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