Re: parallel "return query" is no good

On 2017-03-23 13:03:19 -0400, Robert Haas wrote:
> On Thu, Mar 23, 2017 at 12:50 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> > Commit 7aea8e4f2daa4b39ca9d1309a0c4aadb0f7ed81b allowed a parallel
> > plan to be generated when for a RETURN QUERY or RETURN QUERY EXECUTE
> > statement in a PL/pgsql block.  As it turns out, the analysis that led
> > to this decision was totally wrong-headed, because the plan will
> > always be executed using SPI_cursor_fetch(portal, true, 50), which
> > will cause ExecutePlan() to get invoked with a count of 50, which will
> > cause it to run the parallel plan serially, without workers.
> > Therefore, passing CURSOR_OPT_PARALLEL_OK is a bad idea here; all it
> > can do is cause us to pick a parallel plan that's slow when executed
> > serially instead of the best serial plan.
> >
> > The attached patch fixes it.  I plan to commit this and back-patch it
> > to 9.6, barring objections or better ideas.
> 
> I guess the downside of back-patching this is that it could cause a
> plan change for somebody which ends up being worse.  On the whole,
> serial execution of queries intended to be run in parallel isn't
> likely to work out well, but it's always possible somebody has a cases
> where it happens to be winning, and this could break it.  So maybe I
> should do this only in master?  Thoughts?

I'm +0.5 for backpatching.

- Andres