Re: Parallel execution and prepared statements

On Thu, Nov 17, 2016 at 10:07 AM, Tobias Bussmann <t.bussmann@gmx.net> wrote:
>> Yeah, we could do something like this, perhaps not in exactly this
>> way, but I'm not sure it's a good idea to just execute the parallel
>> plan without workers.
>
> sure, executing parallel plans w/o workers seems a bit of a hack. But:
> - we already do it this way in some other situations

True, but we also try to avoid it whenever possible, because it's
likely to lead to poor performance.

> - the alternative in this special situation would be to _force_ replanning without the CURSOR_OPT_PARALLEL_OK. The
decisionfor replanning is hidden deep within plancache.c and while we could influence it with CURSOR_OPT_CUSTOM_PLAN
thiswouldn't have an effect if the prepared statement doesn't have any parameters. Additionally, influencing the
decisionand generating a non-parallel plan would shift the avg cost calculation used to choose custom or generic plans. 

I think it would be a good idea to come up with a way for a query to
produce both a parallel and a non-parallel plan and pick between them
at execution time.  However, that's more work than I've been willing
to undertake.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company