Re: Query plan not using index for some reason.

On Wed, 2 Oct 2002, Jean-Christian Imbeault wrote:

> One of my SQL is is slow so I tried using EXPLAIN to find out why but
> the query plan is gives seems bad ... it's not using indexes ...
>
> The query is on two tables, both of which have indexes. When I check
> EXPLAIN for the query without the OR clause the planner uses the index.
> When I add the OR clause it uses a seq scan ...
>
> Is the planner right in choosing a seq scan?

Given it wants to use one index in a scan, probably, since I don't think
either of those indexes will help that full condition if I'm guessing the
schema correctly.  I can't see an index on id helping when you also
need to get all the maker_id=53 rows and the one on maker_id doesn't
seem like it'd help the joining of the tables on id.

Hmm, maybe
select products.id as pid from products where maker_id='53'
 union
select products.id as pid from products, rel_genres_movies where
 (rel_genres_movies.minor_id='11' AND rel_genres_movies.prod_id=products.id)"
would run better (or union all if it's safe to have duplicates).

An index on id doesn't help since you also need to get
all the rows where maker_id is 53 and one on maker_id doesn't help join
ids.