2011/2/22 PostgreSQL - Hans-Jürgen Schönig <postg...@cybertec.at>: > how does it work? we try to find suitable statistics for an arbitrary length > list of conditions so that the planner can use it directly rather than > multiplying all the selectivities. this should make estimates a lot more > precise. > the current approach can be extended to work with expressions and well as > "straight" conditions.
/me prepares to go down in flames. Personally, I think the first thing we ought to do is add a real, bona fide planner hint to override the selectivity calculation manually, maybe something like this: WHERE (x < 5 AND y = 1) SELECTIVITY (0.1); Then, having provided a method for the DBA to extinguish the raging flames of searing agony which are consuming them while a crocodile chews off their leg and their boss asks them why they didn't use Oracle, we can continue bikeshedding about the best way of fixing this problem in a more user-transparent fashion. As to the approach you've proposed here, I'm not sure I understand what this is actually doing. Selectivity estimates aren't made directly for predicates; they're made based on MCV and histogram information for predicates. -- 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
