From: Richard Guo <rguo@postgresql.org>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Disable parallel plans for RIGHT_SEMI joins
Date: Thu, 30 Oct 2025 03:00:13 +0000
Message-ID: <E1vEItZ-004B2H-1C@gemulon.postgresql.org> (raw)
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(-)
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To: pgsql-committers@postgresql.org
Cc: rguo@postgresql.org, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Disable parallel plans for RIGHT_SEMI joins
In-Reply-To: <E1vEItZ-004B2H-1C@gemulon.postgresql.org>
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