RE: Autovacuum and visibility maps - Mailing list pgsql-general

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

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 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$

"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.klaver@aklaver.com