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 |
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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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