Re: SELECT COUNT(*) does a scan?

See Oliver's post:

Additionally you can get count to use an index, but you need a where
clause.

Dave
On 8-Sep-05, at 11:22 AM, David Wall wrote:

> When I do an EXPLAIN SELECT COUNT(*) FROM tablename, I noted that
> it does a table scan. I thought PG had some sort of table stat
> that kept track of the current number of rows in a table, but that
> doesn't appear to always be the case.
>
> It seems that right after a VACUUM ANALYZE, that command is very
> fast (on a table with 100,000+ rows), but it can also get quite
> slow, as if a table scan is taking place.
> Does this make sense? Is there an algorithm that says to use the
> stats from analyze only until sufficient updates/inserts/deletes
> have taken place to make them "out of date"?
>
> David
>
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