Matthew Bellew <matthewb@xxxxxxxxxx> writes: > I made have several users encounter performance problems, which all > seem to come down to this problem: multiplying selectivity estimates can > cause tuple estimates to grow very small very quickly, once the estimator > gets to 1 row, the planner may choose plans that are very good ONLY WHEN > there is exactly 1 row (maybe even O(N^large)). Unfortunately, these may > be the worst plans if the estimate is even slightly off (even just > returning 2 or 3 rows versus 1). Yeah, this is a well-known problem. There has been prior discussion along the same lines as you mention (only believe 1-row estimates when it's provably true that there's at most one row), but it hasn't looked like an easy change. See the pgsql-hackers archives for previous threads. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
