UNION and bad performance

From: Olivier Pala <olivier(dot)pala(at)xgs-france(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Cc: Olivier Pala <olivier(dot)pala(at)xgs-france(dot)com>
Subject: UNION and bad performance
Date: 2010-12-09 10:52:14
Message-ID: 4D00B4DE.3030508@xgs-france.com
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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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