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help / color / mirror / Atom feedFrom: Tefft, Michael J <[email protected]>
To: Adrian Klaver <[email protected]>
To: [email protected] <[email protected]>
Subject: RE: Autovacuum and visibility maps
Date: Tue, 3 Dec 2024 18:11:21 +0000
Message-ID: <BN8PR04MB62890EE42B888F3A0B455F76D0362@BN8PR04MB6289.namprd04.prod.outlook.com> (raw)
In-Reply-To: <[email protected]>
References: <BN8PR04MB6289F7099F7B38E5B08D85B7D0362@BN8PR04MB6289.namprd04.prod.outlook.com>
<[email protected]>
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 <[email protected]>
Sent: Tuesday, December 3, 2024 11:57 AM
To: Tefft, Michael J <[email protected]>; [email protected]
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-FO...;
"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*G...;
"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
[email protected]<mailto:[email protected]>
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Subject: RE: Autovacuum and visibility maps
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