[PATCH] Extend ALTER OPERATOR to support adding commutator, negator, hashes, and merges

Hi All,

I've attached a couple of patches to allow ALTER OPERATOR to add
commutators, negators, hashes and merges to operators that lack them.

The need for this arose adding hash functions to the ltree type after the
operator had been created without hash support[1]. There are potential
issues with modifying these attributes that have been discussed
previously[2], but I understand that setting them, if they have not been
set before, is ok.

I belatedly realised that it may not be desirable or necessary to allow
adding commutators and negators in ALTER OPERATOR because the linkage can
already be added when creating a new operator. I don't know what's best, so
I thought I'd post this here and get feedback before removing anything.

The first patch is create_op_fixes_v1.patch and it includes some
refactoring in preparation for the ALTER OPERATOR changes and fixes a
couple of minor bugs in CREATE OPERATOR:
- prevents self negation when filling in/creating an existing shell operator
- remove reference to sort operator in the self negation error message as
the sort attribute appears to be deprecated in Postgres 8.3

The second patch is alter_op_v1.patch which contains the changes to ALTER
OPERATOR and depends on create_op_fixes_v1.patch.

Additionally, I wasn't sure whether it was preferred to fail or succeed on
ALTERs that have no effect, such as adding hashes on an operator that
already allows them or disabling hashes on one that does not. I chose to
raise an error when this happens, on the thinking it was more explicit and
made the code simpler, even though the end result would be what the user
wanted.

Comments appreciated.

Thanks,
Tommy

[1]
https://www.postgresql.org/message-id/flat/CAEhP-W9ZEoHeaP_nKnPCVd_o1c3BAUvq1gWHrq8EbkNRiS9CvQ%40mail.gmail.com
[2] https://www.postgresql.org/message-id/flat/3348985(dot)V7xMLFDaJO(at)dinodell