2011/2/26 PostgreSQL - Hans-Jürgen Schönig <postg...@cybertec.at>: > what we are trying to do is to explicitly store column correlations. so, a > histogram for (a, b) correlation and so on. >

The problem is that we haven't figured out how to usefully store a histogram for <a,b>. Consider the oft-quoted example of a <city,postal-code> -- or <city,zip code> for Americans. A histogram of the tuple is just the same as a histogram on the city. It doesn't tell you how much extra selectivity the postal code or zip code gives you. And if you happen to store a histogram of <postal code, city> by mistake then it doesn't tell you anything at all. We need a data structure that lets us answer the bayesian question "given a city of New York how selective is zip-code = 02139". I don't know what that data structure would be. Heikki and I had a wacky hand-crafted 2D histogram data structure that I suspect doesn't actually work. And someone else did some research on list and came up with a fancy sounding name of a statistics concept that might be what we want. -- greg -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers