| From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
|---|---|
| To: | Mark Mielke <mark(at)mark(dot)mielke(dot)cc> |
| Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Visibility map thoughts |
| Date: | 2007-11-06 17:50:16 |
| Message-ID: | 1194371416.22428.183.camel@dogma.ljc.laika.com |
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| Lists: | pgsql-hackers |
On Tue, 2007-11-06 at 08:01 -0500, Mark Mielke wrote:
> Simon Riggs wrote:
> > On Mon, 2007-11-05 at 09:52 +0000, Heikki Linnakangas wrote:
> > I'm racking my brain trying to think of a query that will benefit from
> > index-only scans without specifically creating covered indexes. Apart
> > from count(*) queries and RI lookups. I can't see RI lookups being much
> > cheaper with this technique, do you see something there
> I'm not sure what RI lookup is. Sorry. :-)
>
> My list would be:
> - EXISTS / NOT EXISTS
> - COUNT(*)
> - Tables that are heavily updated - any case where the index entry often
> maps to a non-visible tuple.
>
> Beyond that, yeah, I cannot think of other benefits.
>
What about range queries or sorts?
The correlation of an index to itself is 100% :)
Regards,
Jeff Davis
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