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

[Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>]

"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.

			regards, tom lane