> Johan Fredriksson <es...@kth.se> writes:
> > Bad plan: https://explain.depesz.com/s/avtZ
> > Good plan: https://explain.depesz.com/s/SJSt
> > Any suggestions on how to make the planner make better decisions for
> > this query?
> Core of the problem looks to be the misestimation here:
>        Index Only Scan using shredder_cgm1 on public.cachedgroupmembers 
> cachedgroupmembers_4
> (cost=0.43..2.33 rows=79 width=8) (actual time=0.020..0.903 rows=1492 
> loops=804)
>          Output: cachedgroupmembers_4.memberid, cachedgroupmembers_4.groupid,
> cachedgroupmembers_4.disabled
>          Index Cond: ((cachedgroupmembers_4.memberid = principals_1.id) AND
> (cachedgroupmembers_4.disabled = 0))
>          Heap Fetches: 5018
> Probably, memberid and disabled are correlated but the planner doesn't
> know that, so it thinks the index condition is way more selective than it
> actually is.  In PG 10, you could very possibly fix that by installing
> extended statistics on that pair of columns.  See
> https://www.postgresql.org/docs/current/static/planner-stats.html#PLANNER-STATS-EXTENDED

I'm not sure what you mean by correlated, but there are only a handful (164 
when I check it) disabled groupmembers out of total 7.5 million.
I'll give CREATE STATISTICS on those columns a shot and see if it gets any 

        / Eskil

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