Re: Need to increase performance of a query

Craig James wrote:
> On 6/10/10 12:34 PM, Anne Rosset wrote:
>> Jochen Erwied wrote:
>>> Thursday, June 10, 2010, 8:36:08 PM you wrote:
>>>
>>>> psrdb=# (SELECT
>>>> psrdb(# MAX(item_rank.rank) AS maxRank
>>>> psrdb(# FROM
>>>> psrdb(# item_rank item_rank
>>>> psrdb(# WHERE
>>>> psrdb(# item_rank.project_id='proj2783'
>>>> psrdb(# AND item_rank.pf_id IS NULL
>>>> psrdb(#
>>>> psrdb(# )
>>>> psrdb-# ORDER BY
>>>> psrdb-# maxRank DESC;
>>>
>>> Don't think it does really matter, but why do you sort a resultset
>>> consisting of only one row?
>>>
>> Sorry, I should have removed the ORDER by (the full query has a union).
>> So without the ORDER by, here are the results:
>> psrdb=# SELECT
>> psrdb-# MAX(item_rank.rank) AS maxRank
>> psrdb-# FROM
>> psrdb-# item_rank item_rank
>> psrdb-# WHERE
>> psrdb-# item_rank.pf_id='plan1408';
>> maxrank
>> -------------
>> 20504000000
>> (1 row)
>>
>> Time: 1.516 ms
>> psrdb=# SELECT
>> psrdb-# MAX(item_rank.rank) AS maxRank
>> psrdb-# FROM
>> psrdb-# item_rank item_rank
>> psrdb-# WHERE
>> psrdb-# item_rank.project_id='proj2783'
>> psrdb-# AND item_rank.pf_id IS NULL;
>> maxrank
>> -------------
>> 20200000000
>> (1 row)
>>
>> Time: 13.177 ms
>>
>> Is there anything that can be done for the second one?
>
> Postgres normally doesn't index NULL values even if the column is
> indexed, so it has to do a table scan when your query includes an IS
> NULL condition.  You need to create an index that includes the "IS
> NULL" condition.
>
>   create index item_rank_null_idx on item_rank(pf_id)
>    where item_rank.pf_id is null;
>
> Craig
>
Hi Craig,
I tried again after adding your suggested index but I didn't see any
improvements: (seems that the index is not used)
psrdb=# explain analyze SELECT
psrdb-#        MAX(item_rank.rank) AS maxRank
psrdb-# FROM
psrdb-#        item_rank item_rank
psrdb-# WHERE
psrdb-#        item_rank.project_id='proj2783'
psrdb-#         AND item_rank.pf_id IS NULL;

QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------------------------------
 Result  (cost=0.17..0.18 rows=1 width=0) (actual time=11.942..11.943
rows=1 loops=1)
   InitPlan
     ->  Limit  (cost=0.00..0.17 rows=1 width=8) (actual
time=11.931..11.932 rows=1 loops=1)
           ->  Index Scan Backward using item_rank_rank on item_rank
(cost=0.00..2933.84 rows=17558 width=8) (actual time=11.926..11.926
rows=1 loops=1)
                 Filter: ((rank IS NOT NULL) AND (pf_id IS NULL) AND
((project_id)::text = 'proj2783'::text))
 Total runtime: 11.988 ms
(6 rows)

Time: 13.654 ms


Thanks,
Anne