Horton

Re: Parallel execution and prepared statements

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Albe Laurenz <laurenz.albe@wien.gv.at>, "Tobias Bussmann *EXTERN*" <t.bussmann@gmx.net>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, Amit Kapila <amit.kapila16@gmail.com>
Date: 2016-12-06T19:00:32Z
Lists: pgsql-hackers

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Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Enable parallelism for prepared statements and extended query protocol.

On Tue, Dec 6, 2016 at 1:07 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.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