Tom, Greg, Robert, Here's my suggestion:
1. First, estimate the cost of the node with a very pessimistic (50%?) selectivity for the calculation. 2. If the cost hits a certain threshold, then run the calculation estimation on the histogram. That way, we avoid the subtransaction and other overhead on very small sets. also: > Trying it on the MCVs makes a lot of sense. I'm not so sure about > trying it on the histogram entries. There's no reason to assume that > those cluster in any way that will be useful. (For example, suppose > that I have the numbers 1 through 10,000 in some particular column and > my expression is col % 100.) Yes, but for seriously skewed column distributions, the difference in frequency between the MCV and a sample "random" distribution will be huge. And it's precisely those distributions which are currently failing in the query planner. -- Josh Berkus PostgreSQL Experts Inc. www.pgexperts.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
