Re: Parallel execution and prepared statements

On Tue, Dec 6, 2016 at 1:07 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> Done.
>
> The comment seems quite confused now:
>
> * If a tuple count was supplied or data is being written to relation, we
> * must force the plan to run without parallelism, because we might exit
> * early.
>
> Exit early is exactly what we *won't* do if writing to an INTO rel, so
> I think this will confuse future readers. I think it should be more like
>
> * If a tuple count was supplied, we must force the plan to run without
> * parallelism, because we might exit early. Also disable parallelism
> * when writing into a relation, because [ uh, why exactly? ]
>
> Considering that the writing would happen at top level of the executor,
> and hence in the parent process, it's not actually clear to me why the
> second restriction is there at all: can't we write tuples to a rel even
> though they came from a parallel worker? In any case, the current wording
> of the comment is a complete fail at explaining this.

Oops. You're right. [ uh, why exactly? ] -> no database changes
whatsoever are allowed while in parallel mode. (This restriction
might be lifted someday, but right now we're stuck with it.)

--
Robert Haas
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