Re: PostgreSQL 12.3 slow index scan chosen

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.

--
Álvaro Herrera https://www.2ndQuadrant.com/
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