On Tue, 2003-07-22 at 19:27, VanL wrote:
> I have three queries that are essentially identical. Two of them run in
> fractions of a second; one of them takes longer than 15 minutes to
> return. (Not sure how long it totally takes, that has been the limit of
> my patience.)
>
> The only difference between these queries is the use of table aliases in
> the sql query. What is happening in postgres that this makes such a
> difference?
You're getting bit by a 'feature' that is supposed to make life easier.
PostgreSQL automatically adds tables you reference to the FROM clause if
they're not listed there already, it then sends a NOTICE saying it did
so. This is supposed to be helpful, here it certainly is not.
What's happening in the below is you reference mm_batch, mm_domain, etc.
which are _different_ than B, D, etc. because aliasing changes what
query processor refers to them as (allowing stuff like self-joins).
So Pg is appending them to the FROM clause, which now looks like this:
FROM mm_batch B, mm_domain D, mm_management_unit M, mm_customer C,
mm_legacy_account LA, mm_target_account TA, mm_batch, mm_domain,
mm_management_unit, mm_customer, mm_legacy_account,
mm_target_account
Yikes. It's now generating an absolutely huge cartesian product between
the result set you want and all those extra tables.
I believe in the upcoming 7.4.x release this won't happen unless you
enable ADD_MISSING_FROM in your conf. Something I certainly won't be
enabling due to this type of situation.
> SLOW ( > 15 minutes):
> select
> mm_batch.name as batch_name,
> mm_domain.name as domain_name,
> mm_management_unit.name as management_unit_name,
> mm_customer.firstname as customer_name,
> mm_legacy_account.username as old_username,
> mm_target_account.username as new_username
> from
> mm_batch B,
> mm_domain D,
> mm_management_unit M,
> mm_customer C,
> mm_legacy_account LA,
> mm_target_account TA
> where
> mm_domain.bid = mm_batch.id
> and mm_domain.mid = mm_management_unit.id
> and mm_domain.cid = mm_customer.id
> and mm_domain.lid = mm_legacy_account.id
> and mm_domain.tid = mm_target_account.id
> and mm_domain.name = 'example.com';
