Re: Extremely slow when query uses GIST exclusion index

Am 29.08.2018 um 20:10 schrieb David:
>
> On Wed, Aug 29, 2018 at 7:25 AM, Andreas Kretschmer
> <andreas(at)a-kretschmer(dot)de <mailto:andreas(at)a-kretschmer(dot)de>> wrote:
>
> Okay, other solution. The problem is the nested loop, we can
> disable that:
>
> test=*# set enable_nestloop to false;
>
>
> Is it OK to keep this off permanently in production?

no, but you can switch off/on per session, for instance. and you can it
set to on after that query.

>
>  Nested Loop  (cost=319.27..776.18 rows=1 width=196) (actual
> time=3.156..334.963 rows=10000 loops=1)
>    Join Filter: (app.group_id = member_span.group_id)
> ->  Hash Join  (cost=319.00..771.00 rows=12 width=104) (actual
> time=3.100..14.040 rows=10000 loops=1)
>
>
> Hm, also, it looks like one of the oddities of this query is that
> PostgreSQL is severely underestimating the cardinality of the join.

ack, that's the main problem here, i think. It leads to the expensive
nested loop. Tbh, i don't have a better suggestion now besides the
workaround with setting nestloop to off.

Regards, Andreas

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