On Sun, Feb 7, 2016 at 4:50 PM, Peter Geoghegan <pg@heroku.com> wrote:
>> I'm not even sure this is necessary. The idea of missing out on
>> producing a single sorted run sounds bad but in practice since we
>> normally do the final merge on the fly there doesn't seem like there's
>> really any difference between reading one tape or reading two or three
>> tapes when outputing the final results. There will be the same amount
>> of I/O happening and a 2-way or 3-way merge for most data types should
>> be basically free.
>
> I basically agree with you, but it seems possible to fix the
> regression (generally misguided though those regressed cases are).
> It's probably easiest to just fix it.
On a related note, we should probably come up with a way of totally
supplanting the work_mem model with something smarter in the next
couple of years. Something that treats memory as a shared resource
even when it's allocated privately, per-process. This external sort
stuff really smooths out the cost function of sorts. ISTM that that
makes the idea of dynamic memory budgets (in place of a one size fits
all work_mem) seem desirable for the first time. That said, I really
don't have a good sense of how to go about moving in that direction at
this point. It seems less than ideal that DBAs have to be so
conservative in sizing work_mem.
--
Peter Geoghegan