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help / color / mirror / Atom feedFrom: Amit Kapila <akapila@postgresql.org>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Reset conflict slot's xmin when a subscription (re)starts retent
Date: Mon, 07 Sep 2026 09:24:20 +0000
Message-ID: <E1x3VaN-00000003pKY-2OXw@gemulon.postgresql.org> (raw)
Reset conflict slot's xmin when a subscription (re)starts retention.
Since the retain_dead_tuples feature uses a single cluster-wide
replication slot across all subscriptions, adding a new subscription on a
database with an older active transaction can require
conflict_detection_slot.xmin to move backward to avoid missing dead tuples
needed for update_deleted conflict detection.
Previously, if a newly created subscription had an older active
transaction than the slot's current xmin, the slot update occurred during
worker apply, leading to an assertion failure.
Fix this by having the launcher track the set of databases with active
tuple-retaining subscriptions. When a database joins this set (either via
a new subscription or when retention restarts), the launcher resets the
slot's xmin to the cluster-wide safe decoding horizon before starting any
workers. Because this horizon accounts for all active transactions across
the entire cluster, it serves as a safe seed without requiring retroactive
backward movement by individual apply workers.
In passing, update the retain_dead_tuples documentation to clarify two
operational boundary cases:
(a) Conflict data continues to accumulate even when a subscription is
disabled or its apply worker is stopped.
(b) track_commit_timestamp must be enabled for conflict detection to
function properly.
Reported-by: Nisha Moond <nisha.moond412@gmail.com>
Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Author: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Nisha Moond <nisha.moond412@gmail.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Discussion: https://postgr.es/m/TY4PR01MB177182F547A62FC2666EC04EC94B72@TY4PR01MB17718.jpnprd01.prod.outlook.com
Backpatch-through: 19, where it was introduced
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/6168c65ddcac4494fb292418cc774897791e0667
Modified Files
--------------
doc/src/sgml/ref/create_subscription.sgml | 31 +++++-
src/backend/replication/logical/launcher.c | 173 ++++++++++++++++++++---------
src/test/subscription/t/035_conflicts.pl | 73 ++++++++++++
3 files changed, 221 insertions(+), 56 deletions(-)
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Subject: Re: pgsql: Reset conflict slot's xmin when a subscription (re)starts retent
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