Question

I’ve got a simple query (postgresql if that matters) that retrieves all items for some_user excluding the ones she has on her wishlist:

select i.* 
from core_item i 
left outer join core_item_in_basket b on (i.id=b.item_id and b.user_id=__some_user__)
where b.on_wishlist is null;

The above query runs in ~50000ms (yep, the number is correct).
If I remove the “b.on_wishlist is null” condition or make it “b.on_wishlist is not null”, the query runs in some 50ms (quite a change).

The query has more joins and conditions but this is irrelevant as only this one slows it down.

Some info on the database size:

• core_items has ~ 10.000 records
• core_user has ~5.000 records
• core_item_in_basket has ~2.000
• records (of which some 50% has
  on_wishlist = true, the rest is null)

I don’t have any indexes (except for ids and foreign keys) on those two tables.

The question is: what should I do to make this run faster? I’ve got a few ideas myself to check out this evening, but I’d like you guys to help if possible, as well.

Thanks!

Answer 1

Sorry for adding 2nd answer, but stackoverflow doesn’t let me format comments properly, and since formatting is essential, I have to post answer.

Couple of options:

1. CREATE INDEX q ON core_item_in_basket (user_id, item_id) WHERE on_wishlist is null;
2. same index, but change order of columns in it.
3. SELECT i.* FROM core_item i WHERE i.id not in (select item_id FROM core_item_in_basket WHERE on_wishlist is null AND user_id = __some_user__); (this query can benefit from index from point #1, but will not benefit from index #2.
4. SELECT * from core_item where id in (select id from core_item EXCEPT select item_id FROM core_item_in_basket WHERE on_wishlist is null AND user_id = __some_user__);

Let us know the results 🙂