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.te...@snapon.com> 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.kla...@aklaver.com> > *Sent:* Tuesday, December 3, 2024 11:57 AM > *To:* Tefft, Michael J <michael.j.te...@snapon.com>; > pgsql-general@lists.postgresql.org > *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$ > > <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$> > > > > "If you have a table whose entire contents are deleted on a periodic > > basis, consider doing it with TRUNCATE rather than using DELETE followed > > by VACUUM. TRUNCATE removes the entire content of the table immediately, > > without requiring a subsequent VACUUM or VACUUM FULL to reclaim the > > now-unused disk space. The disadvantage is that strict MVCC semantics > > are violated." > > > > Combined with this: > > > > https://urldefense.com/v3/__https://www.postgresql.org/docs/current/runtime-config-autovacuum.html*GUC-AUTOVACUUM-VACUUM-INSERT-THRESHOLD__;Iw!!Lf_9VycLqA!mGufXaOdGX6PdXSpHcIUnIF1pe8evFpE7r-l4vJVUcoY--jp8LtF-jWv8YicvFWegi1-_jyxJnNx3YBvbxQOraeerEd0yw$ > > <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_threshold > > > > Specifies the minimum number of updated or deleted tuples needed to > > trigger a VACUUM in any one table. ... > > > > " > > > > I'm going to say the TRUNCATE itself does not trigger an autovacuum. I > > would 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 Klaver > > adrian.kla...@aklaver.com > > > > -- Death to <Redacted>, and butter sauce. Don't boil me, I'm still alive. <Redacted> lobster!
