feat: infer maintained indexes when an LsmWriteSpec omits them (#3748)

## What

`LsmWriteSpec::maintained_indexes` becomes `Option<Vec<String>>`:

| value | meaning |
|---|---|
| `None` (new default) | every index the MemWAL supports, resolved when
the spec is installed |
| `Some([])` | maintain nothing — a scan/filter-only WAL table |
| `Some([..])` | exactly these, taken verbatim |

`with_maintained_indexes` keeps its signature;
`with_no_maintained_indexes()` is new. Surfaced through the remote path
(null on the wire), Python, and Node.

## Why

Callers had to state the maintained set by hand every time, which is
both tedious and easy to get wrong — the common case is "maintain what I
already built."

Resolution filters on `IndexConfig::is_memwal_maintainable`, delegating
to lance's `is_maintainable_index_type`. This is load-bearing rather
than cosmetic: lance does **not** skip an index type its memtable cannot
build, it errors when the shard writer opens, so sweeping up a bitmap
index would fail every memtable claim and leave the table unwritable.
The inferred set excludes those, and an explicit list naming one is now
rejected at spec time instead of at claim time.

## Behavior change

A freshly constructed spec used to maintain **nothing**; it now
maintains **everything supported**. This flipped because napi collapses
`undefined` and `null` to `None`, so TypeScript cannot express "absent
means nothing, null means all" — any other choice makes the bindings
disagree with the wire. The error direction also favors it: an unwanted
maintained index costs memory, while a silently unmaintained one
degrades FTS to an unscored scan.

Three existing tests encoded the old default and are updated rather than
worked around.

## Caveat

The resolved set is a snapshot, not a subscription. An index created
after the spec is installed is not maintained until the spec is unset
and set again. `get_lsm_write_spec` therefore always reports a concrete
list — `None` never round-trips.

## Dependency

Needs a lance release carrying `is_maintainable_index_type`
(lance-format/lance#8095) before this builds against the pinned tag.
Draft until then.

## Testing

38 Rust LSM tests and 10 Python tests pass against a local lance build,
including new coverage that a bitmap index is excluded from inference
and rejected when named, and that `[]` stays distinguishable from null
on the wire.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

python/python/lancedb/_lancedb.pyi

@@ -653,9 +653,10 @@ class LsmWriteSpec:
     def identity(column: str) -> "LsmWriteSpec": ...
     @staticmethod
     def unsharded() -> "LsmWriteSpec": ...
-    def with_maintained_indexes(self, indexes: List[str]) -> "LsmWriteSpec":
-        """Return a copy of this spec asking the MemWAL to keep the named
-        indexes up to date as rows are appended."""
+    def with_maintained_indexes(self, indexes: Optional[List[str]]) -> "LsmWriteSpec":
+        """Set which indexes the MemWAL keeps up to date. None resolves every
+        index on the table at install, failing if one cannot be maintained;
+        a list is verbatim, empty means none."""
         ...
     def with_writer_config_defaults(self, defaults: Dict[str, str]) -> "LsmWriteSpec":
         """Return a copy of this spec recording the given default
@@ -670,7 +671,9 @@ class LsmWriteSpec:
     @property
     def num_buckets(self) -> Optional[int]: ...
     @property
-    def maintained_indexes(self) -> List[str]: ...
+    def maintained_indexes(self) -> Optional[List[str]]:
+        """Indexes the MemWAL keeps up to date, or None for every supported one."""
+        ...
     @property
     def writer_config_defaults(self) -> Dict[str, str]: ...
 

python/python/lancedb/table.py

@@ -4676,6 +4676,13 @@ class AsyncTable:
         via [`set_unenforced_primary_key`]; bucket sharding additionally
         requires it to be the single column being bucketed.
 
+        By default the MemWAL maintains every index on the table, resolved
+        here — a snapshot, so an index created afterwards needs the spec unset
+        and set again. This fails if one cannot be maintained; name the set
+        with ``with_maintained_indexes`` to install anyway. That pins an exact
+        set (a still-building index is rejected, not omitted); ``[]`` maintains
+        none.
+
         Parameters
         ----------
         spec : LsmWriteSpec
@@ -4702,9 +4709,9 @@ class AsyncTable:
 
         Returns ``None`` when the MemWAL LSM write path is not enabled (no
         spec has been set, or it was removed with `unset_lsm_write_spec`).
-        The returned spec — including its ``maintained_indexes`` and
-        ``writer_config_defaults`` — mirrors what was passed to
-        `set_lsm_write_spec`.
+        The returned spec mirrors what was passed to `set_lsm_write_spec`,
+        except that ``maintained_indexes`` always reports the concrete list
+        resolved when the spec was set — ``None`` never round-trips.
         """
         return await self._inner.get_lsm_write_spec()
 

python/python/tests/test_lsm_write_spec.py

@@ -83,7 +83,9 @@ def test_lsm_write_spec_repr():
     assert s.spec_type == "bucket"
     assert s.column == "id"
     assert s.num_buckets == 4
-    assert s.maintained_indexes == []
+    # A fresh spec defers its maintained set to install time.
+    assert s.maintained_indexes is None
+    assert s.with_maintained_indexes([]).maintained_indexes == []
     assert "bucket" in repr(s)
     assert "id" in repr(s)
     assert "4" in repr(s)
@@ -169,18 +171,23 @@ def test_get_lsm_write_spec(tmp_path):
     table.unset_lsm_write_spec()
     assert table.get_lsm_write_spec() is None
 
-    # Identity round-trips (column recovered from the schema).
+    # Identity round-trips (column recovered from the schema). Leaving the
+    # maintained set to be inferred picks up the index on the table, so the
+    # spec reads back naming it rather than as "infer".
     table.set_lsm_write_spec(LsmWriteSpec.identity("id"))
     spec = table.get_lsm_write_spec()
     assert spec.spec_type == "identity"
     assert spec.column == "id"
+    assert spec.maintained_indexes == [idx_name]
     table.unset_lsm_write_spec()
 
-    # Unsharded round-trips (no routing column).
-    table.set_lsm_write_spec(LsmWriteSpec.unsharded())
+    # Unsharded round-trips (no routing column). Opting out is distinct from
+    # the inferred default.
+    table.set_lsm_write_spec(LsmWriteSpec.unsharded().with_maintained_indexes([]))
     spec = table.get_lsm_write_spec()
     assert spec.spec_type == "unsharded"
     assert spec.column is None
+    assert spec.maintained_indexes == []
 
 
 @pytest.mark.asyncio

python/python/tests/test_merge_insert_lsm.py

@@ -544,7 +544,7 @@ def test_lsm_read_fts_unmaintained_index_errors(tmp_path):
     table.create_index("text", config=FTS())
     # No maintained indexes: the active memtable FTS arm cannot serve un-compacted
     # docs, so the search would silently omit them — reject instead.
-    table.set_lsm_write_spec(LsmWriteSpec.unsharded())
+    table.set_lsm_write_spec(LsmWriteSpec.unsharded().with_maintained_indexes([]))
     with pytest.raises(Exception, match="maintained"):
         table.search("fox", query_type="fts", fts_columns="text").to_arrow()
 
@@ -631,7 +631,7 @@ def test_lsm_read_vector_unmaintained_index_errors(tmp_path):
     )
     # Spec with NO maintained indexes: the base vector index's catch-up is untracked,
     # so the scanner rejects rather than risk dropping compacted-but-unindexed rows.
-    table.set_lsm_write_spec(LsmWriteSpec.unsharded())
+    table.set_lsm_write_spec(LsmWriteSpec.unsharded().with_maintained_indexes([]))
     with pytest.raises(Exception, match="maintained"):
         table.search([1.0] * VECTOR_DIM).to_arrow()