Re: planner doesn't use bitmap index

2015-10-29 19:20 GMT+01:00 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:

> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> > -- I was surprised, so following query can use index
> > postgres=# explain select a from test2 where a at time zone
> > 'America/Santiago' >= now() at time zone 'America/Santiago' ;
> > QUERY
> > PLAN
> >
> ══════════════════════════════════════════════════════════════════════════════════════════════
> > Index Only Scan using test2_a_idx on test2 (cost=0.13..12.18 rows=1
> > width=8)
> > Filter: (timezone('America/Santiago'::text, a) >=
> > timezone('America/Santiago'::text, now()))
> > (2 rows)
>
> This plan isn't actually "using" the index in any meaningful way; it's
> applying the where condition as a filter. It happens to be sane to use
> the index as a dumb data source, because it can be an index-only scan, and
> that might (if you're lucky and don't hit too many recheckable rows) be
> cheaper than a seqscan. But we don't consider plain indexscans as worth
> the trouble to consider in such cases, because a full-table plain
> indexscan can never beat a seqscan, either in the planner's cost model or
> in reality.
>
> > why, the index isn't used in this case?
> > postgres=# explain select a,b from test2 where a at time zone
> > 'America/Santiago' >= now() at time zone 'America/Santiago' ;
>
> Can't be an index-only scan because of the use of b, so there's no
> possible way that this can be better than a seqscan.
>

I understand.

Than you for explanation.

Regards

Pavel

>
> regards, tom lane
>