Re: Make COUNT(*) Faster?

Tom Lane wrote:
> Steve Wampler <swampler(at)noao(dot)edu> writes:
>
>>So, leave COUNT(*) alone. But it would be very handy to have a
>>way to get an approximate table size that is more accurate than is
>>provided by a pg_class.reltuples that is only updated on vacuums.
>
> If you want something cheap, you could use the same technique the
> planner uses nowadays: take RelationGetNumberOfBlocks() (which is
> guaranteed accurate) and multiply by reltuples/relpages. I don't
> see anyplace where RelationGetNumberOfBlocks is directly exposed to
> users now, but it'd be trivial to code up a couple of C functions to
> provide this functionality.

Yes - this would be an excellent approximation for my needs! The
solution that Dawid Kuroczko suggested (just call "explain select *
on ..." and parse the result) would be equivalent these days, right?
(I think in the 7.x versions the planner was just using pg_class.reltuples,
which wouldn't have helped.) If true, I can handle that parsing myself
easily enough without exposing RelationGetNumberOfBlocks.

Thanks (Tom and Dawid)!

--
Steve Wampler -- swampler(at)noao(dot)edu
The gods that smiled on your birth are now laughing out loud.