Hi, On 2019-08-20 19:55:56 +0200, Felix Geisendörfer wrote: > > On 20. Aug 2019, at 19:32, Andres Freund <andres@xxxxxxxxxxx> wrote: > > FWIW, that's not a mis-estimate I'm getting on master ;). Obviously > > that doesn't actually address your concern... > > I suppose this is thanks to the new optimizer support functions > mentioned by Michael and Pavel? Right. > > Under-sizing the hashtable just out of caution will have add overhead to > > a lot more common cases. That requires copying data around during > > growth, which is far far from free. Or you can use hashtables that don't > > need to copy, but they're also considerably slower in the more common > > cases. > > How does PostgreSQL currently handle the case where the initial hash > table is under-sized due to the planner having underestimated things? > Are the growth costs getting amortized by using an exponential growth > function? Yes. But that's still far from free. Greetings, Andres Freund
