Re: Why isn't an index scan being used?

Thank you for the help.

> If the "closed_index" index is large than the "state_index", then doing
an Index scan on "closed_index" is going to be costed higher.

FWIW, both indexes appear to be the same size:

select pg_size_pretty(pg_relation_size('state_index'));
1144 kB

select pg_size_pretty(pg_relation_size('closed_index'));
1144 kB

> Most of this likely boils down to random_page_cost being a guess. You may
want to check your effective_cache_size is set to something like 75% of the
machine's memory, and/or tweak random page cost down, if it's set to the
standard 4 setting.

Ok, let me try this.

On Tue, Feb 19, 2019 at 5:51 PM David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
wrote:

> On Wed, 20 Feb 2019 at 13:11, Abi Noda <a(at)abinoda(dot)com> wrote:
> > However, when I index the closed column, a bitmap scan is used instead
> of an index scan, with slightly slower performance. Why isn't an index scan
> being used, given that the exact same number of rows are at play as in my
> query on the state column?
>
> That's down to the planner's cost estimates. Likely it thinks that
> either doing a bitmap scan is cheaper, or close enough that it does
> not matter.
>
> > How do I index closed in a way where an index scan is used?
>
> The costing does account for the size of the index. If the
> "closed_index" index is large than the "state_index", then doing an
> Index scan on "closed_index" is going to be costed higher.
>
> Most of this likely boils down to random_page_cost being a guess. You
> may want to check your effective_cache_size is set to something like
> 75% of the machine's memory, and/or tweak random page cost down, if
> it's set to the standard 4 setting. modern SSDs are pretty fast at
> random reads. HDDs, not so much.
>
> --
> David Rowley http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services
>