feat: add output_schema method to queries (#2717)

This is a helper utility I need for some of my data loader work. It
makes it easy to see the output schema even when a `select` has been
applied.

nodejs/__test__/query.test.ts

@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+import * as tmp from "tmp";
+
+import { type Table, connect } from "../lancedb";
+import {
+  Field,
+  FixedSizeList,
+  Float32,
+  Int64,
+  Schema,
+  Utf8,
+  makeArrowTable,
+} from "../lancedb/arrow";
+import { Index } from "../lancedb/indices";
+
+describe("Query outputSchema", () => {
+  let tmpDir: tmp.DirResult;
+  let table: Table;
+
+  beforeEach(async () => {
+    tmpDir = tmp.dirSync({ unsafeCleanup: true });
+    const db = await connect(tmpDir.name);
+
+    // Create table with explicit schema to ensure proper types
+    const schema = new Schema([
+      new Field("a", new Int64(), true),
+      new Field("text", new Utf8(), true),
+      new Field(
+        "vec",
+        new FixedSizeList(2, new Field("item", new Float32())),
+        true,
+      ),
+    ]);
+
+    const data = makeArrowTable(
+      [
+        { a: 1n, text: "foo", vec: [1, 2] },
+        { a: 2n, text: "bar", vec: [3, 4] },
+        { a: 3n, text: "baz", vec: [5, 6] },
+      ],
+      { schema },
+    );
+    table = await db.createTable("test", data);
+  });
+
+  afterEach(() => {
+    tmpDir.removeCallback();
+  });
+
+  it("should return schema for plain query", async () => {
+    const schema = await table.query().outputSchema();
+
+    expect(schema.fields.length).toBe(3);
+    expect(schema.fields.map((f) => f.name)).toEqual(["a", "text", "vec"]);
+    expect(schema.fields[0].type.toString()).toBe("Int64");
+    expect(schema.fields[1].type.toString()).toBe("Utf8");
+  });
+
+  it("should return schema with dynamic projection", async () => {
+    const schema = await table.query().select({ bl: "a * 2" }).outputSchema();
+
+    expect(schema.fields.length).toBe(1);
+    expect(schema.fields[0].name).toBe("bl");
+    expect(schema.fields[0].type.toString()).toBe("Int64");
+  });
+
+  it("should return schema for vector search with _distance column", async () => {
+    const schema = await table
+      .vectorSearch([1, 2])
+      .select(["a"])
+      .outputSchema();
+
+    expect(schema.fields.length).toBe(2);
+    expect(schema.fields.map((f) => f.name)).toEqual(["a", "_distance"]);
+    expect(schema.fields[0].type.toString()).toBe("Int64");
+    expect(schema.fields[1].type.toString()).toBe("Float32");
+  });
+
+  it("should return schema for FTS search", async () => {
+    await table.createIndex("text", { config: Index.fts() });
+
+    const schema = await table
+      .search("foo", "fts")
+      .select(["a"])
+      .outputSchema();
+
+    // FTS search includes _score column in addition to selected columns
+    expect(schema.fields.length).toBe(2);
+    expect(schema.fields.map((f) => f.name)).toContain("a");
+    expect(schema.fields.map((f) => f.name)).toContain("_score");
+    const aField = schema.fields.find((f) => f.name === "a");
+    expect(aField?.type.toString()).toBe("Int64");
+  });
+
+  it("should return schema for take query", async () => {
+    const schema = await table.takeOffsets([0]).select(["text"]).outputSchema();
+
+    expect(schema.fields.length).toBe(1);
+    expect(schema.fields[0].name).toBe("text");
+    expect(schema.fields[0].type.toString()).toBe("Utf8");
+  });
+
+  it("should return full schema when no select is specified", async () => {
+    const schema = await table.query().outputSchema();
+
+    // Should return all columns
+    expect(schema.fields.length).toBe(3);
+  });
+});