From: | Josh Berkus <josh(at)agliodbs(dot)com> |
---|---|
To: | Greg Stark <gsstark(at)mit(dot)edu> |
Cc: | Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Multi-pass planner |
Date: | 2009-08-21 04:18:52 |
Message-ID: | 4A8E202C.6030008@agliodbs.com |
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Thread: | |
Lists: | pgsql-hackers |
> I don't think it's a bad idea, I just think you have to set your
> expectations pretty low. If the estimates are bad there isn't really
> any plan that will be guaranteed to run quickly.
Well, the way to do this is via a risk-confidence system. That is, each
operation has a level of risk assigned to it; that is, the cost
multiplier if the estimates are wrong. And each estimate has a level of
confidence attached. Then you can divide the risk by the confidence,
and if it exceeds a certain level, you pick another plan which has a
lower risk/confidence level.
However, the amount of extra calculations required for even a simple
query are kind of frightning.
--
Josh Berkus
PostgreSQL Experts Inc.
www.pgexperts.com
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