I have a setup where a 9.6.1 server uses postgres_fdw to connect to a 9.4.9
hot standby server.

I have a DML statement which triggers the error:

ERROR:  XX000: outer pathkeys do not match mergeclauses
LOCATION:  create_mergejoin_plan, createplan.c:3722

The error first starts appearing with this commit (on the local side):

commit aa09cd242fa7e3a694a31f8aed521e80d1e626a4
Author: Robert Haas <rh...@postgresql.org>
Date:   Wed Mar 9 10:51:49 2016 -0500

    postgres_fdw: Consider foreign joining and foreign sorting together.

The version of the remote side does not seem to matter.  I've also promoted
a test instance of the remote from hot standby to master and then upgraded
to 9.6.1, and neither step fixes the issue.

The statement is like this:

explain update foo_local set col3=foo_remote.col3 from foo_remote where
foo_local.id=foo_remote.id and foo_local.id in ('aaa','bbb','ccc','ddd');

Where foo_remote is a pretty complicated view (defined locally) over the
join of 8 foreign tables.

I am having trouble producing a self-contained, disclosable test case for
this.  Small changes causes the error to go away.  On the local side, it
doesn't seem to depend on the contents of the table, only the structure.
But on the remote side, truncating the central table for the query makes
the error go away.

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



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