Re: Large data and slow queries

Greetings,

* Samuel Williams (space(dot)ship(dot)traveller(at)gmail(dot)com) wrote:
> Thanks for all the suggestions Stephen.
>
> > That explain analyze shows a whole ton of heap fetches. When was the
> last time a VACUUM was run on this table, to build the visibility map?
> Without the visibility map being current, an Index-Only Scan, as is
> happening here, can really suck.
>
> I'm using the default 9.6 config, I thought that auto-vacuum was on by default?

Sure, but that doesn't mean the table gets vacuumed. In particular,
actual vacuums are only kicked off when the number of *updated* or
*deleted* tuples passes the autovacuum threshold. If no updates or
deletes are done on the table (iow, it's essentially an insert-only or
insert-mostly table), the autovacuum daemon nevers runs a vacuum on it
(though it'll run analyze's).

https://www.postgresql.org/docs/current/static/runtime-config-autovacuum.html#GUC-AUTOVACUUM-VACUUM-THRESHOLD

Check pg_stat_all_tables to see when/if the table has actually been
vacuumed.

Thanks!

Stephen