From: Michael Paquier <michael@paquier.xyz>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Ignore temporary relations in RelidByRelfilenumber()
Date: Fri, 22 Aug 2025 00:07:59 +0000
Message-ID: <E1upFK3-00137e-05@gemulon.postgresql.org> (raw)
Ignore temporary relations in RelidByRelfilenumber()
Temporary relations may share the same RelFileNumber with a permanent
relation, or other temporary relations associated with other sessions.
Being able to uniquely identify a temporary relation would require
RelidByRelfilenumber() to know about the proc number of the temporary
relation it wants to identify, something it is not designed for since
its introduction in f01d1ae3a104.
There are currently three callers of RelidByRelfilenumber():
- autoprewarm.
- Logical decoding, reorder buffer.
- pg_filenode_relation(), that attempts to find a relation OID based on
a tablespace OID and a RelFileNumber.
This makes the situation problematic particularly for the first two
cases, leading to the possibility of random ERRORs due to
inconsistencies that temporary relations can create in the cache
maintained by RelidByRelfilenumber(). The third case should be less of
an issue, as I suspect that there are few direct callers of
pg_filenode_relation().
The window where the ERRORs are happen is very narrow, requiring an OID
wraparound to create a lookup conflict in RelidByRelfilenumber() with a
temporary table reusing the same OID as another relation already cached.
The problem is easier to reach in workloads with a high OID consumption
rate, especially with a higher number of temporary relations created.
We could get pg_filenode_relation() and RelidByRelfilenumber() to work
with temporary relations if provided the means to identify them with an
optional proc number given in input, but the years have also shown that
we do not have a use case for it, yet. Note that this could not be
backpatched if pg_filenode_relation() needs changes. It is simpler to
ignore temporary relations.
Reported-by: Shenhao Wang <wangsh.fnst@fujitsu.com>
Author: Vignesh C <vignesh21@gmail.com>
Reviewed-By: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Reviewed-By: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-By: Takamichi Osumi <osumi.takamichi@fujitsu.com>
Reviewed-By: Michael Paquier <michael@paquier.xyz>
Reviewed-By: Masahiko Sawada <sawada.mshk@gmail.com>
Reported-By: Shenhao Wang <wangsh.fnst@fujitsu.com>
Discussion: https://postgr.es/m/bbaaf9f9-ebb2-645f-54bb-34d6efc7ac42@fujitsu.com
Backpatch-through: 13
Branch
------
REL_17_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/dcdc95cb4d4ccf3cd798235387e4b789169babad
Modified Files
--------------
doc/src/sgml/func.sgml | 3 ++-
src/backend/utils/adt/dbsize.c | 3 +++
src/backend/utils/cache/relfilenumbermap.c | 8 ++++++++
src/test/regress/expected/alter_table.out | 5 ++++-
src/test/regress/expected/create_table.out | 12 ++++++++++++
src/test/regress/sql/alter_table.sql | 6 ++++--
src/test/regress/sql/create_table.sql | 8 ++++++++
7 files changed, 41 insertions(+), 4 deletions(-)
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To: pgsql-committers@postgresql.org
Cc: michael@paquier.xyz, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Ignore temporary relations in RelidByRelfilenumber()
In-Reply-To: <E1upFK3-00137e-05@gemulon.postgresql.org>
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