Re: MinMaxAggPath vs. parallel-safety

Robert Haas <robertmhaas@gmail.com> writes:
> On Sun, Jun 26, 2016 at 10:35 PM, Noah Misch <noah@leadboat.com> wrote:
>> The above-described topic is currently a PostgreSQL 9.6 open item ("consider
>> whether MinMaxAggPath might fail to be parallel-safe").

> Currently, MinMaxAggPath is never parallel-safe; the question is
> whether we could allow it to be parallel-safe (not, as the current
> phrasing implies, whether it might ever need to be other than
> parallel-safe).

Check.

> It appears to me that the answer is "no", because a
> MinMaxAggPath contains a list of MinMaxAggInfo objects, and there we
> have this:
>         Param      *param;                      /* param for subplan's output */
> Since subplans aren't passed down to parallel workers, no
> MinMaxAggPath can be parallel-safe.   Therefore, I think there's
> nothing to do here right now.  Comments?

Hm.  In principle, this could be made to work, since I don't think it
would be necessary for the Param's value to pass across process
boundaries.  (It could be locally generated within a worker, and then also
consumed within the worker, if the worker's plan looked like a Result with
a subplan attached.)  However, if we don't even pass down the plan trees
for subplans, then I agree that it can't work at the moment.

In any case, this is an optimization opportunity not a bug.  If you want
to kick this can down the road until parallel query is generally smarter
about subplans, that's OK with me.
        regards, tom lane