From: Tom Lane <tgl@sss.pgh.pa.us>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Allow indexscans on partial hash indexes with implied quals.
Date: Thu, 27 Nov 2025 18:10:22 +0000
Message-ID: <E1vOgRh-001hFi-1y@gemulon.postgresql.org> (raw)
Allow indexscans on partial hash indexes with implied quals.
Normally, if a WHERE clause is implied by the predicate of a partial
index, we drop that clause from the set of quals used with the index,
since it's redundant to test it if we're scanning that index.
However, if it's a hash index (or any !amoptionalkey index), this
could result in dropping all available quals for the index's first
key, preventing us from generating an indexscan.
It's fair to question the practical usefulness of this case. Since
hash only supports equality quals, the situation could only arise
if the index's predicate is "WHERE indexkey = constant", implying
that the index contains only one hash value, which would make hash
a really poor choice of index type. However, perhaps there are
other !amoptionalkey index AMs out there with which such cases are
more plausible.
To fix, just don't filter the candidate indexquals this way if
the index is !amoptionalkey. That's a bit hokey because it may
result in testing quals we didn't need to test, but to do it
more accurately we'd have to redundantly identify which candidate
quals are actually usable with the index, something we don't know
at this early stage of planning. Doesn't seem worth the effort.
Reported-by: Sergei Glukhov <s.glukhov@postgrespro.ru>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/e200bf38-6b45-446a-83fd-48617211feff@postgrespro.ru
Backpatch-through: 14
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/5528e8d1046062698b7db179e4280817eaeb5f30
Modified Files
--------------
src/backend/optimizer/path/indxpath.c | 10 ++++++++++
src/test/regress/expected/hash_index.out | 21 +++++++++++++++++++++
src/test/regress/sql/hash_index.sql | 13 +++++++++++++
3 files changed, 44 insertions(+)
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To: pgsql-committers@postgresql.org
Cc: tgl@sss.pgh.pa.us, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Allow indexscans on partial hash indexes with implied quals.
In-Reply-To: <E1vOgRh-001hFi-1y@gemulon.postgresql.org>
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