Re: Problems with "pg.dropped" column after upgrade 9.5 to 9.6

I wrote:
> So I'm pretty tempted to fix it as per attached, that is, teach setrefs.c
> that it's silly to try to convert a Const into a Var. It is sane to
> replace more-complicated expressions with Vars, because that means we save
> re-evaluating those expressions. But a quick look at ExecEvalConst and
> ExecEvalVar shows that a Const is strictly cheaper to execute than a Var,
> so doing such a conversion is actually a pessimization.

After further poking around, I concluded that it'd be a good idea to make
a similar change in set_dummy_tlist_references(), to prevent a failure
if the top plan node below ModifyTable chances to be a Sort or other
non-projecting plan node. I'm not sure such a case can arise today, but
I'm not sure it can't either.

With that, there are a couple more changes in regression test EXPLAIN
output. A full patch that passes "make check-world" is attached.

I'm not sure if it's worth memorializing the specific test case discussed
in this thread as a regression test. It depends enough on the behavior
of SQL function planning that I wouldn't have much confidence in it
continuing to test what we meant it to test going forward.

regards, tom lane