XY Zhan 4148dfef72 feat(lsm): require recorded index catch-up, as an explicit activation (#3911)
> Stacked on #3780. Blocked only on #3922 (`lance` → `v11.0.0-beta.6`),
so CI
> stays red until that lands.

## Missing coverage must mean "not known to be covered"

#3780 caps the SSTable exclusion watermark at an index's recorded
catch-up when
there is one, and silently ignores the case where there is none. On a
table that
requires catch-up, an absent entry means the index is *not* known to
hold the
compacted rows — and the LSM base arm reads base through the index
(`fast_search`, no brute-force tail), so dropping that SSTable loses
those rows
for that query.

```rust
Some(caught_up) => watermark = watermark.min(caught_up),
None if catchup_required => watermark = 0,   // retain everything
None => {}
```

`catchup_required` reads the manifest feature bit directly, and requires
both
words: a half-set manifest is treated as legacy, which is the
conservative side.
Without the bit the field is not maintained at all, so absence carries
no
information and behaviour is unchanged.

## Activation, as a table-level entry point

`Table::require_mem_wal_index_catchup()` performs the one-way switch,
separate
from `set_lsm_write_spec`: a table carrying the bit retains every
generation
until something records catch-up, so it has to follow the deployment of
whatever
repairs coverage, not the creation of the table.

This is a convenience, not the only path — a writer holding the dataset
calls
the equivalent on `DatasetMemWalExt`, which is what the WAL pod does.
Lance
enforces the preconditions either way: the MemWAL index must exist, and
the
table must not already carry `compacted_sstables` from before this
protocol,
since those numbers cannot be validated.

## Still correct after the Lance rework

lance-format/lance#8481 replaced the transmitted `IndexCatchupAdvance`
with a
position derived at commit time from the version a transaction read.
That
changed how a writer earns coverage; it did not change what a reader may
conclude from its absence. The rule here, and the field it reads, are
unchanged.

## Tests

Existing `exclusion_watermarks` unit tests carry the new argument.
Coverage
against a real dataset follows once #3922 lands and this can build.
2026-08-14 09:32:02 -04:00
2023-03-17 18:15:19 -07:00
2025-03-10 09:01:23 -07:00

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