Oliver Mattos <omattos@xxxxxxxxx> writes: >> Can you be more elaborate how you'd want to go about it? > ... If another candidate plan is now lower cost, the current plan would be > terminated[1] by setting a flag instructing each execnode to return as > if it had reached the end of the input, although still caching the > node selectivity values, and the new plan started from scratch. Quite aside from the implementation difficulties you'll have, that approach is a show-stopper right there. You can't just restart from scratch, because we may already have shipped rows to the client, or for DML cases already inserted/updated/deleted rows (and even if you could roll those back, we've possibly fired triggers with unpredictable side effects). Queries containing volatile functions are another no-fly zone for this approach. I can't see any way around that without unacceptable performance costs (i.e. buffering all the rows until we're done) or wire-protocol breakage. I think that a more practical way to address the class of problems you're talking about is to teach the planner to have some notion of worst-case as well as expected-case costs, and then incorporate some perhaps-configurable amount of risk aversion in its choices. regards, tom lane PS: please do not top-post, and do not quote the entire darn thread in each message. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
