Re: postgres_fdw bug in 9.6

Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
> I have a DML statement which triggers the error:
> ERROR: XX000: outer pathkeys do not match mergeclauses
> LOCATION: create_mergejoin_plan, createplan.c:3722

Hmm.

> Any tips on investigating this further in situ? Or is the best option just
> to work harder on a minimal and disclosable test case?

I think we need a test case --- not minimal necessarily, but something
other people can reproduce. You might find that setting enable_hashjoin
and/or enable_nestloop to false makes it easier to provoke the error,
since evidently this requires that we (a) generate a faulty mergejoin Path
and then (b) choose it as the cheapest one, since the error occurs while
converting it to a Plan.

BTW, if you're not doing this in a debug (--enable-cassert) build, it'd
be useful to try it in one. I'm a little suspicious that the root cause
might be a memory-stomp type of problem, ie somebody scribbling on a
pathkey data structure without accounting for it being shared with another
path. It's possible that cassert memory checking would help catch that.

regards, tom lane