Re: Parallel execution and prepared statements

On Wed, Dec 7, 2016 at 12:30 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> 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.)
>

Attached patch changes the comment based on above suggestions.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com