When in doubt, "manually" vacuum and/or analyze.
Maybe even disable autovacuum on that table before the TRUNCATE + INSERT, do the "manual" vacuum-analyze and then re-enable autovacuum. Bonus points for programmatically determining which partitions you're going to insert into, so that you only manually maintain those partitions.
On Tue, Dec 3, 2024 at 1:11 PM Tefft, Michael J <Michael.J.Tefft@xxxxxxxxxx> wrote:
Thanks for the point about truncates versus deletes.
But most of these partitions have over 100k rows, all inserted at once. We have the default setting:
#autovacuum_vacuum_insert_threshold = 1000 # min number of row inserts
So I thought we should be triggering by inserts.
Mike
From: Adrian Klaver <adrian.klaver@xxxxxxxxxxx>
Sent: Tuesday, December 3, 2024 11:57 AM
To: Tefft, Michael J <Michael.J.Tefft@xxxxxxxxxx>; pgsql-general@xxxxxxxxxxxxxxxxxxxx
Subject: Re: Autovacuum and visibility maps
On 12/3/24 08: 32, Tefft, Michael J wrote: > We have some batch queries that had occasionally having degraded > runtimes: from 2 hours degrading to 16 hours, etc. > > Comparing plans from good and bad runs, we saw that the good plans
On 12/3/24 08:32, Tefft, Michael J wrote:> We have some batch queries that had occasionally having degraded> runtimes: from 2 hours degrading to 16 hours, etc.>> Comparing plans from good and bad runs, we saw that the good plans used> index-only scans on table “x”, while the bad plans used index scans.>> Using the pg_visibility utility, we found that all of the 83 partitions> of table “x” were showing zero blocks where all tuples were visible. We> ran a VACUUM on the table; the visibility maps are now clean and the> good plans came back.>> Our question is: why did autovacuum not spare us from this?>> We are using default autovacuum parameters for all except> log_autovacuum_min_duration=5000. These partitions are populated by> processes that do a truncate + a single insert-select.>> We see autovacuum failure (failed to get lock) messages, followed by a> success message, in the log for one of these partitions (the biggest> one) but even that partition showed zero blocks with all tuples visible.>> Are we wrong to expect autovacuum to clean up the visibility map?I have to believe it is due to this:https://urldefense.com/v3/__https://www.postgresql.org/docs/current/routine-vacuuming.html*VACUUM-FOR-SPACE-RECOVERY__;Iw!!Lf_9VycLqA!mGufXaOdGX6PdXSpHcIUnIF1pe8evFpE7r-l4vJVUcoY--jp8LtF-jWv8YicvFWegi1-_jyxJnNx3YBvbxQOracZSxzvbw$"If you have a table whose entire contents are deleted on a periodicbasis, consider doing it with TRUNCATE rather than using DELETE followedby VACUUM. TRUNCATE removes the entire content of the table immediately,without requiring a subsequent VACUUM or VACUUM FULL to reclaim thenow-unused disk space. The disadvantage is that strict MVCC semanticsare violated."Combined with this:https://urldefense.com/v3/__https://www.postgresql.org/docs/current/runtime-config-autovacuum.html*GUC-AUTOVACUUM-VACUUM-INSERT-THRESHOLD__;Iw!!Lf_9VycLqA!mGufXaOdGX6PdXSpHcIUnIF1pe8evFpE7r-l4vJVUcoY--jp8LtF-jWv8YicvFWegi1-_jyxJnNx3YBvbxQOraeerEd0yw$"autovacuum_vacuum_thresholdSpecifies the minimum number of updated or deleted tuples needed totrigger a VACUUM in any one table. ..."I'm going to say the TRUNCATE itself does not trigger an autovacuum. Iwould suggest throwing a manual VACUUM in the table population script.>> postgres=# select version();>> version>> ---------------------------------------------------------------------------------------------------------->> PostgreSQL 14.13 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0> 20210514 (Red Hat 8.5.0-22), 64-bit>> Thank you,>> Mike Tefft>--Adrian Klaveradrian.klaver@xxxxxxxxxxx
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!
