Message-ID: <546B0912.7090206@gmail.com>
Date: Tue, 18 Nov 2014 08:53:38 +0000
From: Tim Dudgeon <tdudgeon.ml@gmail.com>
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To: pgsql-sql@postgresql.org
Subject: Re: slow sub-query problem
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Tom,

thanks. I did a vacuum of the table and unfortunately it didn't help.
But a good spot.

Tim


On 17/11/2014 20:10, Tom Lane wrote:
> Tim Dudgeon <tdudgeon.ml@gmail.com> writes:
>> I'm having problems optimising a query that's very slow due to a sub-query.
> I think it might get better if you could fix this misestimate:
>
>> "              ->  Bitmap Index Scan on idx_sp_property_id
>>    (cost=0.00..33.90 rows=1146 width=0) (actual time=51.656..51.656 rows=811892 loops=366)"
>> "                    Index Cond: (property_id = ANY ('{1,643413,1106201}'::integer[]))"
> 1146 estimated vs 811892 actual is pretty bad, and it doesn't seem like
> this is a very hard case to estimate.  Are the stats for structure_props
> up to date?  Maybe you need to increase the statistics target for the
> property_id column.
>
> Another component of the bad plan choice is this misestimate:
>
>> "  ->  HashAggregate  (cost=1091.73..1091.75 rows=2 width=4) (actual time=2.829..3.212 rows=366 loops=1)"
>> "        Group Key: structure_props_1.structure_id"
> but it might be harder to do anything about that one, since the result
> depends on the property_id being probed; without cross-column statistics
> it may be impossible to do much better.
>
> 			regards, tom lane


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