On Fri, 2008-12-12 at 11:16 -0500, Tom Lane wrote: > Perhaps a better plan is to try to de-emphasize use of ndistinct, > though I concede I have no idea how to do that.
We don't actually care about the accuracy of the ndistinct much, just the accuracy of our answer to the question "given work_mem = X, is it better to use a hash plan". So we just need to scan the table until we can answer that question accurately enough. i.e. a variable sized sample. Perhaps we could store a probability distribution for various values of work_mem, rather than a single ndistinct value. Anyway, definitely handwaving now to stimulate ideas. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
