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compute: fix lost updates in mz_join_core after advance_upper - #37672

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antiguru:fix-join-core-advance-upper
Jul 15, 2026
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compute: fix lost updates in mz_join_core after advance_upper#37672
antiguru merged 1 commit into
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antiguru:fix-join-core-advance-upper

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@antiguruantiguru commented Jul 15, 2026

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Our mz_join_core is a fork of differential's JoinCore::join_core and inherited a correctness bug fixed upstream in TimelyDataflow/differential-dataflow#802 (issue #801).

The operator skipped incoming batches by testing batch.lower() against the mutable acknowledged frontier. advance_upper can advance acknowledged past an in-flight batch when trace merges consolidate the batch's updates away, e.g. an add/remove pair collapsing once logical compaction equates their times. The trace is legitimately empty there only for readers at or beyond the compaction frontier, while our consumers may read finer times where the batch's updates are still real. Testing against acknowledged therefore dropped such batches and silently lost updates.

The fix captures fixed preload_upper frontiers at start-up and gates the skip on them, and advances acknowledged only when a batch sits at or beyond it. This is a one-to-one port of the upstream change.

No regression test: deterministically reproducing the race requires trace merges to consolidate an add/remove pair while a batch is in flight and advance_upper to then jump ahead, which is timing-dependent and not reliable in testdrive/sqllogictest. Upstream #802 added no direct join test either. Correctness rests on mirroring the upstream reasoning.

Note: the alternate DifferentialDataflow join implementation calls upstream join_core from crates.io differential-dataflow 0.24.0, which still carries the bug and will be fixed by a future dependency bump.

🤖 Generated with Claude Code

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antiguru marked this pull request as ready for review July 15, 2026 20:06
@antiguru
antiguru requested a review from a team as a code ownerJuly 15, 2026 20:06
@antiguru@claude
compute: fix lost updates in mz_join_core after advance_upper
35d8bdf
`mz_join_core` skipped incoming batches by testing `batch.lower()`
against the mutable `acknowledged` frontier. `advance_upper` can move
`acknowledged` past an in-flight batch when trace merges consolidate the
batch's updates away, for example an add/remove pair collapsing once
logical compaction equates their times. The trace is legitimately empty
there only for readers at or beyond the compaction frontier, but our
consumers may read finer times where the batch's updates are still real
and unaccounted for. Testing against `acknowledged` therefore dropped
those batches and silently lost updates.
Capture fixed `preload_upper` frontiers at start-up and gate the skip on
them instead, and only advance `acknowledged` forward when a batch sits
at or beyond it. This mirrors the upstream fix in
TimelyDataflow/differential-dataflow#802 (issue #801), which our fork of
`JoinCore::join_core` had inherited.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
@antiguru
antiguruforce-pushed the fix-join-core-advance-upper branch from 0fedd8a to 35d8bdfCompareJuly 15, 2026 20:44
DAlperin
DAlperin approved these changes Jul 15, 2026
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Thanks!

@antiguru

antiguru commented Jul 15, 2026

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Thanks for the review!

@antiguru
antiguru enabled auto-merge (squash) July 15, 2026 20:59
@antiguru
antiguru merged commit de27f5b into MaterializeInc:mainJul 15, 2026
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