From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at>, "Tobias Bussmann *EXTERN*" <t(dot)bussmann(at)gmx(dot)net>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
Subject: | Re: Parallel execution and prepared statements |
Date: | 2016-12-06 19:00:32 |
Message-ID: | CA+TgmoZ+pN1gaENfkLOU_4C_OO5ox8tvaOxVmwTevGn5+TxMGg@mail.gmail.com |
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Lists: | pgsql-hackers |
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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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