| From: | Josh Berkus <josh(at)agliodbs(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Ants Aasma <ants(at)cybertec(dot)at>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Why is indexonlyscan so darned slow? |
| Date: | 2012-05-21 17:30:59 |
| Message-ID: | 4FBA7BD3.2060203@agliodbs.com |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
> 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
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