Re: Default setting for enable_hashagg_disk

On 2020-Jul-10, Peter Geoghegan wrote:

> * The maximum allowable value is 100.0, to protect users from
> accidentally setting hash_mem_multiplier to a value intended to work
> like a work_mem-style KB value (you can't provide an absolute value
> like that directly). This maximum is absurdly high.
>
> I think that it's possible that a small number of users will find it
> useful to set the value of hash_mem_multiplier as high as 5.0. That is
> a very aggressive value, but one that could still make sense with
> certain workloads.

I'm not sure about this bit; sounds a bit like what has been qualified
as "nannyism" elsewhere. Suppose I want to give a hash table 2GB of
memory for whatever reason. If my work_mem is default (4MB) then I
cannot possibly achieve that without altering both settings.

So I propose that maybe we do want a maximum value, but if so it should
be higher than what you propose. I think 10000 is acceptable in that it
doesn't get in the way.

Another point is that if you specify a unit for the multiplier (which is
what users are likely to do for larger values), it'll fail anyway, so
I'm not sure this is such terrible a problem.

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services