Re: Fix error message for MERGE foreign tables

Richard Guo <guofenglinux@gmail.com> writes:
> Or maybe we can make it even earlier, when we expand an RTE for a
> partitioned table and add result tables to leaf_result_relids.

I'm not really on board with injecting command-type-specific logic into
completely unrelated places just so that we can throw an error a bit
earlier.  Alvaro's suggestion of make_modifytable seemed plausible,
not least because it avoids spending any effort when the command
couldn't be MERGE at all.