## What
MemWAL LSM **read** support. When a table has an LSM write spec
(`set_lsm_write_spec`), `merge_insert` upserts live in the MemWAL
active/frozen memtables and flushed SSTables until an external
compaction merges them into the base table, so a normal scan returns
**stale** data. This routes reads through Lance's `LsmScanner` so
queries also surface that in-flight data, deduplicated by primary key
(newest generation wins).
## How
- Adds a **`use_lsm: Option<bool>`** query flag, symmetric with the
`merge_insert` flag:
- **unset** — auto-route through the LSM scanner when the table carries
a write spec
- **`use_lsm(true)`** — force the LSM path; error if there is no spec
- **`use_lsm(false)`** — read the base table only (the escape hatch)
- Plain scan, single-column full-text search, and single-vector ANN all
run through one `LsmScanner` (assembled from on-disk shard manifests
plus the cached writer's in-memory memtables), so a `where` predicate is
honored as a **prefilter** uniformly — including for vector search.
- **Compaction-aware snapshots:** an SSTable generation is dropped only
once it is both compacted into the base table and covered by the arm's
base-index catch-up (`index_catchup`); plain scans use the compaction
watermark alone.
- Query shapes the scanner cannot honor hard-error with guidance to set
`use_lsm(false)`: hybrid, multi/binary vectors, `with_row_id`,
reranking, `order_by`, dynamic/Substrait projection or filters,
`distance_range`, `use_index(false)`, postfilter, take-by-row-id/offset,
reads from a time-traveled version, and an unmaintained or ambiguous
(multiple) FTS/vector index. Namespace-pushdown queries fall back to
local execution when a spec is present; WAL-only writers are handled.
- Exposed across the Rust core and the Python (`use_lsm`) and TypeScript
(`useLsm`) bindings, including `TakeQuery`.
Rebased from Lance `7.2.0-beta.3` to `10.0.0-beta.3`.
## Summary
When an `LsmWriteSpec` is installed on a table (#3396), `merge_insert`
upsert
calls are dispatched through Lance's MemWAL `ShardWriter` (LSM-style
append)
instead of the standard merge path.
- **`use_lsm_write`** — a `merge_insert` builder option, default `true`;
set it
`false` to use the standard path for a call even when a spec is set.
- **`assume_pre_sharded`** — a `merge_insert` builder option, default
`false`;
skips the per-row shard check and routes by the first row only.
- **`close_lsm_writers`** — drains and closes the table's cached MemWAL
shard
writers.
- The `merge_insert` **`on`** columns default to, and are validated
against,
the table's unenforced primary key.
- Shard writers are cached alongside the dataset (in
`DatasetConsistencyWrapper`) and reused for the session.
- `MergeResult` gains **`num_rows`** — on the LSM path the insert/update
breakdown is unknown until compaction, so only the total is reported.
Routing covers all three sharding strategies — bucket (murmur3,
Iceberg-compatible), identity, and unsharded. Each `merge_insert` call
targets
a single shard; the whole input is collected and validated before a
single
atomic `ShardWriter::put`, so a validation failure leaves the MemWAL
untouched.
Bindings: Python (`merge_insert(...).use_lsm_write(...)` /
`.assume_pre_sharded(...)`, `Table.close_lsm_writers`) and TypeScript
(`mergeInsert(...).useLsmWrite(...)` / `.assumePreSharded(...)`,
`Table.closeLsmWriters`).
## Context
Reconstructed from the original #3354 branch onto current `main`: the
branch
predated the #3394 (unenforced primary key) / #3396 (`LsmWriteSpec`)
split and
has been rebuilt on that merged foundation. Depends on Lance
`v7.0.0-beta.13`.
The MemWAL read path (reading un-flushed shard data back into queries)
and
remote (LanceDB Cloud) LSM support are follow-ups.
---------
Co-authored-by: Jack Ye <yezhaoqin@gmail.com>
## Summary
Split out from #3354
Adds `LsmWriteSpec` and `Table::set_lsm_write_spec` /
`unset_lsm_write_spec` to
install and clear the spec that selects Lance's MemWAL LSM-style write
path for
`merge_insert`.
`LsmWriteSpec` offers three sharding strategies, all built on Lance's
`InitializeMemWalBuilder`:
- `LsmWriteSpec::bucket(column, num_buckets)` — hash-bucket sharding by
the
single-column unenforced primary key.
- `LsmWriteSpec::identity(column)` — identity sharding by the raw value
of a
scalar column.
- `LsmWriteSpec::unsharded()` — a single MemWAL shard.
Each can be refined with `with_maintained_indexes(...)` (indexes the
MemWAL
keeps up to date as rows are appended) and
`with_writer_config_defaults(...)`
(default `ShardWriter` configuration recorded in the MemWAL index, so
every
writer starts from the same defaults). All variants require the table to
have
an unenforced primary key.
- `set_lsm_write_spec` installs the spec by initializing the MemWAL
index;
`unset_lsm_write_spec` removes it (dropping the MemWAL index), reverting
to
the standard `merge_insert` path. `unset` is idempotent.
- Bindings: Python (`LsmWriteSpec.bucket` / `.identity` / `.unsharded`,
`set_lsm_write_spec` / `unset_lsm_write_spec`) and TypeScript
(`setLsmWriteSpec` with `specType` `"bucket"` / `"identity"` /
`"unsharded"`). `RemoteTable` returns `NotSupported`.
The actual `merge_insert` LSM dispatch and `ShardWriter` write path are
a
follow-up — this PR only installs and clears the spec.
## Summary
Adds `Table::set_unenforced_primary_key` — records a single column as
the
table's unenforced primary key in Lance schema field metadata.
"Unenforced"
means LanceDB does not check uniqueness on write; the key is metadata
that
`merge_insert` consumes.
- Single-column only; the column must exist and have a supported dtype
(Int32, Int64, Utf8, LargeUtf8, Binary, LargeBinary, FixedSizeBinary).
The
API accepts an iterable for binding ergonomics but requires exactly one
column — compound keys are rejected.
- The primary key is immutable: calling this on a table that already has
an
unenforced primary key is rejected. Concurrent writers racing to set the
key
fail at commit time rather than silently overriding it.
- `RemoteTable` returns `NotSupported`.
- Bindings: Python (`AsyncTable`, `LanceTable`, `RemoteTable`) and
TypeScript
(`Table.setUnenforcedPrimaryKey`).
## Context
Split out from #3354 per review feedback, so the unenforced primary key
and the
`merge_insert` sharding spec land as separate reviewable PRs.
No Lance dependency bump — `main` is already on v7.0.0-beta.10, which
includes
the field-metadata round-trip fix the API relies on. Enforcing
primary-key
immutability at the Lance commit layer (so the cross-column concurrent
race is
also rejected) is a companion Lance change: lance-format/lance#6810.
## Summary
`url::Url::query_pairs_mut()` leaves the URL with `query=Some("")` after
`.clear()` even when the input had no query string. The listing-database
connect path then captured that empty query into
`ListingDatabase::query_string`, and `table_uri()` blindly appended
`?<query>` to every per-table URI — producing URIs like
`s3://bucket/prefix/foo.lance?`.
The trailing `?` is benign for normal table operations, but it breaks
any caller that constructs a sub-path from the table URI. In particular,
MemWAL flushes write to `<table_uri>/_mem_wal/<shard>/<rand>_gen_<n>`,
which `url::Url::parse` then re-parses as `path=<base table>` +
`query=/_mem_wal/...`. `Dataset::write` resolves the base table dataset,
finds it already exists, and fails with `Dataset already exists:
…_gen_1` on the very first MemTable flush (observed deterministically
against S3 across all merge_insert LSM modes; tracked in
[lance-format/lance#6713](https://github.com/lance-format/lance/pull/6715)).
## Fix
Treat `Some("")` query the same as no query when capturing
`query_string`. A real `?foo=bar` query is still propagated unchanged.
Adds a regression test covering both the empty-query and non-empty-query
paths.
## Verification
- `url::Url::parse("s3://bucket/prefix/").query()` → `None`, but after
`query_pairs_mut().clear()` → `Some("")`. Confirmed in a standalone
repro.
- Without this fix, every `table_uri()` for an `s3://`-style connection
ends with `?`, breaking MemWAL and any future sub-path consumer in the
same way.
- New unit test `test_table_uri_url_path_has_no_trailing_question_mark`
exercises both code paths.
Heng Ge
