Reg Me Please wrote:
> While I would not spend resources in fine tuning the count(*), I would
> spend some to underastand why and how the other ones do it better.
>
> Just to be better.
The problem is well understood, and there is extensive discussion in the
mailing lists archives. The basic problem is that with PG's
implementation of MVCC the indexes don't have row visibility
information. The simple solution of adding it to every index entry would
increase index size substantially imposing costs on every index access
and update.
There's a thread in -hackers called "Visibility map thoughts" that is
looking at the situation again and if/how to implement visibility
information in a compact form.
--
Richard Huxton
Archonet Ltd