Re: UNION and bad 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