Re: Default setting for enable_hashagg_disk

On Thu, 9 Apr 2020 at 13:24, Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:

> On Thu, Apr 09, 2020 at 01:48:55PM +0200, Tomas Vondra wrote:
>
> > It it really any different from our enable_* GUCs? Even if you do e.g.
> > enable_sort=off, we may still do a sort. Same for enable_groupagg etc.
>
> Those show that the GUC was disabled by showing disable_cost. That's
> what's
> different about this one.
>

Fwiw in the past this was seen not so much as a positive thing but a bug to
be fixed. We've talked about carrying a boolean "disabled plan" flag which
would be treated as a large cost penalty but not actually be added to the
cost in the plan.

The problems with the disable_cost in the cost are (at least):

1) It causes the resulting costs to be useless for comparing the plan costs
with other plans.

2) It can cause other planning decisions to be distorted in strange
non-linear ways.

--
greg