Extend ALTER OPERATOR to allow setting more optimization attributes.

Allow the COMMUTATOR, NEGATOR, MERGES, and HASHES attributes to be set
by ALTER OPERATOR.  However, we don't allow COMMUTATOR/NEGATOR to be
changed once set, nor allow the MERGES/HASHES flags to be unset once
set.  Changes like that might invalidate plans already made, and
dealing with the consequences seems like more trouble than it's worth.
The main use-case we foresee for this is to allow addition of missed
properties in extension update scripts, such as extending an existing
operator to support hashing.  So only transitions from not-set to set
states seem very useful.

This patch also causes us to reject some incorrect cases that formerly
resulted in inconsistent catalog state, such as trying to set the
commutator of an operator to be some other operator that already has a
(different) commutator.

While at it, move the InvokeObjectPostCreateHook call for CREATE
OPERATOR to not occur until after we've fixed up commutator or negator
links as needed.  The previous ordering could only be justified by
thinking of the OperatorUpd call as a kind of ALTER OPERATOR step;
but we don't call InvokeObjectPostAlterHook therein.  It seems better
to let the hook see the final state of the operator object.

In the documentation, move the discussion of how to establish
commutator pairs from xoper.sgml to the CREATE OPERATOR ref page.

Tommy Pavlicek, reviewed and editorialized a bit by me

Discussion: https://postgr.es/m/CAEhP-W-vGVzf4udhR5M8Bdv88UYnPrhoSkj3ieR3QNrsGQoqdg@mail.gmail.com

src/include/catalog/pg_operator.h

@@ -86,6 +86,11 @@ DECLARE_UNIQUE_INDEX_PKEY(pg_operator_oid_index, 2688, OperatorOidIndexId, pg_op
 DECLARE_UNIQUE_INDEX(pg_operator_oprname_l_r_n_index, 2689, OperatorNameNspIndexId, pg_operator, btree(oprname name_ops, oprleft oid_ops, oprright oid_ops, oprnamespace oid_ops));
 
 
+extern Oid	OperatorLookup(List *operatorName,
+						   Oid leftObjectId,
+						   Oid rightObjectId,
+						   bool *defined);
+
 extern ObjectAddress OperatorCreate(const char *operatorName,
 									Oid operatorNamespace,
 									Oid leftTypeId,
@@ -102,6 +107,16 @@ extern ObjectAddress makeOperatorDependencies(HeapTuple tuple,
 											  bool makeExtensionDep,
 											  bool isUpdate);
 
+extern void OperatorValidateParams(Oid leftTypeId,
+								   Oid rightTypeId,
+								   Oid operResultType,
+								   bool hasCommutator,
+								   bool hasNegator,
+								   bool hasRestrictionSelectivity,
+								   bool hasJoinSelectivity,
+								   bool canMerge,
+								   bool canHash);
+
 extern void OperatorUpd(Oid baseId, Oid commId, Oid negId, bool isDelete);
 
 #endif							/* PG_OPERATOR_H */

src/include/parser/parse_oper.h

@@ -42,6 +42,9 @@ extern Operator compatible_oper(ParseState *pstate, List *op,
 
 /* currently no need for compatible_left_oper/compatible_right_oper */
 
+/* Error reporting support */
+extern const char *op_signature_string(List *op, Oid arg1, Oid arg2);
+
 /* Routines for identifying "<", "=", ">" operators for a type */
 extern void get_sort_group_operators(Oid argtype,
 									 bool needLT, bool needEQ, bool needGT,