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@xxxxxxxxxxxxxx
[mailto:pgsql-performance-owner@xxxxxxxxxxxxxx] On Behalf Of Olivier
Pala
Sent: Donnerstag, 9. Dezember 2010 11:52
To: pgsql-performance@xxxxxxxxxxxxxx
Cc: Olivier Pala
Subject: 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
