Re: [HACKERS] parallel "return query" is no good

On 03/23/2017 10:03 AM, 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 think the greater good of a fix applies here. +1 to 9.6.

>


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