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pgsql: Disable parallel plans for RIGHT_SEMI joins 2+ messages / 1 participants [nested] [flat]
* pgsql: Disable parallel plans for RIGHT_SEMI joins @ 2025-10-30 03:00 Richard Guo <rguo@postgresql.org> 0 siblings, 0 replies; 2+ messages in thread From: Richard Guo @ 2025-10-30 03:00 UTC (permalink / raw) To: pgsql-committers@lists.postgresql.org Disable parallel plans for RIGHT_SEMI joins RIGHT_SEMI joins rely on the HEAP_TUPLE_HAS_MATCH flag to guarantee that only the first match for each inner tuple is considered. However, in a parallel hash join, the inner relation is stored in a shared global hash table that can be probed by multiple workers concurrently. This allows different workers to inspect and set the match flags of the same inner tuples at the same time. If two workers probe the same inner tuple concurrently, both may see the match flag as unset and emit the same tuple, leading to duplicate output rows and violating RIGHT_SEMI join semantics. For now, we disable parallel plans for RIGHT_SEMI joins. In the long term, it may be possible to support parallel execution by performing atomic operations on the match flag, for example using a CAS or similar mechanism. Backpatch to v18, where RIGHT_SEMI join was introduced. Bug: #19094 Reported-by: Lori Corbani <Lori.Corbani@jax.org> Diagnosed-by: Tom Lane <tgl@sss.pgh.pa.us> Author: Richard Guo <guofenglinux@gmail.com> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/19094-6ed410eb5b256abd@postgresql.org Backpatch-through: 18 Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/257ee78341f2657d5c19cdaf0888f843e9bb0c33 Modified Files -------------- src/backend/optimizer/path/joinpath.c | 21 +++++++++++++++------ src/test/regress/expected/join.out | 27 +++++++++++++++++++++++++++ src/test/regress/sql/join.sql | 20 ++++++++++++++++++++ 3 files changed, 62 insertions(+), 6 deletions(-) ^ permalink raw reply [nested|flat] 2+ messages in thread
* pgsql: Disable parallel plans for RIGHT_SEMI joins @ 2025-10-30 03:04 Richard Guo <rguo@postgresql.org> 0 siblings, 0 replies; 2+ messages in thread From: Richard Guo @ 2025-10-30 03:04 UTC (permalink / raw) To: pgsql-committers@lists.postgresql.org Disable parallel plans for RIGHT_SEMI joins RIGHT_SEMI joins rely on the HEAP_TUPLE_HAS_MATCH flag to guarantee that only the first match for each inner tuple is considered. However, in a parallel hash join, the inner relation is stored in a shared global hash table that can be probed by multiple workers concurrently. This allows different workers to inspect and set the match flags of the same inner tuples at the same time. If two workers probe the same inner tuple concurrently, both may see the match flag as unset and emit the same tuple, leading to duplicate output rows and violating RIGHT_SEMI join semantics. For now, we disable parallel plans for RIGHT_SEMI joins. In the long term, it may be possible to support parallel execution by performing atomic operations on the match flag, for example using a CAS or similar mechanism. Backpatch to v18, where RIGHT_SEMI join was introduced. Bug: #19094 Reported-by: Lori Corbani <Lori.Corbani@jax.org> Diagnosed-by: Tom Lane <tgl@sss.pgh.pa.us> Author: Richard Guo <guofenglinux@gmail.com> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/19094-6ed410eb5b256abd@postgresql.org Backpatch-through: 18 Branch ------ REL_18_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/ef6168bafe9be1a5781b2c471ffa4650f31f9a77 Modified Files -------------- src/backend/optimizer/path/joinpath.c | 27 +++++++++++++++++++-------- src/test/regress/expected/join.out | 27 +++++++++++++++++++++++++++ src/test/regress/sql/join.sql | 20 ++++++++++++++++++++ 3 files changed, 66 insertions(+), 8 deletions(-) ^ permalink raw reply [nested|flat] 2+ messages in thread
end of thread, other threads:[~2025-10-30 03:04 UTC | newest] Thread overview: 2+ messages (download: mbox mbox.gz follow: Atom feed) -- links below jump to the message on this page -- 2025-10-30 03:00 pgsql: Disable parallel plans for RIGHT_SEMI joins Richard Guo <rguo@postgresql.org> 2025-10-30 03:04 pgsql: Disable parallel plans for RIGHT_SEMI joins Richard Guo <rguo@postgresql.org>
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