Re: [BUGS] [postgresql 10 beta3] unrecognized node type: 90

I wrote:
> I think that the correct fix probably involves marking each parallel scan
> plan node as dependent on a pseudo executor parameter, which the parent
> Gather or GatherMerge node would flag as being changed on each rescan.
> This would cue the plan layers in between that they cannot optimize on the
> assumption that the leader's instance of the parallel scan will produce
> exactly the same rows as it did last time, even when "nothing else
> changed".  The "wtParam" pseudo parameter that's used for communication
> between RecursiveUnion and its descendant WorkTableScan node is a good
> model for what needs to happen.

Here is a draft patch for this.  It's a bit different from wtParam in
that the special parameter isn't allocated until createplan.c time,
so that we don't eat a parameter slot if we end up choosing a non-parallel
plan; but otherwise things are comparable.

I could use some feedback on whether this is marking dependent child nodes
sanely.  As written, any plan node that's marked parallel_aware is assumed
to need a dependency on the parent Gather or GatherMerge's rescan param
--- and the planner will now bitch if a parallel_aware plan node is not
under any such Gather.  Is this reasonable?  I do not see any
documentation that defines the parallel_aware field with enough clarity
to be very sure about it.

I included the heap_parallelscan_nextpage hack I'm using to make the
original failure reproducible, but that hunk is not meant for commit.
Also, the regression test case is the same as in a2b70c89c.

			regards, tom lane