Re: An "obvious" index not being used

>>>> Daniele Varrazzo <piro@develer.com> wrote:
>
>>      select count(*) from foo
>>      where foo.account_id in (
>>          select id from accounts where system = 'abc');
>
>>   Total runtime: 13412.226 ms
>
> Out of curiosity, how does it do with the logically equivalent?:
>
> select count(*) from foo
> where exists (select * from accounts
>   where accounts.id = foo.account_id
>     and accounts.system = 'abc');

I tried it: it is slower and the query plan still includes the seqscan:

 Aggregate  (cost=44212346.30..44212346.31 rows=1 width=0) (actual
time=21510.468..21510.469 rows=1 loops=1)
   ->  Seq Scan on foo  (cost=0.00..44205704.40 rows=2656760 width=0)
(actual time=0.058..21402.752 rows=92790 loops=1)
         Filter: (subplan)
         SubPlan
           ->  Index Scan using accounts_pkey on accounts  (cost=0.00..8.27
rows=1 width=288) (actual time=0.002..0.002 rows=0 loops=5313519)
                 Index Cond: (id = $0)
                 Filter: (("system")::text = 'abc'::text)
 Total runtime: 21510.531 ms

Here the estimate is even more gross: 2656760 is exactly the 50% of the
records in the table.

--
Daniele Varrazzo - Develer S.r.l.
http://www.develer.com