Re: Contradicting information on the "vacuum threshold"

On 11 August 2017 at 23:54, Erwin Brandstetter <brsaweda(at)gmail(dot)com> wrote:

> On the one hand the manual for autovacuum_analyze_threshold claims here:
>
> https://www.postgresql.org/docs/current/static/runtime-
> config-autovacuum.html#GUC-AUTOVACUUM-ANALYZE-THRESHOLD
>
> > Specifies the minimum number of inserted, updated or deleted tuples
> needed to trigger an ANALYZE in any one table.
>
> Similar in pg_settings.short_desc:
>
> > Minimum number of tuple inserts, updates, or deletes prior to analyze.
>
> "Minimum" indicates "row_count >= vacuum threshold". (Well "prior" makes
> that less clear ..)
>
> On the other hand the manual explains here:
>
> https://www.postgresql.org/docs/current/static/routine-
> vacuuming.html#AUTOVACUUM
>
> > Otherwise, if the number of tuples obsoleted since the last VACUUM
> exceeds the "vacuum threshold", the table is vacuumed.
>
> "Exceeds" indicates "row_count > vacuum threshold".
>
> Actual test results seem to support "row_count > vacuum threshold". See
> test case here:
>
> https://dba.stackexchange.com/questions/181960/tiny-table-
> causes-extreme-performance-degradation-fixed-by-forced-
> vacuum-why/183283#183283
>
> I suggest to either clarify the manual or change the code to actually use
> >= instead of > ... and update the explanation for autovacuum accordingly.
>
> Same for autovacuum_vacuum_threshold.
>

Still incorrect in the manual. The specified number of obsoleted rows does
*not* yet trigger an ANALYZE. Only the next one does. Off-by-one error.

A simple fix could be to replace:
*minimum number of inserted, updated or deleted tuples *
with:
* number of inserted, updated or deleted tuples that must be exceeded*

Regards
Erwin