Re: Why is indexonlyscan so darned slow?

> Anyway, on my machine it seems that the per-tuple CPU costs for SELECT
> COUNT(*) with an index-only scan are something like 10% higher than the
> per-tuple costs with a heap scan.  We might get that down to roughly par
> with some hacking, but it's never going to be vastly better.  The
> argument in favor of index-only scans is mainly about reducing I/O costs
> anyway.

Well, if it's not CPU costs, then something else is eating the time,
since I'm seeing per-tuple COUNT counts on indexes being 400% more than
on heap.

In the airport you said something about index-only scan not scanning the
tuples in leaf page order.   Can you elaborate on that?

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com