Re: Improving count(*)

Richard Huxton <dev(at)archonet(dot)com> writes:
> Might it be possible to apply rule-style rewriting to a clause of an
> ordinary select query? That is, is it prohibitively expensive to get PG
> to recognise
> SELECT count(*) FROM big_table
> and replace it with
> SELECT sum(summary_count) FROM my_materialised_view

> This should allow you to have where-clauses and apply to a range of
> cases. What I fear is that checking to see if the rule applies will cost
> too much on all those queries where it doesn't apply.

There is already code in the optimizer that does similar rewriting
for min/max queries. However, that's a hard-wired transformation.
I don't see any very simple way to provide a user-configurable
equivalent.

regards, tom lane