Up to now, createplan.c attempted to share PARAM_EXEC slots for NestLoopParams across different plan levels, if the same underlying Var was being fed down to different righthand-side subplan trees by different NestLoops. This was, I think, more of an artifact of using subselect.c's PlannerParamItem infrastructure than an explicit design goal, but anyway that was the end result.
This works well enough as long as the plan tree is executing synchronously, but the feature whereby Gather can execute the parallelized subplan locally breaks it. An upper NestLoop node might execute for a row retrieved from a parallel worker, and assign a value for a PARAM_EXEC slot from that row, while the leader's copy of the parallelized subplan is suspended with a different active value of the row the Var comes from. When control eventually returns to the leader's subplan, it gets the wrong answers if the same PARAM_EXEC slot is being used within the subplan, as reported in bug #15577 from Bartosz Polnik.
This is pretty reminiscent of the problem fixed in commit 46c508fbc, and the proper fix seems to be the same: don't try to share PARAM_EXEC slots across different levels of controlling NestLoop nodes.
This requires decoupling NestLoopParam handling from PlannerParamItem handling, although the logic remains somewhat similar. To avoid bizarre division of labor between subselect.c and createplan.c, I decided to move all the param-slot-assignment logic for both cases out of those files and put it into a new file paramassign.c. Hopefully it's a bit better documented now, too.
A regression test case for this might be nice, but we don't know a test case that triggers the problem with a suitably small amount of data.
Back-patch to 9.6 where we added Gather nodes. It's conceivable that related problems exist in older branches; but without some evidence for that, I'll leave the older branches alone.
1db5667bac Avoid sharing PARAM_EXEC slots between different levels of NestLoop.
src/backend/optimizer/plan/createplan.c | 182 +---------
src/backend/optimizer/plan/planner.c | 9 +-
src/backend/optimizer/plan/subselect.c | 396 ++------------------
src/backend/optimizer/util/Makefile | 6 +-
src/backend/optimizer/util/paramassign.c | 599 +++++++++++++++++++++++++++++++
src/include/optimizer/paramassign.h | 34 ++
src/include/optimizer/subselect.h | 5 +-
7 files changed, 678 insertions(+), 553 deletions(-)