RE: Autovacuum and visibility maps

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(dot)klaver(at)aklaver(dot)com>
Sent: Tuesday, December 3, 2024 11:57 AM
To: Tefft, Michael J <Michael(dot)J(dot)Tefft(at)snapon(dot)com>; pgsql-general(at)lists(dot)postgresql(dot)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(dot)klaver(at)aklaver(dot)com<mailto:adrian(dot)klaver(at)aklaver(dot)com>