On Wed, 11 May 2005 16:15:16 -0500, "Jim C. Nasby" <[EMAIL PROTECTED]> wrote: >> This is divided by the number of index columns, so the index correlation >> is estimated to be 0.219. > >That seems like a pretty bad assumption to make.
Any assumption we make without looking at entire index tuples has to be bad. A new GUC variable secondary_correlation introduced by my patch at least gives you a chance to manually control the effects of additional index columns. >> In my tests I got much more plausible results with >> >> 1 - (1 - abs(correlation))^2 > >What's the theory behind that? The same as for csquared -- pure intuition. But the numbers presented in http://archives.postgresql.org/pgsql-hackers/2002-10/msg00072.php seem to imply that in this case my intiution is better ;-) Actually above formula was not proposed in that mail. AFAIR it gives results between p2 and p3. >And I'd still like to know why correlation squared is used. On Wed, 02 Oct 2002 18:48:49 -0400, Tom Lane <[EMAIL PROTECTED]> wrote: |The indexCorrelation^2 algorithm was only a quick hack with no theory |behind it :-(. >It depends on the patches, since this is a production machine. Currently >it's running 7.4.*mumble*, The patch referenced in http://archives.postgresql.org/pgsql-hackers/2003-08/msg00931.php is still available. It doesn't touch too many places and should be easy to review. I'm using it and its predecessors in production for more than two years. Let me know, if the 74b1 version does not apply cleanly to your source tree. Servus Manfred ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
