Re: UNION and bad performance - Mailing list pgsql-performance

From Marc Mamin
Subject Re: UNION and bad performance
Date
Msg-id C4DAC901169B624F933534A26ED7DF31034BB99A@JENMAIL01.ad.intershop.net
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In response to UNION and bad performance  (Olivier Pala <olivier.pala@xgs-france.com>)
Responses Re: UNION and bad performance  (Andreas Kretschmer <akretschmer@spamfence.net>)
List 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@postgresql.org [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of Olivier Pala
Sent: Donnerstag, 9. Dezember 2010 11:52
To: pgsql-performance@postgresql.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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