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://www.postgresql.org/docs/current/routine-vacuuming.html#VACUUM-FOR-SPACE-RECOVERY
"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://www.postgresql.org/docs/current/runtime-config-autovacuum.html#GUC-AUTOVACUUM-VACUUM-INSERT-THRESHOLD
"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@xxxxxxxxxxx
