On Saturday, July 11, 2020, Tom Lane <t...@sss.pgh.pa.us> wrote: > "David G. Johnston" <david.g.johns...@gmail.com> writes: > > On Sat, Jul 11, 2020 at 5:47 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > >> It seems like a lot of the disagreement here is focused on Peter's > >> proposal to make hash_mem_multiplier default to 2.0. But it doesn't > >> seem to me that that's a critical element of the proposal. Why not just > >> make it default to 1.0, thus keeping the default behavior identical > >> to what it is now? > > > If we don't default it to something other than 1.0 we might as well just > > make it memory units and let people decide precisely what they want to > use > > instead of adding the complexity of a multiplier. > > Not sure how that follows? The advantage of a multiplier is that it > tracks whatever people might do to work_mem automatically.
> I was thinking that setting -1 would basically do that. > In general > I'd view work_mem as the base value that people twiddle to control > executor memory consumption. Having to also twiddle this other value > doesn't seem especially user-friendly. I’ll admit I don’t have a feel for what is or is not user-friendly when setting these GUCs in a session to override the global defaults. But as far as the global defaults I say it’s a wash between (32mb, -1) -> (32mb, 48mb) and (32mb, 1.0) -> (32mb, 1.5) If you want 96mb for the session/query hash setting it to 96mb is invariant, whilesetting it to 3.0 means it can change in the future if the system work_mem changes. Knowing the multiplier is 1.5 and choosing 64mb for work_mem in the session is possible but also mutable and has side-effects. If the user is going to set both values to make it invariant we are back to it being a wash. I don’t believe using a multiplier will promote better comprehension for why this setting exists compared to “-1 means use work_mem but you can override a subset if you want.” Is having a session level memory setting be mutable something we want to introduce? Is it more user-friendly? >> If we find that's a poor default, we can always change it later; > >> but it seems to me that the evidence for a higher default is > >> a bit thin at this point. > > > So "your default is 1.0 unless you installed the new database on or after > > 13.4 in which case it's 2.0"? > > What else would be new? See e.g. 848ae330a. (Note I'm not suggesting > that we'd change it in a minor release.) > Minor release update is what I had thought, and to an extent was making possible by not using the multiplier upfront. I agree options are wide open come v14 and beyond. David J.
