Re: timestamp query doesn't use an index ...

On Sun, 21 May 2006, Tom Lane wrote:

> "Marc G. Fournier" <scrappy(at)postgresql(dot)org> writes:
>> I'm trying to figure out some way to speed up the following query:
>
>> select ps2.page_id, ps2.template_component_id, max(ps2.start_time)
>> from page_schedule ps2
>> where ps2.start_time at time zone 'MST7MDT' <= '2006-5-17 8:9:18'
>> group by ps2.page_id, ps2.template_component_id
>
>> Is there some other way I can either write above query *or* do an index,
>> such that it will use the index?
>
> One-sided inequalities frequently *shouldn't* use an index, because
> they're retrieving too much of the table. Are you sure this is fetching
> only a small fraction of the table? Are you using PG 8.1 (8.1 would be
> likely to try to use a bitmap indexscan for this)?
>
> You could experiment with enable_seqscan = off to see if the planner is
> actually wrong about its choice. If so, reducing random_page_cost might
> be the best permanent solution.

vrnprd=# select version();
version
------------------------------------------------------------------------------------------------
PostgreSQL 8.1.3 on i386-portbld-freebsd6.1, compiled by GCC cc (GCC) 3.4.4 [FreeBSD] 20050518
(1 row)

vrnprd=# set enable_seqscan = off;
SET
vrnprd=# explain analyze
select ps2.page_id, ps2.template_component_id, max(ps2.start_time)
from page_schedule ps2
where ps2.start_time at time zone 'MST7MDT' <= '2006-5-17 8:9:18'
group by ps2.page_id, ps2.template_component_id
;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=3509.96..3513.50 rows=283 width=16) (actual time=839.460..839.769 rows=128 loops=1)
-> Bitmap Heap Scan on page_schedule ps2 (cost=573.65..2822.86 rows=91614 width=16) (actual time=149.788..505.438 rows=94798 loops=1)
Recheck Cond: (timezone('MST7MDT'::text, start_time) <= '2006-05-17 08:09:18'::timestamp without time zone)
-> Bitmap Index Scan on start_time_page_schedule (cost=0.00..573.65 rows=91614 width=0) (actual time=127.761..127.761 rows=94798 loops=1)
Index Cond: (timezone('MST7MDT'::text, start_time) <= '2006-05-17 08:09:18'::timestamp without time zone)
Total runtime: 846.604 ms
(6 rows)

vrnprd=#

And yup, it is definitely returning just 128 rows out of the 93k or so:

110 | 419 | 2005-10-26 13:15:00-03
130 | 215 | 2006-04-26 10:15:00-03
(128 rows)

And, not sure how far to reduce random_page_cost, but it was 4 ... tried 2
and 1, and both come up with the same results ... with seqscan enabled, it
does a seqscan :(

I suspected with the <= there wasn't going to be much I could do with
this, but figured I'd make sure there wasn't something that I was
overlooking :(

Thx ...

----
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