From: | Rafael Thofehrn Castro <rafaelthca(at)gmail(dot)com> |
---|---|
To: | pgsql-bugs(at)lists(dot)postgresql(dot)org |
Subject: | Inconsistent increment of pg_stat_database.xact_rollback with logical replication |
Date: | 2024-06-14 22:15:06 |
Message-ID: | CAG0ozMo_xWQn+Avv8jzbbhePGp5OnhdO+YWTkdg4faWSXz0Jzg@mail.gmail.com |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-bugs |
Column xact_rollback from pg_stat_database gets inconsistently incremented
when logical replication is being used (on publisher side).
This can be easily reproduced in latest code from master branch:
- Publisher
postgres=# select xact_commit, xact_rollback from pg_stat_database where
datname = 'postgres';
-[ RECORD 1 ]-+---
xact_commit | 20
xact_rollback | 0
postgres=# insert into t1 values (1);
INSERT 0 1
postgres=# insert into t1 values (2);
INSERT 0 1
postgres=# insert into t1 values (3);
INSERT 0 1
postgres=# insert into t1 values (4);
INSERT 0 1
postgres=# insert into t1 values (5);
INSERT 0 1
postgres=# insert into t1 values (6);
INSERT 0 1
postgres=# insert into t1 values (7);
INSERT 0 1
postgres=# insert into t1 values (8);
INSERT 0 1
postgres=# insert into t1 values (9);
INSERT 0 1
postgres=# insert into t1 values (10);
INSERT 0 1
postgres=# select xact_commit, xact_rollback from pg_stat_database where
datname = 'postgres';
-[ RECORD 1 ]-+---
xact_commit | 33
xact_rollback | 0
- Subscriber
postgres=# alter subscription sub disable;
ALTER SUBSCRIPTION
- Publisher
postgres=# select xact_commit, xact_rollback from pg_stat_database where
datname = 'postgres';
-[ RECORD 1 ]-+---
xact_commit | 36
xact_rollback | 10
What seems to be happening is that the amount of transactions decoded by
the walsender are being added in pg_stat_database.xact_rollback. But these
changes are only flushed to global stats when the walsender gets terminated.
On a quick look look at the source I would suspect that the issue starts
here:
https://github.com/postgres/postgres/blob/master/src/backend/replication/logical/reorderbuffer.c#L2545
All decoded transactions are aborted for cleanup purposes. Following the
source code flow after calling AbortCurrentTransaction() we eventually
reach the part that increments rollback stats here:
https://github.com/postgres/postgres/blob/master/src/backend/utils/activity/pgstat_database.c#L249
This is causing inconsistency in monitoring TPS metric of a database where
we eventually see sudden spikes of TPS in the order of millions.
Regards,
Rafael Castro.
From | Date | Subject | |
---|---|---|---|
Next Message | Lex Vorona | 2024-06-14 22:16:21 | Re: BUG #18510: jsonpath does not support trailing backslash at the end of the query |
Previous Message | David G. Johnston | 2024-06-14 20:31:47 | Re: BUG #18510: jsonpath does not support trailing backslash at the end of the query |