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After initialize_mem_wal, every FTS query logs "ERROR Index __lance_mem_wal has no fields"

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@lance-gatefixer is already working on this.

Since Oct 1, 2026.

  • #9682 by @lance-gatefixer — open

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
86/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python, rust
Domain
databases, search

Research direction

Start in lance rust/lance/src/index.rs around load_scalar_index at lines 2479-2490, then compare the system-index check in lance-index/src/lib.rs:89. Run the reproduction in $S/hunt/datashapes/memwal2.py and verify that FTS queries no longer emit errors for __lance_mem_wal while still returning the expected result.

Written by the indexing model from the issue text.

Description

bug

Bug bash: Ready to fix

Bug bash priority: Low · critical-fix: no
From the FTS for Code bug bash (2026-10-01), findings: datashapes-6.md. Repros ran against released artifacts (pylance 12.0.0 / 13.0.0b18, lancedb 0.39.0 / 0.40.0b12, @lancedb/lancedb 0.40.0-beta.12) unless stated. $S is the bug bash scratch dir.

  • Category: bug
  • Environment: pylance local, Python 3.12, Linux arm64
  • Artifacts: pylance 13.0.0b18 and pylance 12.0.0 (same output)
  • Target tracker: GitHub lance-format/lance

Repro

Script: $S/hunt/datashapes/memwal2.py

ds = lance.write_dataset(pa.table({"id": pa.array([0], pa.int64()), "c": ["def getUserName(): pass"]}), d + "/t")
ds.create_scalar_index("c", "INVERTED", analyzer="code", split_identifiers=True, name="c_idx")
ds = lance.dataset(d + "/t")
ds.initialize_mem_wal(maintained_indexes=["c_idx"], unsharded=True)
ds = lance.dataset(d + "/t")
ds.to_table(full_text_query="user", columns=["id"])

--- init mem_wal
--- FTS query
[2026-10-01T21:14:44Z ERROR lance::index] Index __lance_mem_wal has no fields
[2026-10-01T21:14:44Z ERROR lance::index] Index __lance_mem_wal has no fields
[2026-10-01T21:14:44Z ERROR lance::index] Index __lance_mem_wal has no fields
result [0]
--- list_indices
['__lance_mem_wal', 'c_idx']

The query result is correct, but each FTS query prints three ERROR lines. These look like corruption to users and to log alerting.

Expected behavior

No error log for the system MemWAL index, which never has fields by design.

Suspected root cause

load_scalar_index skips field-less indices but exempts only FRAG_REUSE_INDEX_NAME from the error log (lance rust/lance/src/index.rs:2479-2490). MEM_WAL_INDEX_NAME (__lance_mem_wal, imported at index.rs:32) is also field-less and is not exempted.

Fix direction

Exempt MEM_WAL_INDEX_NAME (or any system index) from that log, for example by reusing the system-index check in lance-index/src/lib.rs:89.

Dominant language
Rust
Stars
7.2k
Forks
882
Avg merge
4d 18h
Merged PRs (30d)
263

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