May be it's time to revisit an old proposal that has failed to catch anybody's attention during the 7.4 beta period: http://archives.postgresql.org/pgsql-hackers/2003-08/msg00937.php
I'm not sure I'd store index correlation in a separate table today. You've invented something better for functional index statistics, AFAIR.
Make it deal with cross-table fk correlations as well :)
Chris
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