Message-ID: <51232A36.6000701@orprovision.com>
Date: Tue, 19 Feb 2013 08:31:02 +0100
From: Andy <andy.gumbrecht@orprovision.com>
Reply-To: andy.gumbrecht@orprovision.com
Organization: OrproVision.com
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To: pgsql-performance@postgresql.org
Subject: Re: Speed of exist
References: <51231D10.1070101@basenlily.me>
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Limit the sub-queries to 1, i.e. :

select 1 from Table2 where Table2.ForeignKey = Table1.PrimaryKey fetch first 1 rows only

Andy.

On 19.02.2013 07:34, Bastiaan Olij wrote:
> Hi All,
>
> Hope someone can help me a little bit here:
>
> I've got a query like the following:
> --
> select Column1, Column2, Column3
> from Table1
> where exists (select 1 from Table2 where Table2.ForeignKey =
> Table1.PrimaryKey)
> or exists (select 1 from Table3 where Table3.ForeignKey = Table1.PrimaryKey)
> --
>
> Looking at the query plan it is doing a sequential scan on both Table2
> and Table3.
>
> If I remove one of the subqueries and turn the query into:
> --
> select Column1, Column2, Column3
> from Table1
> where exists (select 1 from Table2 where Table2.ForeignKey =
> Table1.PrimaryKey)
> --
>
> It is nicely doing an index scan on the index that is on Table2.ForeignKey.
>
> As Table2 and Table3 are rather large the first query takes minutes
> while the second query takes 18ms.
>
> Is there a way to speed this up or an alternative way of selecting
> records from Table1 which have related records in Table2 or Table3 which
> is faster?
>
> Kindest Regards,
>
> Bastiaan Olij
>
>
>

-- 
------------------------------------------------------------------------------------------------------------------------

*Andy Gumbrecht*
Research & Development
Orpro Vision GmbH
Hefehof 24, 31785, Hameln

+49 (0) 5151 809 44 21
+49 (0) 1704 305 671
andy.gumbrecht@orprovision.com
www.orprovision.com


            Orpro Vision GmbH
            Sitz der Gesellschaft: 31785, Hameln
            USt-Id-Nr: DE264453214
            Amtsgericht Hannover HRB204336
            Geschaeftsfuehrer: Roberto Gatti, Massimo Gatti, Adam Shaw

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