Re: BUG #15577: Query returns different results when executed multiple times

On Thu, 10 Jan 2019 at 15:06, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> As to disallow "this plan shape," both leader participation and the
> ability to generate this kind of plan are there because they improve
> performance substantially on some queries. I put a heck of a lot of
> energy into being able to parallelize only part of a query, and into
> allowing for leader participation, and I believe those things make a
> real difference.

I 100% agree that this shape of plan is useful. I guess though, that
whether we keep allowing it or not depends on if a suitable fix
appears for the issue.

One idea that I've been thinking about is some sort of param context
number that we bump during createplan when we encounter a
Gather/GatherMerge path and have assign_param_for_var() pay attention
to that number when deciding if sharing a parameter is okay. How
exactly that context number is calculated and how we pass the value
around I've currently no idea, so please consider this very handwavy.

--
David Rowley http://www.2ndQuadrant.com/
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