| From: | Josh Berkus <josh(at)agliodbs(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| 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>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Stats for multi-column indexes |
| Date: | 2007-03-20 17:12:14 |
| Message-ID: | 460015EE.90900@agliodbs.com |
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| Lists: | pgsql-hackers |
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.
--Josh Berkus
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