UNION and bad performance

[Olivier Pala <olivier.pala@xxxxxxxxxxxxxx>]

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