Allow referring to functions without arguments when unique

In DDL commands referring to an existing function, allow omitting the
argument list if the function name is unique in its schema, per SQL
standard.

This uses the same logic that the regproc type uses for finding
functions by name only.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

src/backend/nodes/copyfuncs.c

@@ -3067,6 +3067,7 @@ _copyObjectWithArgs(const ObjectWithArgs *from)
 
 	COPY_NODE_FIELD(objname);
 	COPY_NODE_FIELD(objargs);
+	COPY_SCALAR_FIELD(args_unspecified);
 
 	return newnode;
 }

src/backend/nodes/equalfuncs.c

@@ -1119,6 +1119,7 @@ _equalObjectWithArgs(const ObjectWithArgs *a, const ObjectWithArgs *b)
 {
 	COMPARE_NODE_FIELD(objname);
 	COMPARE_NODE_FIELD(objargs);
+	COMPARE_SCALAR_FIELD(args_unspecified);
 
 	return true;
 }

src/backend/parser/gram.y

@@ -7202,6 +7202,33 @@ function_with_argtypes:
 					n->objargs = extractArgTypes($2);
 					$$ = n;
 				}
+			/*
+			 * Because of reduce/reduce conflicts, we can't use func_name
+			 * below, but we can write it out the long way, which actually
+			 * allows more cases.
+			 */
+			| type_func_name_keyword
+				{
+					ObjectWithArgs *n = makeNode(ObjectWithArgs);
+					n->objname = list_make1(makeString(pstrdup($1)));
+					n->args_unspecified = true;
+					$$ = n;
+				}
+			| ColId
+				{
+					ObjectWithArgs *n = makeNode(ObjectWithArgs);
+					n->objname = list_make1(makeString($1));
+					n->args_unspecified = true;
+					$$ = n;
+				}
+			| ColId indirection
+				{
+					ObjectWithArgs *n = makeNode(ObjectWithArgs);
+					n->objname = check_func_name(lcons(makeString($1), $2),
+												  yyscanner);
+					n->args_unspecified = true;
+					$$ = n;
+				}
 		;
 
 /*

src/backend/parser/parse_func.c

@@ -1895,8 +1895,10 @@ func_signature_string(List *funcname, int nargs,
 
 /*
  * LookupFuncName
- *		Given a possibly-qualified function name and a set of argument types,
- *		look up the function.
+ *
+ * Given a possibly-qualified function name and optionally a set of argument
+ * types, look up the function.  Pass nargs == -1 to indicate that no argument
+ * types are specified.
  *
  * If the function name is not schema-qualified, it is sought in the current
  * namespace search path.
@@ -1914,6 +1916,35 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 
 	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false, noError);
 
+	/*
+	 * If no arguments were specified, the name must yield a unique candidate.
+	 */
+	if (nargs == -1)
+	{
+		if (clist)
+		{
+			if (clist->next)
+			{
+				if (!noError)
+					ereport(ERROR,
+							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
+							 errmsg("function name \"%s\" is not unique",
+									NameListToString(funcname)),
+							 errhint("Specify the argument list to select the function unambiguously.")));
+			}
+			else
+				return clist->oid;
+		}
+		else
+		{
+			if (!noError)
+				ereport(ERROR,
+						(errcode(ERRCODE_UNDEFINED_FUNCTION),
+						 errmsg("could not find a function named \"%s\"",
+								NameListToString(funcname))));
+		}
+	}
+
 	while (clist)
 	{
 		if (memcmp(argtypes, clist->args, nargs * sizeof(Oid)) == 0)
@@ -1962,7 +1993,7 @@ LookupFuncWithArgs(ObjectWithArgs *func, bool noError)
 		args_item = lnext(args_item);
 	}
 
-	return LookupFuncName(func->objname, argcount, argoids, noError);
+	return LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids, noError);
 }
 
 /*

src/include/nodes/parsenodes.h

@@ -1811,6 +1811,9 @@ typedef struct ObjectWithArgs
 	NodeTag		type;
 	List	   *objname;		/* qualified name of function/operator */
 	List	   *objargs;		/* list of Typename nodes */
+	bool		args_unspecified; /* argument list was omitted, so name must
+								   * be unique (note that objargs == NIL means
+								   * zero args) */
 } ObjectWithArgs;
 
 /*

src/test/regress/expected/create_function_3.out

@@ -218,13 +218,21 @@ SELECT routine_name, ordinal_position, parameter_name, parameter_default
 (7 rows)
 
 DROP FUNCTION functest_IS_1(int, int, text), functest_IS_2(int), functest_IS_3(int);
+-- overload
+CREATE FUNCTION functest_B_2(bigint) RETURNS bool LANGUAGE 'sql'
+       IMMUTABLE AS 'SELECT $1 > 0';
+DROP FUNCTION functest_b_1;
+DROP FUNCTION functest_b_1;  -- error, not found
+ERROR:  could not find a function named "functest_b_1"
+DROP FUNCTION functest_b_2;  -- error, ambiguous
+ERROR:  function name "functest_b_2" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
 -- Cleanups
 DROP SCHEMA temp_func_test CASCADE;
 NOTICE:  drop cascades to 16 other objects
 DETAIL:  drop cascades to function functest_a_1(text,date)
 drop cascades to function functest_a_2(text[])
 drop cascades to function functest_a_3()
-drop cascades to function functest_b_1(integer)
 drop cascades to function functest_b_2(integer)
 drop cascades to function functest_b_3(integer)
 drop cascades to function functest_b_4(integer)
@@ -237,5 +245,6 @@ drop cascades to function functext_f_1(integer)
 drop cascades to function functext_f_2(integer)
 drop cascades to function functext_f_3(integer)
 drop cascades to function functext_f_4(integer)
+drop cascades to function functest_b_2(bigint)
 DROP USER regress_unpriv_user;
 RESET search_path;

src/test/regress/sql/create_function_3.sql

@@ -158,6 +158,14 @@ SELECT routine_name, ordinal_position, parameter_name, parameter_default
 
 DROP FUNCTION functest_IS_1(int, int, text), functest_IS_2(int), functest_IS_3(int);
 
+-- overload
+CREATE FUNCTION functest_B_2(bigint) RETURNS bool LANGUAGE 'sql'
+       IMMUTABLE AS 'SELECT $1 > 0';
+
+DROP FUNCTION functest_b_1;
+DROP FUNCTION functest_b_1;  -- error, not found
+DROP FUNCTION functest_b_2;  -- error, ambiguous
+
 
 -- Cleanups
 DROP SCHEMA temp_func_test CASCADE;