Re: Autovacuum and visibility maps

On 12/3/24 10:11 AM, Tefft, Michael J 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.

From your OP I took the following literally:

"... a single insert-select".

Take a look at the stat table below:

https://www.postgresql.org/docs/current/monitoring-stats.html#MONITORING-PG-STAT-ALL-TABLES-VIEW

pg_stat_all_tables

For given table and see what the *autovacuum* fields return.

You can use the function below to see if there are per table settings
that are overriding the postgresql.conf settings.

https://www.postgresql.org/docs/current/functions-info.html

pg_options_to_table()

Something like:

select pg_options_to_table(reloptions) from pg_class where relname =
'some_table';

>
> 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>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com