Re: Slow query with 'or' clause

philippe wrote:
> explain analyse
> select count(*) from client_contact
> left join client_company using(cli_id)
> where (cli_mail = 'xxx@xxx.xx') OR
> (lower(cli_nom) = 'xxxxxx' and zipcode = '10001');
>
> QUERY PLAN
>
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>  Aggregate  (cost=37523.98..37523.99 rows=1 width=0) (actual
> time=3871.086..3871.087 rows=1 loops=1)
>    ->  Merge Left Join  (cost=0.00..36719.10 rows=321952 width=0) (actual
> time=3871.058..3871.058 rows=0 loops=1)

This is the root of the problem - it's expecting to match over 320000
rows rather than 0.

I'm guessing there's a lot of correlation between cli_mail and cli_nom
(you're expecting them to match the same clients) but the planner
doesn't know this.

If this is a common query, you could try an index on zipcode - that
might cut down the other side.

However, I have to ask why you're using a left-join? Do you really have
rows in client_contact without a matching cli_id in client_company?

--
   Richard Huxton
   Archonet Ltd