Re: Cross-column statistics revisited

"Joshua Tolley" <eggyknap(at)gmail(dot)com> writes:
> Most of the comments on this thread have centered around the questions
> of "what we'd store" and "how we'd use it", which might be better
> phrased as, "The database assumes columns are independent, but we know
> that's not always true. Does this cause enough problems to make it
> worth fixing? How might we fix it?" I have to admit an inability to
> show that it causes problems,

Any small amount of trolling in our archives will turn up plenty of
examples.

It appears to me that a lot of people in this thread are confusing
correlation in the sense of statistical correlation between two
variables with correlation in the sense of how well physically-ordered
a column is. (The latter is actually the same kind of animal, but
always taking one of the two variables to be physical position.)
A bad estimate for physical-position correlation has only limited
impact, as Josh B said upthread; but the other case leads to very
bad rowcount estimates which have *huge* impact on plan choices.

regards, tom lane