Re: Oversight in reparameterize_path_by_child leading to executor crash

Richard, I think it could be useful to put a better commit message
into the patch file, describing both what problem is being fixed and
what the design of the fix is. I gather that the problem is that we
crash if the query contains a partioningwise join and also $SOMETHING,
and the solution is to move reparameterization to happen at
createplan() time but with a precheck that runs during path
generation. Presumably, that means this is more than a minimal bug
fix, because the bug could be fixed without splitting
can-it-be-reparameterized to reparameterize-it in this way. Probably
that's a performance optimization, so maybe it's worth clarifying
whether that's just an independently good idea or whether it's a part
of making the bug fix not regress performance.

I think the macro names in path_is_reparameterizable_by_child could be
better chosen. CHILD_PATH_IS_REPARAMETERIZABLE doesn't convey that the
macro will return from the calling function if not -- it looks like it
just returns a Boolean. Maybe REJECT_IF_PATH_NOT_REPARAMETERIZABLE and
REJECT_IF_PATH_LIST_NOT_REPARAMETERIZABLE or some such.

Another question here is whether we really want to back-patch all of
this or whether it might be better to, as Tom proposed previously,
back-patch a more minimal fix and leave the more invasive stuff for
master.