From: Thomas Munro <tmunro@postgresql.org>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Fix extreme skew detection in Parallel Hash Join.
Date: Thu, 17 Oct 2024 09:34:10 +0000
Message-ID: <E1t1MtW-001DTu-NP@gemulon.postgresql.org> (raw)
Fix extreme skew detection in Parallel Hash Join.
After repartitioning the inner side of a hash join that would have
exceeded the allowed size, we check if all the tuples from a parent
partition moved to one child partition. That is evidence that it
contains duplicate keys and later attempts to repartition will also
fail, so we should give up trying to limit memory (for lack of a better
fallback strategy).
A thinko prevented the check from working correctly in partition 0 (the
one that is partially loaded into memory already). After
repartitioning, we should check for extreme skew if the *parent*
partition's space_exhausted flag was set, not the child partition's.
The consequence was repeated futile repartitioning until per-partition
data exceeded various limits including "ERROR: invalid DSA memory alloc
request size 1811939328", OS allocation failure, or temporary disk space
errors. (We could also do something about some of those symptoms, but
that's material for separate patches.)
This problem only became likely when PostgreSQL 16 introduced support
for Parallel Hash Right/Full Join, allowing NULL keys into the hash
table. Repartitioning always leaves NULL in partition 0, no matter how
many times you do it, because the hash value is all zero bits. That's
unlikely for other hashed values, but they might still have caused
wasted extra effort before giving up.
Back-patch to all supported releases.
Reported-by: Craig Milhiser <craig@milhiser.com>
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Discussion: https://postgr.es/m/CA%2BwnhO1OfgXbmXgC4fv_uu%3DOxcDQuHvfoQ4k0DFeB0Qqd-X-rQ%40mail.gmail.com
Branch
------
REL_15_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/1831545ca1860bc8d6faf9c55bbae92a9177e57e
Modified Files
--------------
src/backend/executor/nodeHash.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-committers@postgresql.org
Cc: tmunro@postgresql.org, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Fix extreme skew detection in Parallel Hash Join.
In-Reply-To: <E1t1MtW-001DTu-NP@gemulon.postgresql.org>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox