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

I wrote:
> Well, if we posit that the problems are limited to false Param-sharing,
> my inclination is to fix it by simplifying rather than adding complexity.
> To wit, let's just assign a new PARAM_EXEC slot for every NestLoopParam
> and never try to share them. The argument that sharing them would work
> for NLPs representing the same Var hinged on the assumption that nested
> NestLoops would necessarily execute "in sync", which we now see is false
> at least when Gathers are in between.

Here's a POC patch for that. It's seriously ugly, but it passes
check-world and I can no longer reproduce the problem using
mockup_bug15577_v2.sql.

At this point assign_nestloop_param_var and
assign_nestloop_param_placeholdervar are dead code, and there's a bunch
of incorrect comments in subselect.c, and I really need to refactor
the division of labor between createplan.c and subselect.c (for one
thing, this is an abuse of the documented purpose of
SS_make_initplan_output_param). But functionally I think it does the
right thing. Please test and verify that you no longer see the race
condition.

regards, tom lane