| From: | Csaba Nagy <nagy(at)ecircle-ag(dot)com> |
|---|---|
| To: | Josh Berkus <josh(at)agliodbs(dot)com> |
| Cc: | Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Richard Huxton <dev(at)archonet(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Mark Kirkwood <markir(at)paradise(dot)net(dot)nz>, Jeff Davis <pgsql(at)j-davis(dot)com>, postgres hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Stats for multi-column indexes |
| Date: | 2007-03-21 09:27:19 |
| Message-ID: | 1174469239.10829.53.camel@coppola.muc.ecircle.de |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
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| Lists: | pgsql-hackers |
On Tue, 2007-03-20 at 18:12, Josh Berkus wrote:
> Tom,
>
> > Actually, I think you don't particularly need stats for that in most
> > cases --- if the planner simply took note that the FK relationship
> > exists, it would know that each row of the FK side joins to exactly
> > one row of the PK side, which in typical cases is sufficient.
>
> Is it? What about the other direction? Currently, doesn't the planner
> assume that the rowcount relationship is 1 to ( child total rows /
> parent total rows) ? That's ok for tables with relatively even
> distribution, but not for skewed ones.
Wouldn't that be improved if the MCVs/histogram of the FK column are
taken into account ? Considering that the FK part is unique, the
skewness in the relationship is completely determined by the FK parts
histogram. That would give at least a lower/upper bound and MCVs to the
relationship.
Cheers,
Csaba.
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