Re: BUG #15865: ALTER TABLE statements causing "relation already exists" errors when some indexes exist

I wrote:
> As before, I attach a patch against HEAD, plus one that assumes e76de8861
> has been reverted first, which is likely easier to review.
> Unlike yesterday, I'm feeling pretty good about this patch now, but it
> still wouldn't hurt for somebody else to go over it.

I started to back-patch this, and soon noticed that the content of the
OCLASS_CONSTRAINT case branch in ATExecAlterColumnType has varied across
versions, which makes copy-and-pasting it seem pretty hazardous. Hence
it seems prudent to do slightly more work and split that code out into
a subroutine rather than having two copies. As attached, which is a
hopefully-final patch for HEAD. As before, it presumes reversion of
e76de8861, because it's a lot easier to see what's going on that way.

BTW ... while working on this, I got annoyed by the fact that
ATExecAlterColumnGenericOptions was inserted, no doubt with the aid of a
dartboard, into the middle of a large group of AlterColumnType-related
functions. Would anyone mind a separate patch to relocate it down
past those, probably just before ATExecChangeOwner?

regards, tom lane