From 2b7112674a4720632b2a97139e7b738caf79f385 Mon Sep 17 00:00:00 2001
From: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>
Date: Thu, 6 Aug 2026 07:24:52 +0000
Subject: [PATCH] fix(python): add legacy V2 table migration

---
 docs/src/python/python.md             |  15 ++
 python/python/lancedb/legacy_v2.py    | 263 ++++++++++++++++++++++++++
 python/python/tests/test_legacy_v2.py | 165 ++++++++++++++++
 3 files changed, 443 insertions(+)
 create mode 100644 python/python/lancedb/legacy_v2.py
 create mode 100644 python/python/tests/test_legacy_v2.py

diff --git a/docs/src/python/python.md b/docs/src/python/python.md
index 36044d35d..716a408fb 100644
--- a/docs/src/python/python.md
+++ b/docs/src/python/python.md
@@ -180,6 +180,21 @@ instead of being materialized with the rest of the row.
 
 ::: lancedb.otel.instrument_lancedb_metrics
 
+## Legacy V2 migration
+
+Tables created with the experimental V2 format in LanceDB Node 0.5.x can be
+rewritten with the legacy PyLance reader. In a dedicated environment, install
+LanceDB normally, then install the legacy reader without its obsolete PyArrow
+upper bound and run the migration:
+
+```shell
+pip install lancedb
+pip install --no-deps pylance==0.12.1
+python -m lancedb.legacy_v2 <database-uri>
+```
+
+::: lancedb.legacy_v2.migrate_legacy_v2_tables
+
 ## Exceptions
 
 ::: lancedb.exceptions.MissingValueError
diff --git a/python/python/lancedb/legacy_v2.py b/python/python/lancedb/legacy_v2.py
new file mode 100644
index 000000000..91e003743
--- /dev/null
+++ b/python/python/lancedb/legacy_v2.py
@@ -0,0 +1,263 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+"""Recovery utilities for the experimental V2 format used by old Node releases."""
+
+from __future__ import annotations
+
+import argparse
+import asyncio
+import inspect
+import warnings
+from collections.abc import Iterable
+from typing import Any
+
+from packaging.version import Version
+
+import lancedb
+
+__all__ = ["migrate_legacy_v2_tables"]
+
+_LEGACY_LANCE_VERSION = Version("0.12.1")
+_LEGACY_V2_ERROR_MARKERS = (
+    "missing columnencoding encoding description",
+    "missing lance.encodings.columnencoding encoding description",
+    "was missing a columnencoding",
+    "rust future panicked",
+    "panic in async function",
+)
+
+
+def _require_legacy_lance() -> Any:
+    try:
+        import lance
+    except ImportError as error:
+        raise RuntimeError(
+            "Legacy V2 migration requires pylance==0.12.1. Install it in a "
+            "dedicated environment with "
+            "`pip install --no-deps pylance==0.12.1`."
+        ) from error
+
+    version = Version(lance.__version__)
+    if version != _LEGACY_LANCE_VERSION:
+        raise RuntimeError(
+            "Legacy V2 migration requires pylance==0.12.1, but found "
+            f"pylance=={version}. Reinstall it with "
+            "`pip install --no-deps --force-reinstall pylance==0.12.1`."
+        )
+    return lance
+
+
+def _exception_messages(error: BaseException) -> Iterable[str]:
+    seen: set[int] = set()
+    current: BaseException | None = error
+    while current is not None and id(current) not in seen:
+        seen.add(id(current))
+        yield str(current).lower()
+        current = current.__cause__ or current.__context__
+
+
+def _is_legacy_v2_error(error: BaseException) -> bool:
+    return any(
+        marker in message
+        for message in _exception_messages(error)
+        for marker in _LEGACY_V2_ERROR_MARKERS
+    )
+
+
+def _list_table_names(db: Any) -> list[str]:
+    list_tables = getattr(db, "list_tables", None)
+    if list_tables is not None:
+        # The deprecated table_names() API defaults to only ten results.
+        return list(list_tables(limit=None).tables)
+
+    # Compatibility for LanceDB 0.16, which was used by the original script.
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore", DeprecationWarning)
+        return list(db.table_names())
+
+
+async def _needs_migration(db: Any, table_name: str) -> bool:
+    table = await db.open_table(table_name)
+    try:
+        # One row is enough to load and validate the data-file metadata.
+        await table.query().limit(1).to_arrow()
+    except (KeyboardInterrupt, SystemExit, GeneratorExit):
+        raise
+    except BaseException as error:
+        if _is_legacy_v2_error(error):
+            return True
+        raise
+    return False
+
+
+async def _create_migrated_table(
+    db: Any, table_name: str, reader: Any, storage_format: str
+) -> Any:
+    parameters = inspect.signature(db.create_table).parameters
+    options: dict[str, Any] = {"mode": "overwrite"}
+    if "data_storage_version" in parameters:
+        # LanceDB 0.16 exposed the format as a direct create_table option.
+        options["data_storage_version"] = storage_format
+    else:
+        options["storage_options"] = {"new_table_data_storage_version": storage_format}
+    return await db.create_table(table_name, reader, **options)
+
+
+async def _migrate_table(
+    source_db: Any,
+    destination_db: Any,
+    table_name: str,
+    storage_format: str,
+) -> int:
+    source_table = source_db.open_table(table_name)
+    source_dataset = source_table.to_lance()
+    source_rows = source_dataset.count_rows()
+    reader = source_dataset.scanner().to_reader()
+
+    migrated_table = await _create_migrated_table(
+        destination_db, table_name, reader, storage_format
+    )
+    migrated_rows = await migrated_table.count_rows()
+    if migrated_rows != source_rows:
+        raise RuntimeError(
+            f"Migration of table {table_name!r} wrote {migrated_rows} rows; "
+            f"expected {source_rows}."
+        )
+
+    # Force the current reader to load data-file metadata before reporting success.
+    await migrated_table.query().limit(1).to_arrow()
+    return migrated_rows
+
+
+async def migrate_legacy_v2_tables(
+    uri: str,
+    *,
+    table_name: str | None = None,
+    destination_uri: str | None = None,
+    storage_format: str = "2.0",
+    show_progress: bool = True,
+) -> list[str]:
+    """Migrate tables written with the incompatible experimental V2 format.
+
+    LanceDB Node 0.5.x could enable an experimental data format when an empty
+    table was created and data was added later. Those files panic older modern
+    readers and are rejected by newer readers. This utility streams them through
+    ``pylance==0.12.1`` and rewrites them in a supported format.
+
+    Install the legacy reader in a dedicated environment before running this
+    function::
+
+        pip install lancedb
+        pip install --no-deps pylance==0.12.1
+
+    ``--no-deps`` is required because the legacy wheel declares an obsolete
+    PyArrow upper bound. The migration uses only its dataset scanner and writes
+    through the current LanceDB package.
+
+    This migration is available only for local/OSS databases, including object
+    storage URIs. It is not supported for LanceDB Cloud ``db://`` connections.
+    In-place migration creates a new table version, so old data remains available
+    for recovery until old versions are cleaned up. Table indices are not copied
+    and should be rebuilt after migration.
+
+    Parameters
+    ----------
+    uri : str
+        Source LanceDB database URI.
+    table_name : str, optional
+        Migrate only this table. By default, inspect every table.
+    destination_uri : str, optional
+        Write to another database. By default, migrate in place.
+    storage_format : str, default "2.0"
+        Data storage format for the rewritten tables. Use ``"0.1"`` for
+        compatibility with older LanceDB releases.
+    show_progress : bool, default True
+        Display progress bars while inspecting and migrating tables.
+
+    Returns
+    -------
+    list of str
+        Names of the migrated tables.
+    """
+    if uri.startswith("db://") or (
+        destination_uri is not None and destination_uri.startswith("db://")
+    ):
+        raise ValueError("Legacy V2 migration is supported only for local/OSS tables")
+
+    # Import and validate before opening or modifying any table.
+    _require_legacy_lance()
+
+    source_db = lancedb.connect(uri)
+    async_source_db = await lancedb.connect_async(uri)
+    destination_db = (
+        async_source_db
+        if destination_uri is None or destination_uri == uri
+        else await lancedb.connect_async(destination_uri)
+    )
+
+    if table_name is not None:
+        table_names = [table_name]
+    else:
+        table_names = _list_table_names(source_db)
+
+    inspection: Iterable[str] = table_names
+    if show_progress:
+        from tqdm.auto import tqdm
+
+        inspection = tqdm(table_names, desc="Checking tables")
+
+    tables_to_migrate = [
+        name for name in inspection if await _needs_migration(async_source_db, name)
+    ]
+
+    migration: Iterable[str] = tables_to_migrate
+    if show_progress:
+        from tqdm.auto import tqdm
+
+        migration = tqdm(tables_to_migrate, desc="Migrating tables")
+
+    migrated = []
+    for name in migration:
+        await _migrate_table(source_db, destination_db, name, storage_format)
+        migrated.append(name)
+    return migrated
+
+
+def _parser() -> argparse.ArgumentParser:
+    parser = argparse.ArgumentParser(
+        description="Migrate tables written with the old experimental V2 format."
+    )
+    parser.add_argument("uri", help="source LanceDB database URI")
+    parser.add_argument("--table-name", help="migrate only this table")
+    parser.add_argument("--destination-uri", help="write to another database URI")
+    parser.add_argument(
+        "--storage-format",
+        default="2.0",
+        help='destination data format (default: "2.0"; use "0.1" for compatibility)',
+    )
+    parser.add_argument(
+        "--no-progress", action="store_true", help="disable progress bars"
+    )
+    return parser
+
+
+def main() -> None:
+    args = _parser().parse_args()
+    migrated = asyncio.run(
+        migrate_legacy_v2_tables(
+            args.uri,
+            table_name=args.table_name,
+            destination_uri=args.destination_uri,
+            storage_format=args.storage_format,
+            show_progress=not args.no_progress,
+        )
+    )
+    if migrated:
+        print(f"Migrated {len(migrated)} table(s): {', '.join(migrated)}")
+    else:
+        print("No legacy V2 tables found")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/python/python/tests/test_legacy_v2.py b/python/python/tests/test_legacy_v2.py
new file mode 100644
index 000000000..88e9fefad
--- /dev/null
+++ b/python/python/tests/test_legacy_v2.py
@@ -0,0 +1,165 @@
+from unittest.mock import AsyncMock, Mock
+
+import pytest
+
+from lancedb import legacy_v2
+
+
+class FakeQuery:
+    def __init__(self, error=None):
+        self.error = error
+
+    def limit(self, _limit):
+        return self
+
+    async def to_arrow(self):
+        if self.error is not None:
+            raise self.error
+        return None
+
+
+class FakeAsyncTable:
+    def __init__(self, rows=2, error=None):
+        self.rows = rows
+        self.error = error
+
+    def query(self):
+        return FakeQuery(self.error)
+
+    async def count_rows(self):
+        return self.rows
+
+
+class FakeDataset:
+    def __init__(self, reader, rows=2):
+        self.reader = reader
+        self.rows = rows
+
+    def count_rows(self):
+        return self.rows
+
+    def scanner(self):
+        scanner = Mock()
+        scanner.to_reader.return_value = self.reader
+        return scanner
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "message",
+    [
+        "rust future panicked: unknown error",
+        "Panic in async function",
+        "Missing ColumnEncoding encoding description",
+        "Missing lance.encodings.ColumnEncoding encoding description",
+        "the column at index 0 was missing a ColumnEncoding",
+    ],
+)
+async def test_needs_migration_recognizes_legacy_reader_errors(message):
+    db = Mock()
+    db.open_table = AsyncMock(return_value=FakeAsyncTable(error=RuntimeError(message)))
+
+    assert await legacy_v2._needs_migration(db, "legacy")
+
+
+@pytest.mark.asyncio
+async def test_needs_migration_propagates_unrelated_errors():
+    db = Mock()
+    db.open_table = AsyncMock(
+        return_value=FakeAsyncTable(error=RuntimeError("permission denied"))
+    )
+
+    with pytest.raises(RuntimeError, match="permission denied"):
+        await legacy_v2._needs_migration(db, "legacy")
+
+
+@pytest.mark.asyncio
+async def test_migration_streams_and_verifies_rows(monkeypatch):
+    reader = object()
+    source_dataset = FakeDataset(reader)
+    source_table = Mock()
+    source_table.to_lance.return_value = source_dataset
+    source_db = Mock()
+    source_db.list_tables.return_value.tables = ["healthy", "legacy"]
+    source_db.open_table.return_value = source_table
+
+    legacy_error = RuntimeError(
+        "Missing lance.encodings.ColumnEncoding encoding description"
+    )
+    async_source_db = Mock()
+
+    async def open_table(name):
+        if name == "legacy":
+            return FakeAsyncTable(error=legacy_error)
+        return FakeAsyncTable()
+
+    async_source_db.open_table = open_table
+    create_calls = []
+
+    async def create_table(name, data, *, mode, storage_options):
+        create_calls.append((name, data, mode, storage_options))
+        return FakeAsyncTable()
+
+    async_source_db.create_table = create_table
+
+    monkeypatch.setattr(legacy_v2, "_require_legacy_lance", Mock())
+    monkeypatch.setattr(legacy_v2.lancedb, "connect", Mock(return_value=source_db))
+
+    async def connect_async(_uri):
+        return async_source_db
+
+    monkeypatch.setattr(legacy_v2.lancedb, "connect_async", connect_async)
+
+    migrated = await legacy_v2.migrate_legacy_v2_tables("/data/db", show_progress=False)
+
+    assert migrated == ["legacy"]
+    source_db.list_tables.assert_called_once_with(limit=None)
+    assert create_calls == [
+        (
+            "legacy",
+            reader,
+            "overwrite",
+            {"new_table_data_storage_version": "2.0"},
+        )
+    ]
+
+
+def test_list_table_names_supports_legacy_connection():
+    db = Mock(spec=["table_names"])
+    db.table_names.return_value = [f"table_{index}" for index in range(12)]
+
+    assert len(legacy_v2._list_table_names(db)) == 12
+
+
+@pytest.mark.asyncio
+async def test_create_table_uses_legacy_storage_parameter():
+    calls = []
+
+    class LegacyConnection:
+        async def create_table(self, name, data, *, mode, data_storage_version=None):
+            calls.append((name, data, mode, data_storage_version))
+            return FakeAsyncTable()
+
+    reader = object()
+    await legacy_v2._create_migrated_table(LegacyConnection(), "legacy", reader, "0.1")
+
+    assert calls == [("legacy", reader, "overwrite", "0.1")]
+
+
+def test_requires_exact_legacy_lance_version(monkeypatch):
+    fake_lance = Mock(__version__="9.0.0")
+    monkeypatch.setitem(__import__("sys").modules, "lance", fake_lance)
+
+    with pytest.raises(RuntimeError, match="requires pylance==0.12.1"):
+        legacy_v2._require_legacy_lance()
+
+
+@pytest.mark.asyncio
+async def test_cloud_migration_is_rejected_before_dependency_check(monkeypatch):
+    require_lance = Mock()
+    monkeypatch.setattr(legacy_v2, "_require_legacy_lance", require_lance)
+
+    with pytest.raises(ValueError, match="only for local/OSS"):
+        await legacy_v2.migrate_legacy_v2_tables("db://example")
+
+    require_lance.assert_not_called()