From: | Josh Berkus <josh(at)agliodbs(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Greg Stark <gsstark(at)mit(dot)edu>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Idea about estimating selectivity for single-column expressions |
Date: | 2009-08-19 19:00:36 |
Message-ID: | 4A8C4BD4.4060509@agliodbs.com |
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Lists: | pgsql-hackers |
Tom, Greg, Robert,
Here's my suggestion:
1. First, estimate the cost of the node with a very pessimistic (50%?)
selectivity for the calculation.
2. If the cost hits a certain threshold, then run the calculation
estimation on the histogram.
That way, we avoid the subtransaction and other overhead on very small sets.
also:
> Trying it on the MCVs makes a lot of sense. I'm not so sure about
> trying it on the histogram entries. There's no reason to assume that
> those cluster in any way that will be useful. (For example, suppose
> that I have the numbers 1 through 10,000 in some particular column and
> my expression is col % 100.)
Yes, but for seriously skewed column distributions, the difference in
frequency between the MCV and a sample "random" distribution will be
huge. And it's precisely those distributions which are currently
failing in the query planner.
--
Josh Berkus
PostgreSQL Experts Inc.
www.pgexperts.com
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