feat: refresh computed columns (#3938)

table.refresh_column("doubled") fills the rows of a declared column that
hold no value, in two passes per fragment: the first scans only the
unfilled
live rows to count exact gains and decide staging, the second streams
the
fragment's physical rows into a standalone column file published in one
DataReplacement -- committed under the dataset's own session -- so peak
memory is bounded by a scan batch. A row that holds a value keeps it;
deleted and already-filled rows never reach the expression, so a poison
value in them cannot fail the refresh. Refresh refuses under an LSM
write
spec, including the mem-wal catch-up flag that outlives unset and marks
retained SSTable rows.

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python/python/tests/test_table.py

@@ -3856,16 +3856,20 @@ async def test_async_search_runs_embedding_on_dedicated_executor(
     )
 
 
-def test_computed_column_declares_all_null(tmp_path):
+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]
 
-    # The declaration is durable field metadata.
-    field = table.schema.field("doubled")
-    assert field.metadata[b"computed_column.expression"] == b"x * 2"
+    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):
@@ -3873,3 +3877,14 @@ def test_computed_column_rejects_transforms_and_computed_together(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]