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Date: Thu, 26 Aug 2010 07:24:20 +0200
From: "A. Kretschmer" <andreas.kretschmer@schollglas.com>
To: pgsql-performance@postgresql.org
Subject: Re: SubQuery Performance
Message-ID: <20100826052419.GB25058@a-kretschmer.de>
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In response to mike :
> Hi All,
> 
> I have a poor performance SQL as following. The table has about 200M
> records, each employee have average 100 records. The query lasts about
> 3 hours. All I want is to update the flag for highest version of each
> client's record. Any suggestion is welcome!
> 
> Thanks,
> 
> Mike
> 
> 
> ====SQL===========
> update empTbl A
> set flag=1
> where
> rec_ver =
> ( select max(rec_ver)
> from empTbl
> where empNo = A.empNo)
> 
> 
> 
> ===Table empTbl=====
> 
> empTbl
> {
> int empNo;
> int flag;
> char[256] empDesc;
> int rec_ver;
> }

Try this:

update empTbl A set flag=1 from (select empno, max(rec_ver) as rec_ver from empTbl group by empno) foo where (a.empno,a.rec_ver) = (foo.empno, foo.rec_ver);

You should create an index on empTbl(empNo,rec_ver).

Please show us the EXPLAIN ANALYSE <query> for both selects.


Regards, Andreas
-- 
Andreas Kretschmer
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