Re: PostgreSQL 12.3 slow index scan chosen

On Mon, Jun 22, 2020 at 03:27:32PM -0400, Alvaro Herrera wrote:
> On 2020-Jun-20, Tom Lane wrote:
>
> > I wrote:
> > > ... oh, now I see: apparently, your filter condition is such that *no*
> > > rows of the objectcustomfieldvalues table get past the filter:
> > >
> > > -> Index Scan using objectcustomfieldvalues3 on objectcustomfieldvalues objectcustomfieldvalues_1 (cost=0.56..807603.40 rows=915 width=4) (actual time=21165.441..21165.441 rows=0 loops=1)
> > > Filter: ((disabled = 0) AND ((largecontent ~~* '%958575%'::text) OR ((content)::text ~~* '%958575%'::text)))
> > > Rows Removed by Filter: 19030904
>
> > You said you'd increased the stats target for
> > objectcustomfieldvalues.objectid, but maybe the real problem is needing
> > to increase the targets for content and largecontent, in hopes of driving
> > down the estimate for how many rows will pass this filter condition.
>
> ... but those on content and largecontent are unanchored conditions --
> are we still able to do any cardinality analysis using those? I thought
> not. Maybe a trigram search would help? See contrib/pg_trgm -- as far
> as I remember that module is able to work with LIKE conditions.
>

Hi Alvaro,

I do have a pg_trgm GIN index on those fields for the search.

Regards,
Ken