Re: Default setting for enable_hashagg_disk

"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Sat, Jul 11, 2020 at 5:47 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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. 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.

>> 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.)

regards, tom lane