Date: Mon, 22 Jun 2020 14:29:06 -0500
From: Kenneth Marshall <ktm@rice.edu>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-performance@lists.postgresql.org
Subject: Re: PostgreSQL 12.3 slow index scan chosen
Message-ID: <20200622192906.GD1497@aart.rice.edu>
References: <1815407.1592677323@sss.pgh.pa.us>
 <20200622192732.GA29072@alvherre.pgsql>
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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