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Re: UNION and bad performance

From: "Marc Mamin" <M(dot)Mamin(at)intershop(dot)de>
To: "Olivier Pala" <olivier(dot)pala(at)xgs-france(dot)com>, <pgsql-performance(at)postgresql(dot)org>
Subject: Re: UNION and bad performance
Date: 2010-12-11 10:27:59
Message-ID: C4DAC901169B624F933534A26ED7DF31034BB99A@JENMAIL01.ad.intershop.net (view raw or flat)
Thread:
Lists: pgsql-performance
Hello,

 

UNION will remove all duplicates, so that the result additionally
requires to be sorted.

Anyway,  for performance issues, you should always start investigation
with explain analyze .

regards,

 

Marc Mamin

 

From: pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Olivier
Pala
Sent: Donnerstag, 9. Dezember 2010 11:52
To: pgsql-performance(at)postgresql(dot)org
Cc: Olivier Pala
Subject: [PERFORM] UNION and bad performance

 

Hi, 

I have a performance trouble with UNION query


First I have this view :

    SELECT a,b,c FROM table1, table2 WHERE jointure AND condition

    Result : 150 000 records in 1~2 s



Then, I adding an UNION into the same view :

    SELECT a,b,c FROM table1, table2 WHERE jointure AND condition
    UNION
    SELECT a,b,c FROM table3

    Result : 150 200 records in 6~7 s


Why, do I have bad performance only for 200 adding records ?

Thanks

SGBD : Postgres 8.3 et 8.4 

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Subject: Re: UNION and bad performance
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Subject: Re: Hardware recommendations

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