Re: support for MERGE

Here's a new version.  Many of the old complaints have been fixed;
particularly, the handling of partitioned tables is now much cleaner and
straightforward.  Amit Langote helped considerably in getting this part
to shape -- thanks for that.  Amit also helped correct the EvalPlanQual
behavior, which wasn't quite up to snuff.

There are a few things that can still be improved here.  For one, I need
to clean up the interactions with table AM (and thus heapam.c etc).
Secondarily, and I'm now not sure that I really want to do it, is change
the representation for executor: instead of creating a fake join between
target and source, perhaps we should have just source, and give
optimizer a separate query to fetch tuples from target.

What I did do is change how the target table is represented from parse
analysis to executor.  For example, in the original code, there were two
RTEs that represented the target table; that is gone.  Now the target
table is always just the query's resultRelation.  This removes a good
number of ugly hacks that had been objected to.

I'll park this in the January commitfest now.

--
Álvaro Herrera              Valdivia, Chile  —  https://www.EnterpriseDB.com/
"Cómo ponemos nuestros dedos en la arcilla del otro. Eso es la amistad; jugar
al alfarero y ver qué formas se pueden sacar del otro" (C. Halloway en
La Feria de las Tinieblas, R. Bradbury)

+    * We maintain separate transaction tables for UPDATE/INSERT/DELETE since
+    * MERGE can run all three actions in a single statement. Note that UPDATE