Re: Query on postgresql 7.4.2 not using index

Tom Lane wrote:
> Arnau <arnaulist@andromeiberica.com> writes:
>
>
>>  Seq Scan on agenda_users_groups  (cost=0.00..53108.45 rows=339675
>>width=8) (actual time=916.903..5763.830 rows=367026 loops=1)
>>    Filter: (group_id = 9::numeric)
>>  Total runtime: 7259.861 ms
>>(3 filas)
>
>
>>espsm_moviltelevision=# select count(*) from agenda_users_groups ;
>>   count
>>---------
>>  2547556
>
>
> So the SELECT is fetching nearly 15% of the rows in the table.  The
> planner is doing *the right thing* to use a seqscan, at least for
> this particular group_id value.


I have done the same tests on 8.1.0.


espsm_moviltelevision=# EXPLAIN ANALYZE SELECT agenda_user_group_id FROM
agenda_users_groups WHERE group_id = 9;
                                                            QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------------------
  Bitmap Heap Scan on agenda_users_groups  (cost=2722.26..30341.78
rows=400361 width=8) (actual time=145.533..680.839 rows=367026 loops=1)
    Recheck Cond: (group_id = 9::numeric)
    ->  Bitmap Index Scan on i_agnusrsgrs_groupid  (cost=0.00..2722.26
rows=400361 width=0) (actual time=142.958..142.958 rows=367026 loops=1)
          Index Cond: (group_id = 9::numeric)
  Total runtime: 1004.966 ms
(5 rows)

espsm_moviltelevision=# EXPLAIN ANALYZE SELECT agenda_user_group_id FROM
agenda_users_groups WHERE group_id::int8 = 9;
                                                           QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------
  Seq Scan on agenda_users_groups  (cost=0.00..60947.43 rows=12777
width=8) (actual time=457.963..2244.928 rows=367026 loops=1)
    Filter: ((group_id)::bigint = 9)
  Total runtime: 2571.496 ms
(3 rows)

espsm_moviltelevision=# EXPLAIN ANALYZE SELECT agenda_user_group_id FROM
agenda_users_groups WHERE group_id::int8 = '9';
                                                           QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------
  Seq Scan on agenda_users_groups  (cost=0.00..60947.43 rows=12777
width=8) (actual time=407.193..2182.880 rows=367026 loops=1)
    Filter: ((group_id)::bigint = 9::bigint)
  Total runtime: 2506.998 ms
(3 rows)

espsm_moviltelevision=# select count(*) from agenda_users_groups ;
   count
---------
  2555437
(1 row)


   Postgresql then uses the index, I don't understand why? in this
server I tried to tune the configuration, it's because of the tuning?
Because it's a newer version of postgresql?


Thanks for all the replies
--
Arnau