On Fri, Dec 17, 2010 at 12:58 PM, Tomas Vondra <t...@fuzzy.cz> wrote: > In the end, all they need to compute an estimate is number of distinct > values for each of the columns (we already have that in pg_stats) and a > number of distinct values for the group of columns in a query. They > really don't need any multidimensional histogram or something like that.
I haven't read the paper yet (sorry) but just off the top of my head, one possible problem here is that our n_distinct estimates aren't always very accurate, especially for large tables. As we've discussed before, making them accurate requires sampling a significant percentage of the table, whereas all of our other statistics can be computed reasonably accurately by sampling a fixed amount of an arbitrarily large table. So it's possible that relying more heavily on n_distinct could turn out worse overall even if the algorithm is better. Not sure if that's an issue here, just throwing it out there... -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
