Richard Huxton escreveu:
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.
Remember that you can always use serial fields to count a table, like:
alter table foo add id serial;
select id from foo order by id desc limit 1;
This should return the same value than count(*), in a few msecs.
--
ACV
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