Re: Better estimates of index correlation

Excerpts from Joshua D. Drake's message of dom mar 13 23:20:01 -0300 2011:
> On Sun, 2011-03-13 at 19:40 -0400, Tom Lane wrote:
> 
> > I'm not planning to do anything about this idea right now, since I'm
> > still hip-deep in collations, but I thought I'd throw it out to get
> > it on the record.
> > 
> > Comments?
> 
> One question: Where is the overhead increase? 

During VACUUM, in the pass that processes indexes.

I think Tom is sligthly confused though: AFAICT this must happen in
btvacuumscan (which does the actual scan), not btvacuumcleanup (which
may not do it, if btbulkdelete did it previously).  Which means it would
be done for each pass over the index when vacuuming a relation, because
I don't see any way for this function to determine whether this is the
last pass we'll do over the index.

It sure would be nice to be able to do it only during the last scan.

-- 
Álvaro Herrera <alvherre@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support