Allow aggregate functions to be VARIADIC.

There's no inherent reason why an aggregate function can't be variadic
(even VARIADIC ANY) if its transition function can handle the case.
Indeed, this patch to add the feature touches none of the planner or
executor, and little of the parser; the main missing stuff was DDL and
pg_dump support.

It is true that variadic aggregates can create the same sort of ambiguity
about parameters versus ORDER BY keys that was complained of when we
(briefly) had both one- and two-argument forms of string_agg().  However,
the policy formed in response to that discussion only said that we'd not
create any built-in aggregates with varying numbers of arguments, not that
we shouldn't allow users to do it.  So the logical extension of that is
we can allow users to make variadic aggregates as long as we're wary about
shipping any such in core.

In passing, this patch allows aggregate function arguments to be named, to
the extent of remembering the names in pg_proc and dumping them in pg_dump.
You can't yet call an aggregate using named-parameter notation.  That seems
like a likely future extension, but it'll take some work, and it's not what
this patch is really about.  Likewise, there's still some work needed to
make window functions handle VARIADIC fully, but I left that for another
day.

initdb forced because of new aggvariadic field in Aggref parse nodes.

src/backend/commands/functioncmds.c

@@ -88,7 +88,6 @@ compute_return_type(TypeName *returnType, Oid languageOid,
 
 	typtup = LookupTypeName(NULL, returnType, NULL);
 
-
 	if (typtup)
 	{
 		if (!((Form_pg_type) GETSTRUCT(typtup))->typisdefined)
@@ -158,22 +157,31 @@ compute_return_type(TypeName *returnType, Oid languageOid,
 }
 
 /*
- * Interpret the parameter list of the CREATE FUNCTION statement.
+ * Interpret the function parameter list of a CREATE FUNCTION or
+ * CREATE AGGREGATE statement.
+ *
+ * Input parameters:
+ * parameters: list of FunctionParameter structs
+ * languageOid: OID of function language (InvalidOid if it's CREATE AGGREGATE)
+ * is_aggregate: needed only to determine error handling
+ * queryString: likewise, needed only for error handling
  *
  * Results are stored into output parameters.  parameterTypes must always
  * be created, but the other arrays are set to NULL if not needed.
  * requiredResultType is set to InvalidOid if there are no OUT parameters,
  * else it is set to the OID of the implied result type.
  */
-static void
-examine_parameter_list(List *parameters, Oid languageOid,
-					   const char *queryString,
-					   oidvector **parameterTypes,
-					   ArrayType **allParameterTypes,
-					   ArrayType **parameterModes,
-					   ArrayType **parameterNames,
-					   List **parameterDefaults,
-					   Oid *requiredResultType)
+void
+interpret_function_parameter_list(List *parameters,
+								  Oid languageOid,
+								  bool is_aggregate,
+								  const char *queryString,
+								  oidvector **parameterTypes,
+								  ArrayType **allParameterTypes,
+								  ArrayType **parameterModes,
+								  ArrayType **parameterNames,
+								  List **parameterDefaults,
+								  Oid *requiredResultType)
 {
 	int			parameterCount = list_length(parameters);
 	Oid		   *inTypes;
@@ -223,6 +231,12 @@ examine_parameter_list(List *parameters, Oid languageOid,
 							(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
 						   errmsg("SQL function cannot accept shell type %s",
 								  TypeNameToString(t))));
+				/* We don't allow creating aggregates on shell types either */
+				else if (is_aggregate)
+					ereport(ERROR,
+							(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
+							 errmsg("aggregate cannot accept shell type %s",
+									TypeNameToString(t))));
 				else
 					ereport(NOTICE,
 							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
@@ -246,9 +260,16 @@ examine_parameter_list(List *parameters, Oid languageOid,
 			aclcheck_error_type(aclresult, toid);
 
 		if (t->setof)
-			ereport(ERROR,
-					(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
-					 errmsg("functions cannot accept set arguments")));
+		{
+			if (is_aggregate)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
+						 errmsg("aggregates cannot accept set arguments")));
+			else
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
+						 errmsg("functions cannot accept set arguments")));
+		}
 
 		/* handle input parameters */
 		if (fp->mode != FUNC_PARAM_OUT && fp->mode != FUNC_PARAM_TABLE)
@@ -890,13 +911,16 @@ CreateFunction(CreateFunctionStmt *stmt, const char *queryString)
 	 * Convert remaining parameters of CREATE to form wanted by
 	 * ProcedureCreate.
 	 */
-	examine_parameter_list(stmt->parameters, languageOid, queryString,
-						   &parameterTypes,
-						   &allParameterTypes,
-						   &parameterModes,
-						   &parameterNames,
-						   &parameterDefaults,
-						   &requiredResultType);
+	interpret_function_parameter_list(stmt->parameters,
+									  languageOid,
+									  false,	/* not an aggregate */
+									  queryString,
+									  &parameterTypes,
+									  &allParameterTypes,
+									  &parameterModes,
+									  &parameterNames,
+									  &parameterDefaults,
+									  &requiredResultType);
 
 	if (stmt->returnType)
 	{