColumnResult:
+        """
+        Fill the rows of a computed column that hold no value yet.
+
+        Declared with ``add_columns(computed=...)``, a column starts empty and
+        gets its values here. Rows appended since the last refresh are filled
+        by the next one; rows already filled are left as they are, so the call
+        is idempotent and does not observe a mutated input.
+
+        Local tables only; LanceDB Cloud and Enterprise raise
+        ``NotImplementedError``.
+
+        Parameters
+        ----------
+        column: str
+            The name of the computed column to fill.
+
+        Returns
+        -------
+        RefreshColumnResult
+            The number of rows filled and the new version of the table.
+        """
+        return await self._inner.refresh_column(column)
+
    async def alter_columns(
        self, *alterations: Iterable[dict[str, Any]]
    ) -> AlterColumnsResult:
@@ -3854,3 +3854,37 @@ async def test_async_search_runs_embedding_on_dedicated_executor(
    assert all(name.startswith("lancedb-embedding") for name in captured_threads), (
        f"embedding ran off the dedicated executor: {captured_threads}"
    )
+
+
+def test_computed_column_declare_and_refresh(tmp_path):
+    db = lancedb.connect(tmp_path)
+    table = db.create_table("computed", [{"x": 1}, {"x": 2}])
+
+    table.add_columns(computed={"doubled": "x * 2"})
+    assert table.to_arrow()["doubled"].to_pylist() == [None, None]
+
+    result = table.refresh_column("doubled")
+    assert result.rows_filled == 2
+    assert sorted(table.to_arrow()["doubled"].to_pylist()) == [2, 4]
+
+    table.add([{"x": 5}])
+    assert table.refresh_column("doubled").rows_filled == 1
+    assert sorted(table.to_arrow()["doubled"].to_pylist()) == [2, 4, 10]
+
+
+def test_computed_column_rejects_transforms_and_computed_together(tmp_path):
+    db = lancedb.connect(tmp_path)
+    table = db.create_table("computed_mixed", [{"x": 1}])
+    with pytest.raises(ValueError):
+        table.add_columns({"a": "x + 1"}, computed={"b": "x * 2"})
+
+
+@pytest.mark.asyncio
+async def test_computed_column_async(tmp_path):
+    db = await lancedb.connect_async(tmp_path)
+    table = await db.create_table("computed_async", [{"x": 3}])
+
+    await table.add_columns(computed={"tripled": "x * 3"})
+    await table.refresh_column("tripled")
+
+    assert (await table.to_arrow())["tripled"].to_pylist() == [9]
@@ -16,7 +16,8 @@ use query::{FTSQuery, HybridQuery, Query, VectorQuery};
 use session::Session;
 use table::{
    AddColumnsResult, AddResult, AlterColumnsResult, DeleteResult, DropColumnsResult, FtsToken,
-    LsmWriteSpec, MergeResult, PyBlobFile, Table, UpdateFieldMetadataResult, UpdateResult,
+    LsmWriteSpec, MergeResult, PyBlobFile, RefreshColumnResult, Table, UpdateFieldMetadataResult,
+    UpdateResult,
 };

 pub mod arrow;
@@ -57,6 +58,7 @@ pub fn _lancedb(_py: Python, m: &Bound<'_, PyModule>) -> PyResult<()> {
    m.add_class::<VectorQuery>()?;
    m.add_class::<RecordBatchStream>()?;
    m.add_class::<AddColumnsResult>()?;
+    m.add_class::<RefreshColumnResult>()?;
    m.add_class::<AlterColumnsResult>()?;
    m.add_class::<UpdateFieldMetadataResult>()?;
    m.add_class::<AddResult>()?;
@@ -415,6 +415,32 @@ pub struct AddColumnsResult {
    pub version: u64,
 }

+#[pyclass(get_all, from_py_object)]
+#[derive(Clone, Debug)]
+pub struct RefreshColumnResult {
+    pub rows_filled: u64,
+    pub version: u64,
+}
+
+#[pymethods]
+impl RefreshColumnResult {
+    pub fn __repr__(&self) -> String {
+        format!(
+            "RefreshColumnResult(rows_filled={}, version={})",
+            self.rows_filled, self.version
+        )
+    }
+}
+
+impl From<lancedb::table::RefreshColumnResult> for RefreshColumnResult {
+    fn from(result: lancedb::table::RefreshColumnResult) -> Self {
+        Self {
+            rows_filled: result.rows_filled,
+            version: result.version,
+        }
+    }
+}
+
 #[pymethods]
 impl AddColumnsResult {
    pub fn __repr__(&self) -> String {
@@ -1510,6 +1536,29 @@ impl Table {
        })
    }

+    pub fn add_computed_columns(
+        self_: PyRef<'_, Self>,
+        columns: Vec<(String, String)>,
+    ) -> PyResult<Bound<'_, PyAny>> {
+        let inner = self_.inner_ref()?.clone();
+        future_into_py(self_.py(), async move {
+            let mut builder = inner.add_columns();
+            for (name, expression) in columns {
+                builder = builder.computed(name, expression);
+            }
+            let result = builder.execute().await.infer_error()?;
+            Ok(AddColumnsResult::from(result))
+        })
+    }
+
+    pub fn refresh_column(self_: PyRef<'_, Self>, column: String) -> PyResult<Bound<'_, PyAny>> {
+        let inner = self_.inner_ref()?.clone();
+        future_into_py(self_.py(), async move {
+            let result = inner.refresh_column(column).await.infer_error()?;
+            Ok(RefreshColumnResult::from(result))
+        })
+    }
+
    pub fn add_columns_with_schema(
        self_: PyRef<'_, Self>,
        schema: PyArrowType<Schema>,
@@ -71,6 +71,14 @@ pub enum Error {
    IndexNotFound { name: String },
    #[snafu(display("Embedding function '{name}' was not found. : {reason}"))]
    EmbeddingFunctionNotFound { name: String, reason: String },
+    #[snafu(display("Column '{name}' was not found"))]
+    ColumnNotFound { name: String },
+    #[snafu(display("Column '{name}' already exists"))]
+    ColumnAlreadyExists { name: String },
+    #[snafu(display("Column '{name}' is not a computed column"))]
+    NotAComputedColumn { name: String },
+    #[snafu(display("Invalid expression for column '{column}': {message}"))]
+    InvalidExpression { column: String, message: String },

    #[snafu(display("Table '{name}' already exists"))]
    TableAlreadyExists { name: String },
@@ -2706,6 +2706,13 @@ impl<S: HttpSend> BaseTable for RemoteTable<S> {

                Ok(result)
            }
+            // A declaration reaches here as AllNulls, which the remote protocol
+            // has no representation for.
+            NewColumnTransform::AllNulls(_) => {
+                return Err(Error::NotSupported {
+                    message: "computed columns are supported only on local tables".into(),
+                });
+            }
            _ => {
                return Err(Error::NotSupported {
                    message: "Only SQL expressions are supported for adding columns".into(),
@@ -6455,6 +6462,37 @@ mod tests {
        assert_eq!(result.version, if old_server { 0 } else { 43 });
    }

+    /// Computed columns are local-only. Both halves say so here rather than
+    /// reaching the wire and failing somewhere less legible.
+    #[tokio::test]
+    async fn test_computed_columns_are_refused() {
+        let table = Table::new_with_handler("my_table", |request| -> http::Response<String> {
+            panic!("unexpected request: {}", request.url().path())
+        });
+
+        let declared = Arc::new(Schema::new(vec![Field::new(
+            "doubled",
+            DataType::Int32,
+            true,
+        )]));
+        let err = table
+            .add_columns()
+            .transform(NewColumnTransform::AllNulls(declared))
+            .execute()
+            .await
+            .unwrap_err();
+        assert!(
+            matches!(&err, Error::NotSupported { message } if message.contains("local tables")),
+            "{err:?}"
+        );
+
+        let err = table.refresh_column("doubled").await.unwrap_err();
+        assert!(
+            matches!(&err, Error::NotSupported { message } if message.contains("local tables")),
+            "{err:?}"
+        );
+    }
+
    #[tokio::test]
    async fn test_prewarm_index() {
        let table = Table::new_with_handler("my_table", |request| {
@@ -69,6 +69,7 @@ pub mod add_columns;
 mod add_data;
 pub mod branch_merge;
 pub mod checkpoint;
+pub mod computed_columns;
 mod create_index;
 pub mod datafusion;
 pub(crate) mod dataset;
@@ -78,6 +79,7 @@ pub mod merge;
 pub mod optimize;
 mod primary_key;
 pub mod query;
+pub mod refresh;
 pub mod schema_evolution;
 pub mod update;
 pub mod write_progress;
@@ -91,6 +93,9 @@ pub use branch_merge::{
    MergeBranchResult, MergeBranchStatus, MergePreview, RowCountSummary,
 };
 pub use chrono::Duration;
+pub use computed_columns::{
+    ComputedColumn, ComputedColumnKind, computed_column_from_field, computed_columns,
+};
 pub use delete::DeleteResult;
 use futures::future::join_all;
 pub use lance::dataset::refs::{BranchContents, Ref, TagContents, Tags as LanceTags};
@@ -98,6 +103,7 @@ pub use lance::dataset::scanner::DatasetRecordBatchStream;
 pub use lance_index::optimize::OptimizeOptions;
 pub use lsm_stats::{BucketStats, GenerationStats, LsmStats, MemtableStats};
 pub use optimize::{CompactionOptions, OptimizeAction, OptimizeStats};
+pub use refresh::RefreshColumnResult;
 pub use schema_evolution::{
    AddColumnsResult, AlterColumnsResult, DropColumnsResult, FieldMetadataUpdate,
    UpdateFieldMetadataResult,
@@ -782,6 +788,14 @@ pub trait BaseTable: std::fmt::Display + std::fmt::Debug + Send + Sync {
        transforms: NewColumnTransform,
        read_columns: Option<Vec<String>>,
    ) -> Result<AddColumnsResult>;
+    /// Fill a computed column's unfilled rows.
+    ///
+    /// The default returns `NotSupported`; Lance-backed tables override it.
+    async fn refresh_column(&self, _column: &str) -> Result<RefreshColumnResult> {
+        Err(Error::NotSupported {
+            message: "computed columns are supported only on local tables".into(),
+        })
+    }
    /// Alter columns in the table.
    async fn alter_columns(&self, alterations: &[ColumnAlteration]) -> Result<AlterColumnsResult>;
    /// Drop columns from the table.
@@ -1674,6 +1688,29 @@ impl Table {
        AddColumnsBuilder::new(self.inner.clone())
    }

+    /// Fill the fragments of a computed column that hold no values yet.
+    ///
+    /// Declared with
+    /// [`AddColumnsBuilder::computed`](add_columns::AddColumnsBuilder::computed),
+    /// a column starts empty and gets its values here. Fragments appended
+    /// since the last refresh are filled by the next one; fragments already
+    /// filled are left as they are, so the call is idempotent and does not
+    /// observe a mutated input.
+    ///
+    /// Local tables only.
+    ///
+    /// ```
+    /// # use lancedb::Table;
+    /// # async fn refresh(table: &Table) -> Result<(), Box<dyn std::error::Error>> {
+    /// let result = table.refresh_column("doubled").await?;
+    /// println!("filled {} rows at version {}", result.rows_filled, result.version);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn refresh_column(&self, column: impl AsRef<str>) -> Result<RefreshColumnResult> {
+        self.inner.refresh_column(column.as_ref()).await
+    }
+
    /// Change a column's name or nullability.
    pub async fn alter_columns(
        &self,
@@ -3341,6 +3378,12 @@ impl BaseTable for NativeTable {
        Ok(result)
    }

+    async fn refresh_column(&self, column: &str) -> Result<RefreshColumnResult> {
+        let result = refresh::execute_refresh_column(self, column).await?;
+        self.bump_freshness();
+        Ok(result)
+    }
+
    async fn alter_columns(&self, alterations: &[ColumnAlteration]) -> Result<AlterColumnsResult> {
        let result = schema_evolution::execute_alter_columns(self, alterations).await?;
        self.bump_freshness();
@@ -8,6 +8,7 @@ use std::sync::Arc;
 use lance::dataset::NewColumnTransform;

 use super::BaseTable;
+use super::computed_columns;
 use super::schema_evolution::AddColumnsResult;
 use crate::{Error, Result};

@@ -15,6 +16,7 @@ use crate::{Error, Result};
 pub struct AddColumnsBuilder {
    parent: Arc<dyn BaseTable>,
    transform: Option<NewColumnTransform>,
+    computed: Vec<(String, String)>,
    read_columns: Option<Vec<String>>,
 }

@@ -23,6 +25,7 @@ impl std::fmt::Debug for AddColumnsBuilder {
        f.debug_struct("AddColumnsBuilder")
            .field("parent", &self.parent)
            .field("has_transform", &self.transform.is_some())
+            .field("computed", &self.computed)
            .field("read_columns", &self.read_columns)
            .finish()
    }
@@ -33,19 +36,57 @@ impl AddColumnsBuilder {
        Self {
            parent,
            transform: None,
+            computed: Vec::new(),
            read_columns: None,
        }
    }

-    /// Set how the new columns' values are produced. Required.
+    /// Set how the new columns' values are produced.
    pub fn transform(mut self, transform: NewColumnTransform) -> Self {
        self.transform = Some(transform);
        self
    }

+    /// Add a column defined by `expression`, evaluated by a later refresh
+    /// rather than by this commit. Its type and inputs are derived from the
+    /// expression.
+    ///
+    /// The column is committed with no values, so declaring one costs the same
+    /// on an empty table as on a large one. Rows get values from
+    /// [`Table::refresh_column`](super::Table::refresh_column), which fills
+    /// every fragment that has none -- including fragments appended since the
+    /// last refresh.
+    ///
+    /// Refresh does not revisit a fragment it has filled, so mutating an input
+    /// leaves the value computed at fill time; recomputing means dropping the
+    /// column and declaring it again. An input cannot be renamed, retyped or
+    /// dropped while a declaration reads it, since the expression names it.
+    ///
+    /// Local tables only: LanceDB Cloud and Enterprise reject a declaration
+    /// with `NotSupported`.
+    ///
+    /// ```
+    /// # use lancedb::Table;
+    /// # async fn declare(table: &Table) -> Result<(), Box<dyn std::error::Error>> {
+    /// table
+    ///     .add_columns()
+    ///     .computed("doubled", "x * 2")
+    ///     .execute()
+    ///     .await?;
+    /// let filled = table.refresh_column("doubled").await?;
+    /// println!("filled {} rows", filled.rows_filled);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn computed(mut self, name: impl Into<String>, expression: impl Into<String>) -> Self {
+        self.computed.push((name.into(), expression.into()));
+        self
+    }
+
    /// Limit which existing columns a [`NewColumnTransform::BatchUDF`] mapper
-    /// receives. Every other transform determines what it reads, so setting
-    /// this alongside one is an error rather than a silent no-op.
+    /// receives. Every other transform, and a computed column, determines what
+    /// it reads, so setting this alongside one is an error rather than a silent
+    /// no-op.
    pub fn read_columns(mut self, columns: impl IntoIterator<Item = impl Into<String>>) -> Self {
        self.read_columns = Some(columns.into_iter().map(Into::into).collect());
        self
@@ -56,24 +97,43 @@ impl AddColumnsBuilder {
        let Self {
            parent,
            transform,
+            computed,
            read_columns,
        } = self;

-        let Some(transform) = transform else {
-            return Err(Error::InvalidInput {
-                message: "add_columns requires a transform".into(),
-            });
-        };
-
-        if read_columns.is_some() && !matches!(transform, NewColumnTransform::BatchUDF(_)) {
-            return Err(Error::InvalidInput {
-                message: "read_columns applies only to a BatchUDF transform; \
-                          every other transform determines what it reads"
+        match (transform, computed.is_empty()) {
+            (None, true) => Err(Error::InvalidInput {
+                message: "add_columns requires a transform or a computed column".into(),
+            }),
+            // The two commit through different transforms, so one call covering
+            // both would be two commits and could half-apply.
+            (Some(_), false) => Err(Error::InvalidInput {
+                message: "add_columns cannot mix a transform with computed columns; \
+                          they cannot be added atomically in one call"
                    .into(),
-            });
+            }),
+            (Some(transform), true) => {
+                if read_columns.is_some() && !matches!(transform, NewColumnTransform::BatchUDF(_)) {
+                    return Err(Error::InvalidInput {
+                        message: "read_columns applies only to a BatchUDF transform; \
+                                  every other transform determines what it reads"
+                            .into(),
+                    });
+                }
+                parent.add_columns(transform, read_columns).await
+            }
+            (None, false) => {
+                if read_columns.is_some() {
+                    return Err(Error::InvalidInput {
+                        message: "read_columns applies only to a BatchUDF transform; \
+                                  a computed column's inputs come from its expression"
+                            .into(),
+                    });
+                }
+                let transform = computed_columns::declare(parent.schema().await?, &computed)?;
+                parent.add_columns(transform, None).await
+            }
        }
-
-        parent.add_columns(transform, read_columns).await
    }
 }

@@ -85,8 +145,8 @@ mod tests {
    use arrow_schema::{DataType, Field, Schema};
    use lance::dataset::{BatchUDF, NewColumnTransform};

-    use crate::Table;
    use crate::connect;
+    use crate::{Error, Table};

    async fn table_with_two_columns(name: &str) -> Table {
        let conn = connect("memory://").execute().await.unwrap();
@@ -98,10 +158,7 @@ mod tests {
    async fn test_requires_a_transform() {
        let table = table_with_two_columns("no_transform").await;
        let err = table.add_columns().execute().await.unwrap_err();
-        assert!(
-            err.to_string().contains("requires a transform"),
-            "got: {err}"
-        );
+        assert!(matches!(err, Error::InvalidInput { .. }));
    }

    #[tokio::test]
@@ -117,7 +174,7 @@ mod tests {
            .execute()
            .await
            .unwrap_err();
-        assert!(err.to_string().contains("BatchUDF"), "got: {err}");
+        assert!(matches!(err, Error::InvalidInput { .. }));

        let schema = table.schema().await.unwrap();
        assert!(
@@ -126,6 +183,47 @@ mod tests {
        );
    }

+    #[tokio::test]
+    async fn test_mixing_transform_and_computed_is_rejected() {
+        let table = table_with_two_columns("mixed_add").await;
+        let err = table
+            .add_columns()
+            .transform(NewColumnTransform::SqlExpressions(vec![(
+                "eager".into(),
+                "x * 2".into(),
+            )]))
+            .computed("lazy", "x * 3")
+            .execute()
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::InvalidInput { .. }));
+
+        let schema = table.schema().await.unwrap();
+        assert!(schema.field_with_name("eager").is_err());
+        assert!(schema.field_with_name("lazy").is_err());
+    }
+
+    #[tokio::test]
+    async fn test_read_columns_with_computed_is_rejected() {
+        let table = table_with_two_columns("read_cols_computed").await;
+        let err = table
+            .add_columns()
+            .computed("doubled", "x * 2")
+            .read_columns(["x"])
+            .execute()
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::InvalidInput { .. }));
+        assert!(
+            table
+                .schema()
+                .await
+                .unwrap()
+                .field_with_name("doubled")
+                .is_err()
+        );
+    }
+
    #[tokio::test]
    async fn test_read_columns_limits_what_a_batch_udf_sees() {
        let table = table_with_two_columns("read_cols_udf").await;
@@ -0,0 +1,705 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+//! Computed columns.
+//!
+//! A computed column is defined by a rule rather than by values supplied at
+//! write time. Declaring one commits the column carrying that rule in field
+//! metadata but no data, so the cost does not scale with the table; a later
+//! refresh fills the rows.
+//!
+//! The rule is tagged by kind ([`ComputedColumnKind`]) because kinds differ in
+//! where the column's type and inputs come from. A SQL expression is
+//! self-describing -- both are derived from the expression, so a caller writes
+//! neither -- while a kind resolved through a registry cannot be typed without
+//! consulting it. Only SQL exists today; the tag is what lets another kind be
+//! added without a second reading of the same key.
+//!
+//! [`computed_columns`] and [`computed_column_from_field`] read declarations
+//! back off a schema.
+
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use arrow_schema::{Field as ArrowField, Schema as ArrowSchema, SchemaRef};
+use lance::dataset::NewColumnTransform;
+use lance_datafusion::planner::Planner;
+
+use crate::{Error, Result};
+
+/// Field metadata key marking a column as computed. The value is `"true"`.
+pub const COMPUTED_COLUMN_META_KEY: &str = "computed_column";
+
+/// Field metadata key naming the kind of rule that defines the column.
+pub const KIND_META_KEY: &str = "computed_column.kind";
+
+/// Field metadata key holding the SQL expression that defines the column.
+pub const EXPRESSION_META_KEY: &str = "computed_column.expression";
+
+/// Field metadata key holding the column's inputs, as a JSON array of names.
+pub const INPUTS_META_KEY: &str = "computed_column.inputs";
+
+/// Value of [`KIND_META_KEY`] for a column defined by a SQL expression.
+pub const SQL_KIND: &str = "sql";
+
+/// The rule that defines a computed column's values.
+///
+/// Non-exhaustive: a kind added later is an additive change, and a caller that
+/// only handles the kinds it knows keeps compiling.
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum ComputedColumnKind {
+    /// A SQL expression evaluated by DataFusion. It is the whole definition:
+    /// the column's type and its inputs are both derived from it.
+    Sql {
+        /// The expression.
+        expression: String,
+    },
+    /// A kind this version does not understand, written by a newer one.
+    ///
+    /// Reported rather than hidden so a caller can tell a column it cannot
+    /// refresh apart from one that was never computed. Nothing produces this.
+    Unrecognized {
+        /// The kind as it was found in the metadata.
+        kind: String,
+    },
+}
+
+/// A computed column's declaration, as read back from field metadata.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ComputedColumn {
+    /// Name of the computed column.
+    pub name: String,
+    /// The rule that defines it.
+    pub kind: ComputedColumnKind,
+    /// Columns the rule reads, recorded at declaration time.
+    ///
+    /// Outside the kind because every kind has inputs and the consumers that
+    /// use them -- refresh planning, dependency ordering -- do not care which
+    /// kind produced them. Where they come from does differ, and that is
+    /// settled at declaration: derived from a SQL expression, supplied by the
+    /// caller for a kind that cannot be parsed.
+    pub inputs: Vec<String>,
+}
+
+/// Build the field metadata recording a SQL binding.
+fn computed_column_metadata(expression: &str, inputs: &[String]) -> HashMap<String, String> {
+    HashMap::from([
+        (COMPUTED_COLUMN_META_KEY.to_string(), "true".to_string()),
+        (KIND_META_KEY.to_string(), SQL_KIND.to_string()),
+        (EXPRESSION_META_KEY.to_string(), expression.to_string()),
+        (
+            INPUTS_META_KEY.to_string(),
+            serde_json::to_string(inputs).unwrap_or_else(|_| "[]".to_string()),
+        ),
+    ])
+}
+
+/// Read a field's computed-column declaration, if it carries one.
+///
+/// A field flagged computed but carrying no kind, or a SQL one missing its
+/// expression, is not a computed column here: without the rule there is
+/// nothing to refresh from, so it is reported as absent rather than as a
+/// half-formed declaration. An unrecognized kind is different -- the rule is
+/// there and intact, this version just cannot act on it -- and comes back as
+/// [`ComputedColumnKind::Unrecognized`].
+pub fn computed_column_from_field(field: &ArrowField) -> Option<ComputedColumn> {
+    let metadata = field.metadata();
+    if metadata.get(COMPUTED_COLUMN_META_KEY).map(String::as_str) != Some("true") {
+        return None;
+    }
+    let kind = match metadata.get(KIND_META_KEY)?.as_str() {
+        SQL_KIND => ComputedColumnKind::Sql {
+            expression: metadata.get(EXPRESSION_META_KEY)?.clone(),
+        },
+        other => ComputedColumnKind::Unrecognized {
+            kind: other.to_string(),
+        },
+    };
+    let inputs = metadata
+        .get(INPUTS_META_KEY)
+        .and_then(|raw| serde_json::from_str::<Vec<String>>(raw).ok())
+        .unwrap_or_default();
+    Some(ComputedColumn {
+        name: field.name().clone(),
+        kind,
+        inputs,
+    })
+}
+
+/// Read every computed-column declaration carried by `schema`, in field order.
+///
+/// Introspection is a pure read of the schema the caller already holds, the
+/// way a SQL catalog reports a generation expression as another column of
+/// `information_schema.columns`.
+pub fn computed_columns(schema: &ArrowSchema) -> Vec<ComputedColumn> {
+    schema
+        .fields()
+        .iter()
+        .filter_map(|field| computed_column_from_field(field))
+        .collect()
+}
+
+/// Reject a schema change to a column some declaration reads.
+///
+/// A binding is SQL text naming its inputs, so renaming, retyping or dropping
+/// one leaves an expression that no longer resolves. Refusing the change keeps
+/// a declaration that survived [`plan`] evaluable for as long as it exists.
+///
+/// Paths are compared at their root: a declaration reading `metadata` is
+/// invalidated by a change to `metadata.age` just as surely.
+pub(crate) fn ensure_not_an_input(schema: &ArrowSchema, paths: &[&str]) -> Result<()> {
+    let root = |path: &str| path.split('.').next().unwrap_or(path).to_string();
+    for declaration in computed_columns(schema) {
+        for path in paths {
+            // A declaration does not read itself, so it is free to be dropped
+            // or renamed along with its binding.
+            if declaration.name == root(path) {
+                continue;
+            }
+            if declaration
+                .inputs
+                .iter()
+                .any(|input| root(input) == root(path))
+            {
+                return Err(Error::InvalidInput {
+                    message: format!(
+                        "column '{}' is read by computed column '{}'; drop that column first",
+                        path, declaration.name
+                    ),
+                });
+            }
+        }
+    }
+    Ok(())
+}
+
+/// Resolve `(name, expression)` pairs against `schema` into fields carrying
+/// their bindings.
+///
+/// Everything that can be known statically is checked here rather than at
+/// refresh time: that the expression parses, that every column it reads
+/// exists, and that the target name is free. A declaration that survives this
+/// is one a refresh can always act on.
+pub(crate) fn plan(schema: SchemaRef, columns: &[(String, String)]) -> Result<Vec<ArrowField>> {
+    if columns.is_empty() {
+        return Err(Error::InvalidInput {
+            message: "at least one computed column is required".into(),
+        });
+    }
+
+    let planner = Planner::new(schema.clone());
+    let mut fields = Vec::with_capacity(columns.len());
+    let mut declared: Vec<&str> = Vec::with_capacity(columns.len());
+
+    for (name, expression) in columns {
+        if schema.field_with_name(name).is_ok() || declared.contains(&name.as_str()) {
+            return Err(Error::ColumnAlreadyExists { name: name.clone() });
+        }
+
+        let expr = planner
+            .parse_expr(expression)
+            .and_then(|expr| planner.optimize_expr(expr))
+            .map_err(|e| Error::InvalidExpression {
+                column: name.clone(),
+                message: e.to_string(),
+            })?;
+
+        let mut inputs = Planner::column_names_in_expr(&expr);
+        inputs.sort();
+        inputs.dedup();
+
+        // Resolved here rather than left to the planner so an unknown column
+        // names itself in the error instead of surfacing as a plan failure.
+        let mut indices = Vec::with_capacity(inputs.len());
+        for input in &inputs {
+            let index = schema
+                .index_of(input)
+                .map_err(|_| Error::InvalidExpression {
+                    column: name.clone(),
+                    message: format!("unknown column '{input}'"),
+                })?;
+            indices.push(index);
+        }
+
+        // Physical expressions address columns by position, so the planner
+        // that types the expression has to be built on the projected schema
+        // the refresh will actually read.
+        let read_schema =
+            Arc::new(
+                schema
+                    .project(&indices)
+                    .map_err(|e| Error::InvalidExpression {
+                        column: name.clone(),
+                        message: e.to_string(),
+                    })?,
+            );
+        let physical = Planner::new(read_schema.clone())
+            .create_physical_expr(&expr)
+            .map_err(|e| Error::InvalidExpression {
+                column: name.clone(),
+                message: e.to_string(),
+            })?;
+        let data_type =
+            physical
+                .data_type(read_schema.as_ref())
+                .map_err(|e| Error::InvalidExpression {
+                    column: name.clone(),
+                    message: e.to_string(),
+                })?;
+
+        // Declared columns start entirely null, so nullability is a property
+        // of the declaration rather than of what the expression yields.
+        fields.push(
+            ArrowField::new(name, data_type, true)
+                .with_metadata(computed_column_metadata(expression, &inputs)),
+        );
+        declared.push(name);
+    }
+
+    Ok(fields)
+}
+
+/// Build the transform that declares `columns` against `schema`.
+///
+/// An all-null column is how a binding with no values yet is carried into a
+/// commit; that it is spelled `AllNulls` is a detail of the commit, not of the
+/// column, which is why this is internal and
+/// [`AddColumnsBuilder::computed`](super::AddColumnsBuilder::computed) is the
+/// public way in.
+pub(crate) fn declare(
+    schema: SchemaRef,
+    columns: &[(String, String)],
+) -> Result<NewColumnTransform> {
+    let fields = plan(schema, columns)?;
+    Ok(NewColumnTransform::AllNulls(Arc::new(ArrowSchema::new(
+        fields,
+    ))))
+}
+
+/// Commit a declaration of a kind this version does not produce, the way a
+/// newer lancedb would leave one behind. Shared with the refresh tests, which
+/// need the same column to check that refresh refuses it.
+#[cfg(test)]
+pub(super) async fn add_foreign_kind(table: &crate::Table, name: &str, kind: &str) {
+    use arrow_schema::DataType;
+
+    let field = ArrowField::new(name, DataType::Int32, true).with_metadata(HashMap::from([
+        (COMPUTED_COLUMN_META_KEY.to_string(), "true".to_string()),
+        (KIND_META_KEY.to_string(), kind.to_string()),
+        (INPUTS_META_KEY.to_string(), r#"["x"]"#.to_string()),
+    ]));
+    table
+        .add_columns()
+        .transform(NewColumnTransform::AllNulls(Arc::new(ArrowSchema::new(
+            vec![field],
+        ))))
+        .execute()
+        .await
+        .unwrap();
+}
+
+#[cfg(test)]
+mod tests {
+    use arrow_array::record_batch;
+    use arrow_schema::DataType;
+    use futures::TryStreamExt;
+    use lance::dataset::ColumnAlteration;
+
+    use super::*;
+    use crate::connect;
+    use crate::query::{ExecutableQuery, QueryBase, Select};
+    use crate::{Error, Table};
+
+    async fn table_with_ints(name: &str) -> Table {
+        let conn = connect("memory://").execute().await.unwrap();
+        let batch = record_batch!(("x", Int32, [1, 2, 3])).unwrap();
+        conn.create_table(name, batch).execute().await.unwrap()
+    }
+
+    /// Declare `columns` the way a caller would: plan the expressions, then
+    /// add them through the ordinary column API.
+    async fn add_computed(table: &Table, columns: &[(String, String)]) -> Result<u64> {
+        let mut builder = table.add_columns();
+        for (name, expression) in columns {
+            builder = builder.computed(name, expression);
+        }
+        Ok(builder.execute().await?.version)
+    }
+
+    async fn declared(table: &Table) -> Vec<ComputedColumn> {
+        computed_columns(table.schema().await.unwrap().as_ref())
+    }
+
+    #[tokio::test]
+    async fn test_declare_infers_type_and_inputs() {
+        let table = table_with_ints("declare_infers").await;
+        let initial = table.version().await.unwrap();
+
+        let version = add_computed(&table, &[("doubled".into(), "x * 2".into())])
+            .await
+            .unwrap();
+        assert!(version > initial);
+
+        let schema = table.schema().await.unwrap();
+        let field = schema.field_with_name("doubled").unwrap();
+        assert_eq!(field.data_type(), &DataType::Int32);
+        assert!(field.is_nullable());
+
+        assert_eq!(
+            declared(&table).await,
+            vec![ComputedColumn {
+                name: "doubled".into(),
+                kind: ComputedColumnKind::Sql {
+                    expression: "x * 2".into()
+                },
+                inputs: vec!["x".into()],
+            }]
+        );
+    }
+
+    /// The binding reaches the schema only if `AllNulls` carries per-field
+    /// metadata through the commit. The whole representation rests on it.
+    #[tokio::test]
+    async fn test_all_nulls_preserves_field_metadata() {
+        let table = table_with_ints("metadata_survives").await;
+        add_computed(&table, &[("doubled".into(), "x * 2".into())])
+            .await
+            .unwrap();
+
+        let schema = table.schema().await.unwrap();
+        let metadata = schema.field_with_name("doubled").unwrap().metadata();
+        assert_eq!(
+            metadata.get(COMPUTED_COLUMN_META_KEY).map(String::as_str),
+            Some("true")
+        );
+        assert_eq!(metadata.get(KIND_META_KEY).map(String::as_str), Some("sql"));
+        assert_eq!(
+            metadata.get(EXPRESSION_META_KEY).map(String::as_str),
+            Some("x * 2")
+        );
+        assert_eq!(
+            metadata.get(INPUTS_META_KEY).map(String::as_str),
+            Some(r#"["x"]"#)
+        );
+    }
+
+    #[tokio::test]
+    async fn test_declared_column_is_all_null() {
+        let table = table_with_ints("declare_is_null").await;
+        add_computed(&table, &[("doubled".into(), "x * 2".into())])
+            .await
+            .unwrap();
+
+        let batches = table
+            .query()
+            .select(Select::columns(&["doubled"]))
+            .execute()
+            .await
+            .unwrap()
+            .try_collect::<Vec<_>>()
+            .await
+            .unwrap();
+
+        let total: usize = batches.iter().map(|b| b.num_rows()).sum();
+        assert_eq!(total, 3);
+        for batch in &batches {
+            assert_eq!(batch["doubled"].null_count(), batch.num_rows());
+        }
+    }
+
+    #[tokio::test]
+    async fn test_unknown_column_fails_at_declare_time() {
+        let table = table_with_ints("unknown_input").await;
+        let err = add_computed(&table, &[("bad".into(), "missing + 1".into())])
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::InvalidExpression { column, .. } if column == "bad"));
+
+        let schema = table.schema().await.unwrap();
+        assert!(schema.field_with_name("bad").is_err());
+    }
+
+    #[tokio::test]
+    async fn test_unparsable_expression_fails_at_declare_time() {
+        let table = table_with_ints("bad_syntax").await;
+        let err = add_computed(&table, &[("bad".into(), "x *".into())])
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::InvalidExpression { column, .. } if column == "bad"));
+        assert!(
+            table
+                .schema()
+                .await
+                .unwrap()
+                .field_with_name("bad")
+                .is_err()
+        );
+    }
+
+    /// A user-defined function is an expression like any other; only its
+    /// resolution is missing. When a registry-aware planner exists this
+    /// becomes a supported declaration rather than a new API.
+    #[tokio::test]
+    async fn test_unregistered_function_is_rejected_for_now() {
+        let table = table_with_ints("udf_not_yet").await;
+        let err = add_computed(&table, &[("vec".into(), "embed(x)".into())])
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::InvalidExpression { column, .. } if column == "vec"));
+        assert!(
+            table
+                .schema()
+                .await
+                .unwrap()
+                .field_with_name("vec")
+                .is_err()
+        );
+    }
+
+    #[tokio::test]
+    async fn test_existing_column_name_is_rejected() {
+        let table = table_with_ints("name_taken").await;
+        let err = add_computed(&table, &[("x".into(), "x * 2".into())])
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::ColumnAlreadyExists { name } if name == "x"));
+        assert!(declared(&table).await.is_empty());
+    }
+
+    #[tokio::test]
+    async fn test_constant_expression_needs_no_inputs() {
+        let table = table_with_ints("constant").await;
+        add_computed(&table, &[("answer".into(), "42".into())])
+            .await
+            .unwrap();
+
+        let declared = declared(&table).await;
+        assert_eq!(declared.len(), 1);
+        assert!(declared[0].inputs.is_empty());
+    }
+
+    #[tokio::test]
+    async fn test_multiple_columns_in_one_commit() {
+        let table = table_with_ints("multi").await;
+        let initial = table.version().await.unwrap();
+
+        add_computed(
+            &table,
+            &[
+                ("plus".into(), "x + 1".into()),
+                ("squared".into(), "x * x".into()),
+            ],
+        )
+        .await
+        .unwrap();
+
+        assert_eq!(table.version().await.unwrap(), initial + 1);
+        let declared = declared(&table).await;
+        assert_eq!(declared.len(), 2);
+        assert_eq!(declared[0].name, "plus");
+        assert_eq!(declared[1].name, "squared");
+    }
+
+    #[tokio::test]
+    async fn test_duplicate_declaration_in_one_call_is_rejected() {
+        let table = table_with_ints("dupe").await;
+        let err = add_computed(
+            &table,
+            &[
+                ("dup".into(), "x + 1".into()),
+                ("dup".into(), "x + 2".into()),
+            ],
+        )
+        .await
+        .unwrap_err();
+        assert!(matches!(err, Error::ColumnAlreadyExists { name } if name == "dup"));
+        assert!(declared(&table).await.is_empty());
+    }
+
+    /// A column added by an ordinary transform is materialized, not bound, so
+    /// it carries no declaration to report.
+    #[tokio::test]
+    async fn test_ordinary_columns_are_not_reported_as_computed() {
+        let table = table_with_ints("plain").await;
+        assert!(declared(&table).await.is_empty());
+
+        table
+            .add_columns()
+            .transform(NewColumnTransform::SqlExpressions(vec![(
+                "eager".into(),
+                "x * 2".into(),
+            )]))
+            .execute()
+            .await
+            .unwrap();
+        assert!(declared(&table).await.is_empty());
+    }
+
+    /// Built-in functions type the column the same way an operator does.
+    #[tokio::test]
+    async fn test_builtin_function_inference() {
+        let conn = connect("memory://").execute().await.unwrap();
+        let batch = record_batch!(("name", Utf8, ["ada", "grace"]), ("n", Int32, [-1, 2])).unwrap();
+        let table = conn
+            .create_table("builtins", batch)
+            .execute()
+            .await
+            .unwrap();
+
+        add_computed(
+            &table,
+            &[
+                ("shout".into(), "upper(name)".into()),
+                ("width".into(), "length(name)".into()),
+                ("magnitude".into(), "abs(n)".into()),
+            ],
+        )
+        .await
+        .unwrap();
+
+        let schema = table.schema().await.unwrap();
+        assert_eq!(
+            schema.field_with_name("shout").unwrap().data_type(),
+            &DataType::Utf8
+        );
+        assert_eq!(
+            schema.field_with_name("magnitude").unwrap().data_type(),
+            &DataType::Int32
+        );
+        // length() returns a width-dependent integer type; assert it is one
+        // rather than pinning which.
+        assert!(
+            schema
+                .field_with_name("width")
+                .unwrap()
+                .data_type()
+                .is_integer()
+        );
+
+        let declared = declared(&table).await;
+        assert_eq!(declared.len(), 3);
+        assert_eq!(declared[0].inputs, vec!["name".to_string()]);
+        assert_eq!(declared[2].inputs, vec!["n".to_string()]);
+    }
+
+    /// The reason the kind is tagged: a declaration written by a newer version
+    /// has to read back as a computed column this one cannot evaluate, not as
+    /// an ordinary column. Reported as absent it would be refreshable by
+    /// nothing and redeclarable over, silently.
+    #[tokio::test]
+    async fn test_unrecognized_kind_is_reported_rather_than_hidden() {
+        let table = table_with_ints("foreign_kind").await;
+        super::add_foreign_kind(&table, "embedding", "udf").await;
+
+        assert_eq!(
+            declared(&table).await,
+            vec![ComputedColumn {
+                name: "embedding".into(),
+                kind: ComputedColumnKind::Unrecognized { kind: "udf".into() },
+                inputs: vec!["x".into()],
+            }]
+        );
+
+        let err = add_computed(&table, &[("embedding".into(), "x * 2".into())])
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::ColumnAlreadyExists { name } if name == "embedding"));
+    }
+
+    /// A kind is what makes a declaration readable at all, so the flag alone
+    /// is half-formed in the same way a missing expression is.
+    #[test]
+    fn test_flag_without_a_kind_is_not_a_declaration() {
+        let field =
+            ArrowField::new("half", DataType::Int32, true).with_metadata(HashMap::from([(
+                COMPUTED_COLUMN_META_KEY.to_string(),
+                "true".to_string(),
+            )]));
+        assert_eq!(computed_column_from_field(&field), None);
+    }
+
+    /// A SQL declaration is its expression; without one there is nothing to
+    /// refresh from.
+    #[test]
+    fn test_sql_kind_without_an_expression_is_not_a_declaration() {
+        let field = ArrowField::new("half", DataType::Int32, true).with_metadata(HashMap::from([
+            (COMPUTED_COLUMN_META_KEY.to_string(), "true".to_string()),
+            (KIND_META_KEY.to_string(), SQL_KIND.to_string()),
+        ]));
+        assert_eq!(computed_column_from_field(&field), None);
+    }
+
+    #[tokio::test]
+    async fn test_inputs_are_deduplicated_and_sorted() {
+        let conn = connect("memory://").execute().await.unwrap();
+        let batch = record_batch!(("b", Int32, [1, 2]), ("a", Int32, [3, 4])).unwrap();
+        let table = conn.create_table("dedupe", batch).execute().await.unwrap();
+
+        add_computed(&table, &[("total".into(), "b + a + b".into())])
+            .await
+            .unwrap();
+
+        assert_eq!(
+            declared(&table).await[0].inputs,
+            vec!["a".to_string(), "b".to_string()]
+        );
+    }
+
+    #[tokio::test]
+    async fn test_dropping_an_input_is_refused() {
+        let table = table_with_ints("drop_input").await;
+        add_computed(&table, &[("doubled".into(), "x * 2".into())])
+            .await
+            .unwrap();
+
+        let err = table.drop_columns(&["x"]).await.unwrap_err();
+        assert!(
+            matches!(&err, Error::InvalidInput { message } if message.contains("doubled")),
+            "{err:?}"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_renaming_an_input_is_refused() {
+        let table = table_with_ints("rename_input").await;
+        add_computed(&table, &[("doubled".into(), "x * 2".into())])
+            .await
+            .unwrap();
+
+        let err = table
+            .alter_columns(&[ColumnAlteration::new("x".into()).rename("y".into())])
+            .await
+            .unwrap_err();
+        assert!(
+            matches!(&err, Error::InvalidInput { message } if message.contains("doubled")),
+            "{err:?}"
+        );
+    }
+
+    /// Nothing resolves against nullability, so it is not a rebinding.
+    #[tokio::test]
+    async fn test_altering_an_input_nullability_is_allowed() {
+        let table = table_with_ints("nullable_input").await;
+        add_computed(&table, &[("doubled".into(), "x * 2".into())])
+            .await
+            .unwrap();
+
+        table
+            .alter_columns(&[ColumnAlteration::new("x".into()).set_nullable(true)])
+            .await
+            .unwrap();
+    }
+
+    /// A declaration does not read itself, so it travels with its binding.
+    #[tokio::test]
+    async fn test_dropping_the_computed_column_is_allowed() {
+        let table = table_with_ints("drop_computed").await;
+        add_computed(&table, &[("doubled".into(), "x * 2".into())])
+            .await
+            .unwrap();
+
+        table.drop_columns(&["doubled"]).await.unwrap();
+        assert!(declared(&table).await.is_empty());
+    }
+}
@@ -0,0 +1,523 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+//! Filling computed columns.
+//!
+//! A row without a value gets one; a row that has one keeps it. Refresh is
+//! therefore idempotent and does not observe input mutation -- once a row is
+//! filled, changing what the expression reads leaves the stored result alone.
+//!
+//! Convergence comes from staging nothing when nothing would change, so an
+//! expression yielding null settles after one pass rather than re-selecting
+//! the same rows forever. Fragments that already cover the column and hold no
+//! nulls are skipped without evaluating it at all.
+
+use std::sync::Arc;
+
+use arrow_array::RecordBatch;
+use arrow_schema::Schema as ArrowSchema;
+use futures::{TryStreamExt, stream};
+use lance::Dataset;
+use lance::dataset::WriteDestination;
+use lance::dataset::fragment::FileFragment;
+use lance::dataset::transaction::Operation;
+use lance_core::ROW_ID;
+use lance_core::datatypes::Schema as LanceSchema;
+use serde::{Deserialize, Serialize};
+
+use super::NativeTable;
+use super::computed_columns::{ComputedColumnKind, computed_column_from_field};
+use crate::{Error, Result};
+
+/// Alias the expression is projected under, so its result and the column's
+/// current values can be read side by side.
+const COMPUTED_ALIAS: &str = "__lancedb_computed";
+
+/// The result of refreshing a computed column.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, Default)]
+pub struct RefreshColumnResult {
+    /// Rows that had a value computed.
+    #[serde(default)]
+    pub rows_filled: u64,
+    /// The commit version associated with the operation.
+    #[serde(default)]
+    pub version: u64,
+}
+
+/// Internal implementation of the refresh logic.
+pub(crate) async fn execute_refresh_column(
+    table: &NativeTable,
+    column: &str,
+) -> Result<RefreshColumnResult> {
+    table.dataset.ensure_mutable()?;
+    let dataset = table.dataset.get().await?;
+
+    let expression = declared_expression(&dataset, column)?;
+    let field = dataset
+        .schema()
+        .field(column)
+        .ok_or_else(|| Error::ColumnNotFound {
+            name: column.to_string(),
+        })?;
+    // The dataset's own field, so the identity write_column checks against the
+    // manifest holds by construction.
+    let column_schema = LanceSchema {
+        fields: vec![field.clone()],
+        metadata: Default::default(),
+    };
+
+    let mut rows_filled = 0u64;
+    let mut replacements = Vec::new();
+    for fragment in fragments_to_consider(&dataset, column, field.id).await? {
+        let Some((filled, values)) =
+            fill_fragment(&dataset, &fragment, column, &expression).await?
+        else {
+            continue;
+        };
+        rows_filled += filled;
+        replacements.push(
+            fragment
+                .write_column(stream::iter(values.into_iter().map(Ok)), &column_schema)
+                .await?,
+        );
+    }
+
+    if replacements.is_empty() {
+        return Ok(RefreshColumnResult {
+            rows_filled: 0,
+            version: dataset.version().version,
+        });
+    }
+
+    let read_version = dataset.version().version;
+    let new_dataset = Dataset::commit(
+        WriteDestination::Dataset(dataset.clone()),
+        Operation::DataReplacement { replacements },
+        Some(read_version),
+        None,
+        None,
+        Arc::new(Default::default()),
+        false,
+    )
+    .await?;
+
+    let version = new_dataset.version().version;
+    table.dataset.update(new_dataset);
+    Ok(RefreshColumnResult {
+        rows_filled,
+        version,
+    })
+}
+
+/// The SQL expression `column` is declared with.
+fn declared_expression(dataset: &Dataset, column: &str) -> Result<String> {
+    let schema = ArrowSchema::from(dataset.schema());
+    let field = schema
+        .field_with_name(column)
+        .map_err(|_| Error::ColumnNotFound {
+            name: column.to_string(),
+        })?;
+    let declaration =
+        computed_column_from_field(field).ok_or_else(|| Error::NotAComputedColumn {
+            name: column.to_string(),
+        })?;
+    match declaration.kind {
+        ComputedColumnKind::Sql { expression } => Ok(expression),
+        ComputedColumnKind::Unrecognized { kind } => Err(Error::NotSupported {
+            message: format!(
+                "computed column '{column}' is defined by '{kind}', which this version of \
+                 lancedb cannot evaluate"
+            ),
+        }),
+    }
+}
+
+/// Quote `name` as a lance SQL identifier.
+///
+/// Lance's dialect delimits with backticks, so a double-quoted name would
+/// parse as a string literal rather than a column.
+fn quote_identifier(name: &str) -> String {
+    format!("`{}`", name.replace('`', "``"))
+}
+
+/// Fragments that could hold a row needing a value.
+///
+/// A fragment whose data files do not carry the field cannot hold one that
+/// does. One that carries it is asked, since a row rewrite -- an update, or a
+/// compaction folding an unfilled fragment into a filled one -- can leave
+/// nulls behind a covering file.
+async fn fragments_to_consider(
+    dataset: &Dataset,
+    column: &str,
+    field_id: i32,
+) -> Result<Vec<FileFragment>> {
+    let unfilled = format!("{} IS NULL", quote_identifier(column));
+    let mut considered = Vec::new();
+    for fragment in dataset.get_fragments() {
+        let covered = fragment
+            .metadata()
+            .files
+            .iter()
+            .any(|file| file.fields.contains(&field_id));
+        if !covered || fragment.count_rows(Some(unfilled.clone())).await? > 0 {
+            considered.push(fragment);
+        }
+    }
+    Ok(considered)
+}
+
+/// Compute one fragment's column, keeping every value it already holds.
+///
+/// `Ok(None)` when no live row gained a value, which is what keeps a refresh
+/// from restaging a fragment whose expression yields null. Deleted rows are
+/// carried through so the values line up positionally with the fragment's data
+/// files; they are never read back, but the column file has to cover them.
+async fn fill_fragment(
+    dataset: &Dataset,
+    fragment: &FileFragment,
+    column: &str,
+    expression: &str,
+) -> Result<Option<(u64, Vec<RecordBatch>)>> {
+    let mut scanner = dataset.scan();
+    scanner
+        .with_fragments(vec![fragment.metadata().clone()])
+        .with_row_id()
+        .include_deleted_rows()
+        .project_with_transform(&[
+            (column, quote_identifier(column).as_str()),
+            (COMPUTED_ALIAS, expression),
+        ])?;
+
+    let projected = Arc::new(ArrowSchema::new(vec![
+        ArrowSchema::from(dataset.schema())
+            .field_with_name(column)
+            .map_err(|_| Error::ColumnNotFound {
+                name: column.to_string(),
+            })?
+            .clone(),
+    ]));
+
+    let missing = |name: &str| Error::Runtime {
+        message: format!("refreshing {column} produced no {name} column"),
+    };
+
+    let mut filled = 0u64;
+    let mut values = Vec::new();
+    let mut batches = scanner.try_into_stream().await?;
+    while let Some(batch) = batches.try_next().await? {
+        let existing = batch
+            .column_by_name(column)
+            .ok_or_else(|| missing(column))?;
+        let computed = batch
+            .column_by_name(COMPUTED_ALIAS)
+            .ok_or_else(|| missing("expression"))?;
+        let row_ids = batch
+            .column_by_name(ROW_ID)
+            .ok_or_else(|| missing(ROW_ID))?;
+
+        // A row is filled only if it gains a value: an expression yielding null
+        // leaves it as unfilled as it was, which is what lets a refresh settle.
+        // A deleted row has a null row id; its value is written but not counted.
+        let unfilled = arrow::compute::is_null(existing.as_ref())?;
+        filled += (0..unfilled.len())
+            .filter(|i| unfilled.value(*i) && row_ids.is_valid(*i) && computed.is_valid(*i))
+            .count() as u64;
+
+        let merged = arrow_select::zip::zip(&unfilled, computed, existing)?;
+        values.push(RecordBatch::try_new(projected.clone(), vec![merged])?);
+    }
+
+    Ok((filled > 0).then_some((filled, values)))
+}
+
+#[cfg(test)]
+mod tests {
+    use arrow_array::{Int32Array, record_batch};
+    use futures::TryStreamExt;
+
+    use crate::connect;
+    use crate::query::{ExecutableQuery, QueryBase, Select};
+    use crate::{Error, Result, Table};
+
+    async fn table_with(name: &str, values: Vec<i32>) -> Table {
+        let conn = connect("memory://").execute().await.unwrap();
+        let batch = record_batch!(("x", Int32, values)).unwrap();
+        conn.create_table(name, batch).execute().await.unwrap()
+    }
+
+    async fn declare_doubled(table: &Table) -> Result<u64> {
+        Ok(table
+            .add_columns()
+            .computed("doubled", "x * 2")
+            .execute()
+            .await?
+            .version)
+    }
+
+    async fn read(table: &Table, column: &str) -> Vec<Option<i32>> {
+        let batches = table
+            .query()
+            .select(Select::columns(&[column]))
+            .execute()
+            .await
+            .unwrap()
+            .try_collect::<Vec<_>>()
+            .await
+            .unwrap();
+        let mut values: Vec<Option<i32>> = batches
+            .iter()
+            .flat_map(|batch| {
+                batch[column]
+                    .as_any()
+                    .downcast_ref::<Int32Array>()
+                    .unwrap()
+                    .iter()
+                    .collect::<Vec<_>>()
+            })
+            .collect();
+        values.sort();
+        values
+    }
+
+    async fn append(table: &Table, values: Vec<i32>) {
+        let batch = record_batch!(("x", Int32, values)).unwrap();
+        table.add(batch).execute().await.unwrap();
+    }
+
+    #[tokio::test]
+    async fn test_refresh_fills_a_declared_column() {
+        let table = table_with("refresh_fills", vec![1, 2, 3]).await;
+        let declared = declare_doubled(&table).await.unwrap();
+        assert_eq!(read(&table, "doubled").await, vec![None, None, None]);
+
+        let result = table.refresh_column("doubled").await.unwrap();
+        assert!(result.version > declared);
+        assert_eq!(result.rows_filled, 3);
+        assert_eq!(
+            read(&table, "doubled").await,
+            vec![Some(2), Some(4), Some(6)]
+        );
+    }
+
+    /// Values written after the last refresh must be reachable by another one.
+    #[tokio::test]
+    async fn test_refresh_fills_rows_appended_since_the_last_refresh() {
+        let table = table_with("refresh_appended", vec![1, 2]).await;
+        declare_doubled(&table).await.unwrap();
+        table.refresh_column("doubled").await.unwrap();
+
+        append(&table, vec![5, 6]).await;
+        assert_eq!(
+            read(&table, "doubled").await,
+            vec![None, None, Some(2), Some(4)]
+        );
+
+        let result = table.refresh_column("doubled").await.unwrap();
+        assert_eq!(result.rows_filled, 2);
+        assert_eq!(
+            read(&table, "doubled").await,
+            vec![Some(2), Some(4), Some(10), Some(12)]
+        );
+    }
+
+    #[tokio::test]
+    async fn test_refresh_with_nothing_to_fill() {
+        let table = table_with("refresh_noop", vec![1, 2, 3]).await;
+        declare_doubled(&table).await.unwrap();
+        table.refresh_column("doubled").await.unwrap();
+
+        let again = table.refresh_column("doubled").await.unwrap();
+        assert_eq!(again.rows_filled, 0);
+        assert_eq!(
+            read(&table, "doubled").await,
+            vec![Some(2), Some(4), Some(6)]
+        );
+    }
+
+    /// A row is filled only by gaining a value, so an expression yielding null
+    /// settles at once instead of re-selecting the same rows forever. Nothing
+    /// is staged, so the version does not move either.
+    #[tokio::test]
+    async fn test_refresh_converges_on_a_null_result() {
+        let table = table_with("refresh_null_result", vec![1, 2, 3]).await;
+        let declared = table
+            .add_columns()
+            .computed("maybe", "nullif(x, x)")
+            .execute()
+            .await
+            .unwrap()
+            .version;
+
+        let first = table.refresh_column("maybe").await.unwrap();
+        assert_eq!(first.rows_filled, 0);
+        assert_eq!(first.version, declared);
+        assert_eq!(read(&table, "maybe").await, vec![None, None, None]);
+
+        let again = table.refresh_column("maybe").await.unwrap();
+        assert_eq!(again.rows_filled, 0);
+        assert_eq!(again.version, declared);
+    }
+
+    /// The contract's boundary: a filled fragment is not revisited, so
+    /// mutating an input leaves the value computed at fill time.
+    #[tokio::test]
+    async fn test_refresh_does_not_observe_input_mutation() {
+        let table = table_with("refresh_mutation", vec![1]).await;
+        declare_doubled(&table).await.unwrap();
+        table.refresh_column("doubled").await.unwrap();
+        assert_eq!(read(&table, "doubled").await, vec![Some(2)]);
+
+        table.update().column("x", "3").execute().await.unwrap();
+
+        let again = table.refresh_column("doubled").await.unwrap();
+        assert_eq!(again.rows_filled, 0);
+        assert_eq!(read(&table, "doubled").await, vec![Some(2)]);
+    }
+
+    /// A row rewrite before the first refresh materializes the declared
+    /// column as null behind a covering data file. Those rows are still
+    /// unfilled and a later refresh has to reach them.
+    #[tokio::test]
+    async fn test_update_before_the_first_refresh() {
+        let table = table_with("refresh_update_first", vec![1]).await;
+        declare_doubled(&table).await.unwrap();
+
+        table.update().column("x", "3").execute().await.unwrap();
+
+        let result = table.refresh_column("doubled").await.unwrap();
+        assert_eq!(result.rows_filled, 1);
+        assert_eq!(read(&table, "doubled").await, vec![Some(6)]);
+    }
+
+    /// The contract holds row by row, not fragment by fragment: revisiting a
+    /// fragment to fill one row must not recompute a filled row sitting beside
+    /// it, even where the input behind it has since changed.
+    #[tokio::test]
+    async fn test_refresh_does_not_recompute_a_filled_row_beside_an_unfilled_one() {
+        let table = table_with("refresh_mixed", vec![1, 2]).await;
+        declare_doubled(&table).await.unwrap();
+        table.refresh_column("doubled").await.unwrap();
+
+        append(&table, vec![5]).await;
+        table
+            .update()
+            .column("x", "100")
+            .only_if("x = 1")
+            .execute()
+            .await
+            .unwrap();
+        table
+            .optimize(crate::table::OptimizeAction::Compact {
+                options: crate::table::CompactionOptions::default(),
+                remap_options: None,
+            })
+            .await
+            .unwrap();
+
+        let result = table.refresh_column("doubled").await.unwrap();
+        assert_eq!(result.rows_filled, 1);
+        // 2 is the mutated row keeping the value it was filled with, not 200.
+        assert_eq!(
+            read(&table, "doubled").await,
+            vec![Some(2), Some(4), Some(10)]
+        );
+    }
+
+    /// Filling a fragment must not disturb the values it already holds, which
+    /// is what makes a compaction-mixed fragment safe to revisit.
+    #[tokio::test]
+    async fn test_refresh_preserves_already_filled_rows() {
+        let table = table_with("refresh_preserves", vec![1, 2]).await;
+        declare_doubled(&table).await.unwrap();
+        table.refresh_column("doubled").await.unwrap();
+
+        append(&table, vec![5]).await;
+        table
+            .optimize(crate::table::OptimizeAction::Compact {
+                options: crate::table::CompactionOptions::default(),
+                remap_options: None,
+            })
+            .await
+            .unwrap();
+
+        let result = table.refresh_column("doubled").await.unwrap();
+        assert_eq!(result.rows_filled, 1);
+        assert_eq!(
+            read(&table, "doubled").await,
+            vec![Some(2), Some(4), Some(10)]
+        );
+    }
+
+    #[tokio::test]
+    async fn test_refresh_leaves_deleted_rows_alone() {
+        let table = table_with("refresh_deleted", vec![1, 2, 3, 4]).await;
+        declare_doubled(&table).await.unwrap();
+        table.delete("x = 2").await.unwrap();
+
+        let result = table.refresh_column("doubled").await.unwrap();
+        assert_eq!(result.rows_filled, 3);
+        assert_eq!(
+            read(&table, "doubled").await,
+            vec![Some(2), Some(6), Some(8)]
+        );
+    }
+
+    #[tokio::test]
+    async fn test_refresh_a_constant_expression() {
+        let table = table_with("refresh_constant", vec![1, 2, 3]).await;
+        table
+            .add_columns()
+            .computed("answer", "42")
+            .execute()
+            .await
+            .unwrap();
+
+        let result = table.refresh_column("answer").await.unwrap();
+        assert_eq!(result.rows_filled, 3);
+    }
+
+    /// A name needing quotes reaches the evaluator intact: it is carried as a
+    /// projection alias, never spliced into SQL text.
+    #[tokio::test]
+    async fn test_refresh_a_column_whose_name_needs_quoting() {
+        let table = table_with("refresh_quoted", vec![1, 2, 3]).await;
+        table
+            .add_columns()
+            .computed("double value", "x * 2")
+            .execute()
+            .await
+            .unwrap();
+
+        let result = table.refresh_column("double value").await.unwrap();
+        assert_eq!(result.rows_filled, 3);
+        assert_eq!(
+            read(&table, "double value").await,
+            vec![Some(2), Some(4), Some(6)]
+        );
+    }
+
+    #[tokio::test]
+    async fn test_refresh_rejects_a_plain_column() {
+        let table = table_with("refresh_plain", vec![1, 2, 3]).await;
+        let err = table.refresh_column("x").await.unwrap_err();
+        assert!(matches!(err, Error::NotAComputedColumn { name } if name == "x"));
+    }
+
+    #[tokio::test]
+    async fn test_refresh_rejects_an_unknown_column() {
+        let table = table_with("refresh_missing", vec![1, 2, 3]).await;
+        let err = table.refresh_column("nope").await.unwrap_err();
+        assert!(matches!(err, Error::ColumnNotFound { name } if name == "nope"));
+    }
+
+    /// A declaration of a kind this version cannot evaluate is refused by
+    /// name, rather than mistaken for a plain column or fed to the SQL path.
+    #[tokio::test]
+    async fn test_refresh_rejects_a_kind_it_cannot_evaluate() {
+        let table = table_with("refresh_foreign", vec![1, 2, 3]).await;
+        super::super::computed_columns::add_foreign_kind(&table, "embedding", "udf").await;
+
+        let err = table.refresh_column("embedding").await.unwrap_err();
+        assert!(matches!(err, Error::NotSupported { message } if message.contains("udf")));
+    }
+}
@@ -8,11 +8,13 @@
 //! - [`alter_columns`](execute_alter_columns): Rename columns, change types, or modify nullability
 //! - [`drop_columns`](execute_drop_columns): Remove columns from the table

+use arrow_schema::Schema as ArrowSchema;
 use lance::dataset::{ColumnAlteration, NewColumnTransform};
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;

 use super::NativeTable;
+use super::computed_columns;
 use crate::Result;

 /// The result of an add columns operation.
@@ -116,6 +118,14 @@ pub(crate) async fn execute_alter_columns(
 ) -> Result<AlterColumnsResult> {
    table.dataset.ensure_mutable()?;
    let mut dataset = (*table.dataset.get().await?).clone();
+    // Nullability is not part of what an expression resolves against, so only
+    // a rename or a retype can invalidate a binding.
+    let rebinding = alterations
+        .iter()
+        .filter(|alteration| alteration.rename.is_some() || alteration.data_type.is_some())
+        .map(|alteration| alteration.path.as_str())
+        .collect::<Vec<_>>();
+    computed_columns::ensure_not_an_input(&ArrowSchema::from(dataset.schema()), &rebinding)?;
    dataset.alter_columns(alterations).await?;
    let version = dataset.version().version;
    table.dataset.update(dataset);
@@ -131,6 +141,7 @@ pub(crate) async fn execute_drop_columns(
 ) -> Result<DropColumnsResult> {
    table.dataset.ensure_mutable()?;
    let mut dataset = (*table.dataset.get().await?).clone();
+    computed_columns::ensure_not_an_input(&ArrowSchema::from(dataset.schema()), columns)?;
    dataset.drop_columns(columns).await?;
    let version = dataset.version().version;
    table.dataset.update(dataset);
Author	SHA1	Message	Date
Wyatt Alt	c463ca1503	docs: state what a computed column promises The declaration API shipped without a runnable example, and none of the three binding docs said when values appear, what happens to them when an input changes, which schema operations a declaration blocks, or that the feature is local-only. Those are the questions a caller has to answer before using it. Adds Rust doctests on both entry points and the same semantics to the Python and TypeScript parameter docs, plus a worked Python example. Also adds the abstract refresh_column that both concrete Python tables already implemented, so the surface is declared in one place and the cross-references resolve.	2026-08-12 21:01:11 -07:00
Wyatt Alt	79e9dffd06	fix: make computed columns explicitly local-only Both operations were advertised on remote tables and neither could work. A declaration reaches the wire as AllNulls, which RemoteTable::add_columns does not accept, and refresh_column fell through to the BaseTable default; the TypeScript wrapper reached the same surface. Callers got errors that named neither the feature nor the reason. The remote protocol has no representation for a stored expression, and what one should look like is not settled -- the server persists a declaration under a different vocabulary. So this states the boundary rather than guessing at a wire format: an explicit AllNulls arm, a default that names computed columns, and NotImplementedError raised in Python before the round trip.	2026-08-12 20:00:18 -07:00
Wyatt Alt	c3efc320a6	fix(rust): refuse schema changes that invalidate a computed column A declaration records the columns its expression reads, but nothing consulted them: renaming an input left an expression naming a column that no longer exists, and the failure surfaced at refresh time as a plan error rather than at the operation that caused it. Dropping or retyping an input did the same. alter_columns and drop_columns now reject a change to a column some declaration reads. Nullability is not part of what an expression resolves against, so it stays allowed. A declaration does not read itself and so travels with its own binding, and paths compare at their root, since a change to `metadata.age` invalidates an expression reading `metadata` just as surely. Binding to field ids instead would leave the expression text naming the old column, so it would need rewriting stored SQL on every rename. Refusing the operation is what a generated column does elsewhere.	2026-08-12 20:00:18 -07:00
Wyatt Alt	8d6dea6313	fix(rust): fill computed columns row by row Refresh selected rows with `{column} IS NULL` and rewrote them through an UPDATE, which made the output value double as the record of whether the row had been computed. Two consequences: an expression yielding null re-selected the same rows on every run and reported them as filled forever, and the target name was interpolated into SQL unquoted, so a column named `double value` could be declared and never refreshed. Filling is now per fragment. Each fragment that could hold an unfilled row has the expression evaluated over its physical rows and the result written as a standalone column file, published together in one DataReplacement. A row that already holds a value keeps it -- the computed and current values are merged on the is-null mask -- and a row counts as filled only when it gains a value, so a fragment where nothing would change is never staged and a null expression settles after one pass. Which fragments are worth looking at comes from the manifest first: one whose data files do not carry the field cannot hold a filled row. A fragment that does carry it is still asked, because a row rewrite -- an update, or a compaction folding an unfilled fragment into a filled one -- leaves nulls behind a covering file. That case is the reason coverage alone is not the marker; the test for it fails against a coverage-only implementation. Names now reach the evaluator through a projection alias or a backtick-quoted identifier, lance's dialect having no other way to spell one -- a double-quoted name parses as a string literal.	2026-08-12 20:00:18 -07:00
Wyatt Alt	5a2d3f39e2	chore: update lance dependency to v11.0.0-beta.7 Filling a computed column needs FileFragment::write_column, which lands in this release.	2026-08-12 20:00:18 -07:00
Wyatt Alt	219f41339d	refactor(rust): tag a computed column's definition by kind ComputedColumn carried a bare expression string, which asserts that every computed column is a SQL expression. That holds for the only kind there is, but it is the wrong shape for the next one: a column defined by a registered function cannot be typed by parsing its definition, so its type and its inputs have a different provenance than a SQL column's. A single string has nowhere to say which it is. The definition is now ComputedColumnKind, non-exhaustive so another kind is additive, and the metadata carries a matching computed_column.kind tag beside the payload. Tagging the persisted form is the point -- the Rust type stays cheap to change and field metadata does not, and a second kind distinguished only by which keys happen to be present would leave every reader sniffing the shape. An unrecognized kind reads back as Unrecognized rather than as absent. A newer version's declaration is a computed column this one cannot evaluate, not a plain column: reported as absent it would be redeclarable over and would fail refresh as "not a computed column". Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>	2026-08-12 18:48:48 -07:00
Wyatt Alt	5982eebbb3	feat(nodejs): expose computed columns and refreshColumn addComputedColumns declares columns defined by a SQL expression, and refreshColumn fills a computed column's unfilled rows: await table.addComputedColumns([{ name: "doubled", valueSql: "x * 2" }]); await table.refreshColumn("doubled");	2026-08-12 18:48:48 -07:00
Wyatt Alt	5a1c839382	feat(python): expose computed columns and refresh_column add_columns gains a computed= mapping of column name to SQL expression, and refresh_column fills a computed column's unfilled rows. Both are available on the sync and async tables: table.add_columns(computed={"doubled": "x * 2"}) table.refresh_column("doubled") transforms and computed are mutually exclusive, since they commit through different paths and could half-apply.	2026-08-12 18:48:48 -07:00
Wyatt Alt	e40e073a9d	feat(rust): fill computed columns with refresh_column Declaring a computed column stores its expression but computes nothing, so until now the column stayed null with no way to fill it. refresh_column evaluates the expression over the rows that still hold no value and commits the results: table.refresh_column("doubled").await? Rows without a value are the ones to fill, which also makes the operation idempotent and resumable after a failure: refreshing again picks up whatever did not land. A row whose expression evaluates to null is indistinguishable from an unfilled one and is recomputed, which costs work but cannot change the result. Values written after a refresh are reachable by the next one, which is the case that matters -- an expression column populated once and then appended to would otherwise read null for every later row forever.	2026-08-12 18:48:48 -07:00
Wyatt Alt	a47c22b26e	feat(rust): declare computed columns through add_columns A computed column is a column defined by a SQL expression rather than by values supplied at write time, so it is added through add_columns like any other: table.add_columns().computed("doubled", "x * 2").execute().await? Declaring does not compute. The column is committed carrying its expression in field metadata but no data, which makes declaration cost the same on a large table as on an empty one and leaves a single code path that ever produces values. A later refresh fills it. The expression is the whole definition: both the result type and the input columns are derived from it with lance-datafusion's planner, so a caller writes neither, and the two can never disagree the way a hand-declared input list can. Everything statically knowable is rejected at declare time rather than deferred: an expression that does not parse, one referencing a column that does not exist, a name already in use, and the same name declared twice in one call. The binding lives in three field metadata keys: virtual_column marks the column, virtual_column.expression holds it, and virtual_column.inputs holds the parsed inputs as a JSON array. computed_columns() reads them back off a schema, the way a SQL catalog reports a generation expression as another column of its information schema, so introspection needs no round trip. A transform and computed columns cannot be combined in one call, since they commit through different transforms and could half-apply. Only functions the query engine already knows can be named; resolving a user-defined one needs a planner aware of the function registry, which does not exist yet.	2026-08-12 18:48:48 -07:00