BUG #15572: Misleading message reported by "Drop function operation" on DB with functions having same name

diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 4661fc4f62..070701044b 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -2053,12 +2053,15 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		{
 			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.")));
+				/*
+				 * Raise an error regardless of if noError is set if there are
+				 * multiple matching functions.
+				 */
+				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;
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index b80c5ed2d8..dd4e6c0f6d 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -138,6 +138,16 @@ DROP FUNCTION test_function_exists(int, text, int[]);
 ERROR:  function test_function_exists(integer, text, integer[]) does not exist
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 NOTICE:  function test_function_exists(pg_catalog.int4,text,pg_catalog.int4[]) does not exist, skipping
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+ERROR:  function name "test_ambiguous_funcname" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 ERROR:  aggregate test_aggregate_exists(*) does not exist
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index c1d30bc4a5..33f513e436 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -153,6 +153,17 @@ DROP FUNCTION IF EXISTS test_function_exists();
 DROP FUNCTION test_function_exists(int, text, int[]);
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+
+
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 DROP AGGREGATE IF EXISTS test_aggregate_exists(*);

diff --git a/src/backend/commands/amcmds.c b/src/backend/commands/amcmds.c
index 1ab2e469ee..784d7553e8 100644
--- a/src/backend/commands/amcmds.c
+++ b/src/backend/commands/amcmds.c
@@ -254,7 +254,7 @@ lookup_index_am_handler_func(List *handler_name, char amtype)
 				 errmsg("handler function is not specified")));
 
 	/* handlers have one argument of type internal */
-	handlerOid = LookupFuncName(handler_name, 1, funcargtypes, false);
+	handlerOid = LookupFuncName(handler_name, 1, funcargtypes, false, false);
 
 	/* check that handler has the correct return type */
 	switch (amtype)
diff --git a/src/backend/commands/conversioncmds.c b/src/backend/commands/conversioncmds.c
index 5afe86712f..122573464f 100644
--- a/src/backend/commands/conversioncmds.c
+++ b/src/backend/commands/conversioncmds.c
@@ -77,7 +77,7 @@ CreateConversionCommand(CreateConversionStmt *stmt)
 	 * a qualified name.
 	 */
 	funcoid = LookupFuncName(func_name, sizeof(funcargs) / sizeof(Oid),
-							 funcargs, false);
+							 funcargs, false, false);
 
 	/* Check it returns VOID, else it's probably the wrong function */
 	if (get_func_rettype(funcoid) != VOIDOID)
diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index e508334981..34f72114fa 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -239,7 +239,7 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 						stmt->trigname)));
 
 	/* Find and validate the trigger function. */
-	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false, false);
 	funcrettype = get_func_rettype(funcoid);
 	if (funcrettype != EVTTRIGGEROID)
 		ereport(ERROR,
diff --git a/src/backend/commands/foreigncmds.c b/src/backend/commands/foreigncmds.c
index 5ada4a8594..16c9242241 100644
--- a/src/backend/commands/foreigncmds.c
+++ b/src/backend/commands/foreigncmds.c
@@ -484,7 +484,8 @@ lookup_fdw_handler_func(DefElem *handler)
 		return InvalidOid;
 
 	/* handlers have no arguments */
-	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false);
+	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false,
+								false);
 
 	/* check that handler has correct return type */
 	if (get_func_rettype(handlerOid) != FDW_HANDLEROID)
@@ -511,7 +512,8 @@ lookup_fdw_validator_func(DefElem *validator)
 	funcargtypes[0] = TEXTARRAYOID;
 	funcargtypes[1] = OIDOID;
 
-	return LookupFuncName((List *) validator->arg, 2, funcargtypes, false);
+	return LookupFuncName((List *) validator->arg, 2, funcargtypes, false,
+						  false);
 	/* validator's return value is ignored, so we don't check the type */
 }
 
diff --git a/src/backend/commands/operatorcmds.c b/src/backend/commands/operatorcmds.c
index f7c00e2f48..ade1a23550 100644
--- a/src/backend/commands/operatorcmds.c
+++ b/src/backend/commands/operatorcmds.c
@@ -208,7 +208,7 @@ DefineOperator(List *names, List *parameters)
 		typeId[1] = typeId2;
 		nargs = 2;
 	}
-	functionOid = LookupFuncName(functionName, nargs, typeId, false);
+	functionOid = LookupFuncName(functionName, nargs, typeId, false, false);
 
 	/*
 	 * We require EXECUTE rights for the function.  This isn't strictly
@@ -271,7 +271,7 @@ ValidateRestrictionEstimator(List *restrictionName)
 	typeId[2] = INTERNALOID;	/* args list */
 	typeId[3] = INT4OID;		/* varRelid */
 
-	restrictionOid = LookupFuncName(restrictionName, 4, typeId, false);
+	restrictionOid = LookupFuncName(restrictionName, 4, typeId, false, false);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(restrictionOid) != FLOAT8OID)
@@ -312,12 +312,12 @@ ValidateJoinEstimator(List *joinName)
 	 * arguments, but we still allow the old 4-argument form. Try the
 	 * preferred form first.
 	 */
-	joinOid = LookupFuncName(joinName, 5, typeId, true);
+	joinOid = LookupFuncName(joinName, 5, typeId, true, false);
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 4, typeId, true);
+		joinOid = LookupFuncName(joinName, 4, typeId, true, false);
 	/* If not found, reference the 5-argument signature in error msg */
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 5, typeId, false);
+		joinOid = LookupFuncName(joinName, 5, typeId, false, false);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(joinOid) != FLOAT8OID)
diff --git a/src/backend/commands/proclang.c b/src/backend/commands/proclang.c
index b010d23466..2fe4d84997 100644
--- a/src/backend/commands/proclang.c
+++ b/src/backend/commands/proclang.c
@@ -107,7 +107,7 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * return type.
 		 */
 		funcname = SystemFuncName(pltemplate->tmplhandler);
-		handlerOid = LookupFuncName(funcname, 0, funcargtypes, true);
+		handlerOid = LookupFuncName(funcname, 0, funcargtypes, true, false);
 		if (OidIsValid(handlerOid))
 		{
 			funcrettype = get_func_rettype(handlerOid);
@@ -155,7 +155,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplinline);
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			inlineOid = LookupFuncName(funcname, 1, funcargtypes, true,
+									   false);
 			if (!OidIsValid(inlineOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplinline,
@@ -197,7 +198,7 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplvalidator);
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			valOid = LookupFuncName(funcname, 1, funcargtypes, true, false);
 			if (!OidIsValid(valOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplvalidator,
@@ -262,7 +263,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * Lookup the PL handler function and check that it is of the expected
 		 * return type
 		 */
-		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes, false);
+		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes, false,
+									false);
 		funcrettype = get_func_rettype(handlerOid);
 		if (funcrettype != LANGUAGE_HANDLEROID)
 		{
@@ -291,7 +293,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plinline)
 		{
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes, false);
+			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes, false,
+									   false);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
@@ -301,7 +304,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plvalidator)
 		{
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes, false);
+			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes, false,
+									false);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index 2daffae8cd..b1a4850406 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -669,7 +669,7 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 	 * Find and validate the trigger function.
 	 */
 	if (!OidIsValid(funcoid))
-		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false, false);
 	if (!isInternal)
 	{
 		aclresult = pg_proc_aclcheck(funcoid, GetUserId(), ACL_EXECUTE);
diff --git a/src/backend/commands/tsearchcmds.c b/src/backend/commands/tsearchcmds.c
index cda21675f0..41f2ab3f58 100644
--- a/src/backend/commands/tsearchcmds.c
+++ b/src/backend/commands/tsearchcmds.c
@@ -109,7 +109,7 @@ get_ts_parser_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, false, false);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
@@ -680,7 +680,7 @@ get_ts_template_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, false, false);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
diff --git a/src/backend/commands/typecmds.c b/src/backend/commands/typecmds.c
index 9ca30b0443..b4f9c8897a 100644
--- a/src/backend/commands/typecmds.c
+++ b/src/backend/commands/typecmds.c
@@ -1699,28 +1699,28 @@ findTypeInputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = CSTRINGOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 
 	if (!OidIsValid(procOid))
 	{
 		argList[1] = OIDOID;
 		argList[2] = INT4OID;
 
-		procOid = LookupFuncName(procname, 3, argList, true);
+		procOid = LookupFuncName(procname, 3, argList, true, false);
 	}
 
 	if (OidIsValid(procOid))
@@ -1765,14 +1765,14 @@ findTypeOutputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 
 	if (OidIsValid(procOid))
 	{
@@ -1814,14 +1814,14 @@ findTypeReceiveFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1844,7 +1844,7 @@ findTypeSendFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1867,7 +1867,7 @@ findTypeTypmodinFunction(List *procname)
 	 */
 	argList[0] = CSTRINGARRAYOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1894,7 +1894,7 @@ findTypeTypmodoutFunction(List *procname)
 	 */
 	argList[0] = INT4OID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1921,7 +1921,7 @@ findTypeAnalyzeFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1997,7 +1997,7 @@ findRangeCanonicalFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
@@ -2039,7 +2039,7 @@ findRangeSubtypeDiffFunction(List *procname, Oid subtype)
 	argList[0] = subtype;
 	argList[1] = subtype;
 
-	procOid = LookupFuncName(procname, 2, argList, true);
+	procOid = LookupFuncName(procname, 2, argList, true, false);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index 6963922b0e..752f1cc43a 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -934,7 +934,7 @@ transformRangeTableSample(ParseState *pstate, RangeTableSample *rts)
 	 */
 	funcargtypes[0] = INTERNALOID;
 
-	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, true);
+	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, true, false);
 
 	/* we want error to complain about no-such-method, not no-such-function */
 	if (!OidIsValid(handlerOid))
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 4661fc4f62..6f69d360f6 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -2032,17 +2032,21 @@ func_signature_string(List *funcname, int nargs,
  * namespace search path.
  *
  * If the function is not found, we return InvalidOid if noError is true,
- * else raise an error.
+ * else raise an error.  missing_ok is similar to noError except we still
+ * raise an error for an ambigious function, regardless of what missing_ok is
+ * set to.
  */
 Oid
-LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
+LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError,
+			   bool missing_ok)
 {
 	FuncCandidateList clist;
 
 	/* Passing NULL for argtypes is no longer allowed */
 	Assert(argtypes);
 
-	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false, noError);
+	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false,
+								  noError || missing_ok);
 
 	/*
 	 * If no arguments were specified, the name must yield a unique candidate.
@@ -2065,7 +2069,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		}
 		else
 		{
-			if (!noError)
+			if (!noError && !missing_ok)
 				ereport(ERROR,
 						(errcode(ERRCODE_UNDEFINED_FUNCTION),
 						 errmsg("could not find a function named \"%s\"",
@@ -2080,7 +2084,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		clist = clist->next;
 	}
 
-	if (!noError)
+	if (!noError && !missing_ok)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
 				 errmsg("function %s does not exist",
@@ -2135,10 +2139,11 @@ LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 
 	/*
 	 * When looking for a function or routine, we pass noError through to
-	 * LookupFuncName and let it make any error messages.  Otherwise, we make
-	 * our own errors for the aggregate and procedure cases.
+	 * LookupFuncName's missing_ok parameter and let it make any error
+	 * messages.  Otherwise, we make our own errors for the aggregate and
+	 * procedure cases.
 	 */
-	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids,
+	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids, false,
 						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? noError : true);
 
 	if (objtype == OBJECT_FUNCTION)
diff --git a/src/include/parser/parse_func.h b/src/include/parser/parse_func.h
index ff1477e64f..4e8f7af0f3 100644
--- a/src/include/parser/parse_func.h
+++ b/src/include/parser/parse_func.h
@@ -63,7 +63,7 @@ extern const char *func_signature_string(List *funcname, int nargs,
 					  List *argnames, const Oid *argtypes);
 
 extern Oid LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
-			   bool noError);
+			   bool noError, bool missing_ok);
 extern Oid LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
 				   bool noError);
 
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index b80c5ed2d8..dd4e6c0f6d 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -138,6 +138,16 @@ DROP FUNCTION test_function_exists(int, text, int[]);
 ERROR:  function test_function_exists(integer, text, integer[]) does not exist
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 NOTICE:  function test_function_exists(pg_catalog.int4,text,pg_catalog.int4[]) does not exist, skipping
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+ERROR:  function name "test_ambiguous_funcname" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 ERROR:  aggregate test_aggregate_exists(*) does not exist
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index c1d30bc4a5..ad4ef76460 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -153,6 +153,16 @@ DROP FUNCTION IF EXISTS test_function_exists();
 DROP FUNCTION test_function_exists(int, text, int[]);
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 DROP AGGREGATE IF EXISTS test_aggregate_exists(*);

diff --git a/src/backend/commands/amcmds.c b/src/backend/commands/amcmds.c
index c84507b5d0..9f2bad2afd 100644
--- a/src/backend/commands/amcmds.c
+++ b/src/backend/commands/amcmds.c
@@ -254,7 +254,7 @@ lookup_index_am_handler_func(List *handler_name, char amtype)
 				 errmsg("handler function is not specified")));
 
 	/* handlers have one argument of type internal */
-	handlerOid = LookupFuncName(handler_name, 1, funcargtypes, false);
+	handlerOid = LookupFuncName(handler_name, 1, funcargtypes, false, false);
 
 	/* check that handler has the correct return type */
 	switch (amtype)
diff --git a/src/backend/commands/conversioncmds.c b/src/backend/commands/conversioncmds.c
index 5afe86712f..122573464f 100644
--- a/src/backend/commands/conversioncmds.c
+++ b/src/backend/commands/conversioncmds.c
@@ -77,7 +77,7 @@ CreateConversionCommand(CreateConversionStmt *stmt)
 	 * a qualified name.
 	 */
 	funcoid = LookupFuncName(func_name, sizeof(funcargs) / sizeof(Oid),
-							 funcargs, false);
+							 funcargs, false, false);
 
 	/* Check it returns VOID, else it's probably the wrong function */
 	if (get_func_rettype(funcoid) != VOIDOID)
diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index adb77d8f69..91c1ea08c5 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -238,7 +238,7 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 						stmt->trigname)));
 
 	/* Find and validate the trigger function. */
-	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false, false);
 	funcrettype = get_func_rettype(funcoid);
 	if (funcrettype != EVTTRIGGEROID)
 		ereport(ERROR,
diff --git a/src/backend/commands/foreigncmds.c b/src/backend/commands/foreigncmds.c
index 413ce3fcb6..02ed460847 100644
--- a/src/backend/commands/foreigncmds.c
+++ b/src/backend/commands/foreigncmds.c
@@ -484,7 +484,8 @@ lookup_fdw_handler_func(DefElem *handler)
 		return InvalidOid;
 
 	/* handlers have no arguments */
-	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false);
+	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false,
+								false);
 
 	/* check that handler has correct return type */
 	if (get_func_rettype(handlerOid) != FDW_HANDLEROID)
@@ -511,7 +512,8 @@ lookup_fdw_validator_func(DefElem *validator)
 	funcargtypes[0] = TEXTARRAYOID;
 	funcargtypes[1] = OIDOID;
 
-	return LookupFuncName((List *) validator->arg, 2, funcargtypes, false);
+	return LookupFuncName((List *) validator->arg, 2, funcargtypes, false,
+						  false);
 	/* validator's return value is ignored, so we don't check the type */
 }
 
diff --git a/src/backend/commands/operatorcmds.c b/src/backend/commands/operatorcmds.c
index 7b98e4b910..86a37e18e3 100644
--- a/src/backend/commands/operatorcmds.c
+++ b/src/backend/commands/operatorcmds.c
@@ -208,7 +208,7 @@ DefineOperator(List *names, List *parameters)
 		typeId[1] = typeId2;
 		nargs = 2;
 	}
-	functionOid = LookupFuncName(functionName, nargs, typeId, false);
+	functionOid = LookupFuncName(functionName, nargs, typeId, false, false);
 
 	/*
 	 * We require EXECUTE rights for the function.  This isn't strictly
@@ -271,7 +271,7 @@ ValidateRestrictionEstimator(List *restrictionName)
 	typeId[2] = INTERNALOID;	/* args list */
 	typeId[3] = INT4OID;		/* varRelid */
 
-	restrictionOid = LookupFuncName(restrictionName, 4, typeId, false);
+	restrictionOid = LookupFuncName(restrictionName, 4, typeId, false, false);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(restrictionOid) != FLOAT8OID)
@@ -312,12 +312,12 @@ ValidateJoinEstimator(List *joinName)
 	 * arguments, but we still allow the old 4-argument form. Try the
 	 * preferred form first.
 	 */
-	joinOid = LookupFuncName(joinName, 5, typeId, true);
+	joinOid = LookupFuncName(joinName, 5, typeId, true, false);
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 4, typeId, true);
+		joinOid = LookupFuncName(joinName, 4, typeId, true, false);
 	/* If not found, reference the 5-argument signature in error msg */
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 5, typeId, false);
+		joinOid = LookupFuncName(joinName, 5, typeId, false, false);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(joinOid) != FLOAT8OID)
diff --git a/src/backend/commands/proclang.c b/src/backend/commands/proclang.c
index c2e9e41c07..7319c3d922 100644
--- a/src/backend/commands/proclang.c
+++ b/src/backend/commands/proclang.c
@@ -106,7 +106,7 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * return type.
 		 */
 		funcname = SystemFuncName(pltemplate->tmplhandler);
-		handlerOid = LookupFuncName(funcname, 0, funcargtypes, true);
+		handlerOid = LookupFuncName(funcname, 0, funcargtypes, true, false);
 		if (OidIsValid(handlerOid))
 		{
 			funcrettype = get_func_rettype(handlerOid);
@@ -154,7 +154,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplinline);
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			inlineOid = LookupFuncName(funcname, 1, funcargtypes, true,
+									   false);
 			if (!OidIsValid(inlineOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplinline,
@@ -196,7 +197,7 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplvalidator);
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			valOid = LookupFuncName(funcname, 1, funcargtypes, true, false);
 			if (!OidIsValid(valOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplvalidator,
@@ -261,7 +262,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * Lookup the PL handler function and check that it is of the expected
 		 * return type
 		 */
-		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes, false);
+		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes, false,
+									false);
 		funcrettype = get_func_rettype(handlerOid);
 		if (funcrettype != LANGUAGE_HANDLEROID)
 		{
@@ -290,7 +292,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plinline)
 		{
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes, false);
+			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes, false,
+									   false);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
@@ -300,7 +303,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plvalidator)
 		{
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes, false);
+			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes, false,
+									false);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index 0b245a613e..d6fcac7bdb 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -667,7 +667,7 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 	 * Find and validate the trigger function.
 	 */
 	if (!OidIsValid(funcoid))
-		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false, false);
 	if (!isInternal)
 	{
 		aclresult = pg_proc_aclcheck(funcoid, GetUserId(), ACL_EXECUTE);
diff --git a/src/backend/commands/tsearchcmds.c b/src/backend/commands/tsearchcmds.c
index 8e5eec22b5..7c1c7deb50 100644
--- a/src/backend/commands/tsearchcmds.c
+++ b/src/backend/commands/tsearchcmds.c
@@ -108,7 +108,7 @@ get_ts_parser_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, false, false);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
@@ -679,7 +679,7 @@ get_ts_template_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, false, false);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
diff --git a/src/backend/commands/typecmds.c b/src/backend/commands/typecmds.c
index fa7161ef9d..15b256895e 100644
--- a/src/backend/commands/typecmds.c
+++ b/src/backend/commands/typecmds.c
@@ -1699,28 +1699,28 @@ findTypeInputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = CSTRINGOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 
 	if (!OidIsValid(procOid))
 	{
 		argList[1] = OIDOID;
 		argList[2] = INT4OID;
 
-		procOid = LookupFuncName(procname, 3, argList, true);
+		procOid = LookupFuncName(procname, 3, argList, true, false);
 	}
 
 	if (OidIsValid(procOid))
@@ -1765,14 +1765,14 @@ findTypeOutputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 
 	if (OidIsValid(procOid))
 	{
@@ -1814,14 +1814,14 @@ findTypeReceiveFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1844,7 +1844,7 @@ findTypeSendFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1867,7 +1867,7 @@ findTypeTypmodinFunction(List *procname)
 	 */
 	argList[0] = CSTRINGARRAYOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1894,7 +1894,7 @@ findTypeTypmodoutFunction(List *procname)
 	 */
 	argList[0] = INT4OID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1921,7 +1921,7 @@ findTypeAnalyzeFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1997,7 +1997,7 @@ findRangeCanonicalFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
@@ -2039,7 +2039,7 @@ findRangeSubtypeDiffFunction(List *procname, Oid subtype)
 	argList[0] = subtype;
 	argList[1] = subtype;
 
-	procOid = LookupFuncName(procname, 2, argList, true);
+	procOid = LookupFuncName(procname, 2, argList, true, false);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index c6ce1011e2..e5cc88b245 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -933,7 +933,7 @@ transformRangeTableSample(ParseState *pstate, RangeTableSample *rts)
 	 */
 	funcargtypes[0] = INTERNALOID;
 
-	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, true);
+	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, true, false);
 
 	/* we want error to complain about no-such-method, not no-such-function */
 	if (!OidIsValid(handlerOid))
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 5222231b51..c2c7a9cacb 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -2032,17 +2032,21 @@ func_signature_string(List *funcname, int nargs,
  * namespace search path.
  *
  * If the function is not found, we return InvalidOid if noError is true,
- * else raise an error.
+ * else raise an error.  missing_ok is similar to noError except we still
+ * raise an error for an ambigious function, regardless of what missing_ok is
+ * set to.
  */
 Oid
-LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
+LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError,
+			   bool missing_ok)
 {
 	FuncCandidateList clist;
 
 	/* Passing NULL for argtypes is no longer allowed */
 	Assert(argtypes);
 
-	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false, noError);
+	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false,
+								  noError || missing_ok);
 
 	/*
 	 * If no arguments were specified, the name must yield a unique candidate.
@@ -2065,7 +2069,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		}
 		else
 		{
-			if (!noError)
+			if (!noError && !missing_ok)
 				ereport(ERROR,
 						(errcode(ERRCODE_UNDEFINED_FUNCTION),
 						 errmsg("could not find a function named \"%s\"",
@@ -2080,7 +2084,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		clist = clist->next;
 	}
 
-	if (!noError)
+	if (!noError && !missing_ok)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
 				 errmsg("function %s does not exist",
@@ -2135,10 +2139,11 @@ LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 
 	/*
 	 * When looking for a function or routine, we pass noError through to
-	 * LookupFuncName and let it make any error messages.  Otherwise, we make
-	 * our own errors for the aggregate and procedure cases.
+	 * LookupFuncName's missing_ok parameter and let it make any error
+	 * messages.  Otherwise, we make our own errors for the aggregate and
+	 * procedure cases.
 	 */
-	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids,
+	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids, false,
 						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? noError : true);
 
 	if (objtype == OBJECT_FUNCTION)
diff --git a/src/include/parser/parse_func.h b/src/include/parser/parse_func.h
index ff1477e64f..4e8f7af0f3 100644
--- a/src/include/parser/parse_func.h
+++ b/src/include/parser/parse_func.h
@@ -63,7 +63,7 @@ extern const char *func_signature_string(List *funcname, int nargs,
 					  List *argnames, const Oid *argtypes);
 
 extern Oid LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
-			   bool noError);
+			   bool noError, bool missing_ok);
 extern Oid LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
 				   bool noError);
 
diff --git a/src/pl/tcl/pltcl.c b/src/pl/tcl/pltcl.c
index bfbf62305c..05a4b2af16 100644
--- a/src/pl/tcl/pltcl.c
+++ b/src/pl/tcl/pltcl.c
@@ -616,7 +616,7 @@ call_pltcl_start_proc(Oid prolang, bool pltrusted)
 
 	/* Parse possibly-qualified identifier and look up the function */
 	namelist = stringToQualifiedNameList(start_proc);
-	procOid = LookupFuncName(namelist, 0, fargtypes, false);
+	procOid = LookupFuncName(namelist, 0, fargtypes, false, false);
 
 	/* Current user must have permission to call function */
 	aclresult = pg_proc_aclcheck(procOid, GetUserId(), ACL_EXECUTE);
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index b80c5ed2d8..dd4e6c0f6d 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -138,6 +138,16 @@ DROP FUNCTION test_function_exists(int, text, int[]);
 ERROR:  function test_function_exists(integer, text, integer[]) does not exist
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 NOTICE:  function test_function_exists(pg_catalog.int4,text,pg_catalog.int4[]) does not exist, skipping
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+ERROR:  function name "test_ambiguous_funcname" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 ERROR:  aggregate test_aggregate_exists(*) does not exist
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index c1d30bc4a5..ad4ef76460 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -153,6 +153,16 @@ DROP FUNCTION IF EXISTS test_function_exists();
 DROP FUNCTION test_function_exists(int, text, int[]);
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 DROP AGGREGATE IF EXISTS test_aggregate_exists(*);

diff --git a/src/backend/commands/amcmds.c b/src/backend/commands/amcmds.c
index c84507b5d0..9f2bad2afd 100644
--- a/src/backend/commands/amcmds.c
+++ b/src/backend/commands/amcmds.c
@@ -254,7 +254,7 @@ lookup_index_am_handler_func(List *handler_name, char amtype)
 				 errmsg("handler function is not specified")));
 
 	/* handlers have one argument of type internal */
-	handlerOid = LookupFuncName(handler_name, 1, funcargtypes, false);
+	handlerOid = LookupFuncName(handler_name, 1, funcargtypes, false, false);
 
 	/* check that handler has the correct return type */
 	switch (amtype)
diff --git a/src/backend/commands/conversioncmds.c b/src/backend/commands/conversioncmds.c
index 5afe86712f..122573464f 100644
--- a/src/backend/commands/conversioncmds.c
+++ b/src/backend/commands/conversioncmds.c
@@ -77,7 +77,7 @@ CreateConversionCommand(CreateConversionStmt *stmt)
 	 * a qualified name.
 	 */
 	funcoid = LookupFuncName(func_name, sizeof(funcargs) / sizeof(Oid),
-							 funcargs, false);
+							 funcargs, false, false);
 
 	/* Check it returns VOID, else it's probably the wrong function */
 	if (get_func_rettype(funcoid) != VOIDOID)
diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index adb77d8f69..91c1ea08c5 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -238,7 +238,7 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 						stmt->trigname)));
 
 	/* Find and validate the trigger function. */
-	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false, false);
 	funcrettype = get_func_rettype(funcoid);
 	if (funcrettype != EVTTRIGGEROID)
 		ereport(ERROR,
diff --git a/src/backend/commands/foreigncmds.c b/src/backend/commands/foreigncmds.c
index 413ce3fcb6..02ed460847 100644
--- a/src/backend/commands/foreigncmds.c
+++ b/src/backend/commands/foreigncmds.c
@@ -484,7 +484,8 @@ lookup_fdw_handler_func(DefElem *handler)
 		return InvalidOid;
 
 	/* handlers have no arguments */
-	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false);
+	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false,
+								false);
 
 	/* check that handler has correct return type */
 	if (get_func_rettype(handlerOid) != FDW_HANDLEROID)
@@ -511,7 +512,8 @@ lookup_fdw_validator_func(DefElem *validator)
 	funcargtypes[0] = TEXTARRAYOID;
 	funcargtypes[1] = OIDOID;
 
-	return LookupFuncName((List *) validator->arg, 2, funcargtypes, false);
+	return LookupFuncName((List *) validator->arg, 2, funcargtypes, false,
+						  false);
 	/* validator's return value is ignored, so we don't check the type */
 }
 
diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c
index 4f62e48d98..b7f9b46d4d 100644
--- a/src/backend/commands/functioncmds.c
+++ b/src/backend/commands/functioncmds.c
@@ -658,7 +658,7 @@ interpret_func_support(DefElem *defel)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procName, 1, argList, true);
+	procOid = LookupFuncName(procName, 1, argList, false, true);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
diff --git a/src/backend/commands/operatorcmds.c b/src/backend/commands/operatorcmds.c
index 7b98e4b910..86a37e18e3 100644
--- a/src/backend/commands/operatorcmds.c
+++ b/src/backend/commands/operatorcmds.c
@@ -208,7 +208,7 @@ DefineOperator(List *names, List *parameters)
 		typeId[1] = typeId2;
 		nargs = 2;
 	}
-	functionOid = LookupFuncName(functionName, nargs, typeId, false);
+	functionOid = LookupFuncName(functionName, nargs, typeId, false, false);
 
 	/*
 	 * We require EXECUTE rights for the function.  This isn't strictly
@@ -271,7 +271,7 @@ ValidateRestrictionEstimator(List *restrictionName)
 	typeId[2] = INTERNALOID;	/* args list */
 	typeId[3] = INT4OID;		/* varRelid */
 
-	restrictionOid = LookupFuncName(restrictionName, 4, typeId, false);
+	restrictionOid = LookupFuncName(restrictionName, 4, typeId, false, false);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(restrictionOid) != FLOAT8OID)
@@ -312,12 +312,12 @@ ValidateJoinEstimator(List *joinName)
 	 * arguments, but we still allow the old 4-argument form. Try the
 	 * preferred form first.
 	 */
-	joinOid = LookupFuncName(joinName, 5, typeId, true);
+	joinOid = LookupFuncName(joinName, 5, typeId, true, false);
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 4, typeId, true);
+		joinOid = LookupFuncName(joinName, 4, typeId, true, false);
 	/* If not found, reference the 5-argument signature in error msg */
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 5, typeId, false);
+		joinOid = LookupFuncName(joinName, 5, typeId, false, false);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(joinOid) != FLOAT8OID)
diff --git a/src/backend/commands/proclang.c b/src/backend/commands/proclang.c
index 59c4e8dfd0..f0d0bc374d 100644
--- a/src/backend/commands/proclang.c
+++ b/src/backend/commands/proclang.c
@@ -106,7 +106,7 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * return type.
 		 */
 		funcname = SystemFuncName(pltemplate->tmplhandler);
-		handlerOid = LookupFuncName(funcname, 0, funcargtypes, true);
+		handlerOid = LookupFuncName(funcname, 0, funcargtypes, true, false);
 		if (OidIsValid(handlerOid))
 		{
 			funcrettype = get_func_rettype(handlerOid);
@@ -155,7 +155,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplinline);
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			inlineOid = LookupFuncName(funcname, 1, funcargtypes, true,
+									   false);
 			if (!OidIsValid(inlineOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplinline,
@@ -198,7 +199,7 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplvalidator);
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			valOid = LookupFuncName(funcname, 1, funcargtypes, true, false);
 			if (!OidIsValid(valOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplvalidator,
@@ -264,7 +265,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * Lookup the PL handler function and check that it is of the expected
 		 * return type
 		 */
-		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes, false);
+		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes, false,
+									false);
 		funcrettype = get_func_rettype(handlerOid);
 		if (funcrettype != LANGUAGE_HANDLEROID)
 		{
@@ -293,7 +295,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plinline)
 		{
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes, false);
+			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes, false,
+									   false);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
@@ -303,7 +306,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plvalidator)
 		{
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes, false);
+			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes, false,
+									false);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index 0b245a613e..d6fcac7bdb 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -667,7 +667,7 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 	 * Find and validate the trigger function.
 	 */
 	if (!OidIsValid(funcoid))
-		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false, false);
 	if (!isInternal)
 	{
 		aclresult = pg_proc_aclcheck(funcoid, GetUserId(), ACL_EXECUTE);
diff --git a/src/backend/commands/tsearchcmds.c b/src/backend/commands/tsearchcmds.c
index 8e5eec22b5..7c1c7deb50 100644
--- a/src/backend/commands/tsearchcmds.c
+++ b/src/backend/commands/tsearchcmds.c
@@ -108,7 +108,7 @@ get_ts_parser_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, false, false);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
@@ -679,7 +679,7 @@ get_ts_template_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, false, false);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
diff --git a/src/backend/commands/typecmds.c b/src/backend/commands/typecmds.c
index 448926db12..46a1270470 100644
--- a/src/backend/commands/typecmds.c
+++ b/src/backend/commands/typecmds.c
@@ -1700,28 +1700,28 @@ findTypeInputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = CSTRINGOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 
 	if (!OidIsValid(procOid))
 	{
 		argList[1] = OIDOID;
 		argList[2] = INT4OID;
 
-		procOid = LookupFuncName(procname, 3, argList, true);
+		procOid = LookupFuncName(procname, 3, argList, true, false);
 	}
 
 	if (OidIsValid(procOid))
@@ -1766,14 +1766,14 @@ findTypeOutputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 
 	if (OidIsValid(procOid))
 	{
@@ -1815,14 +1815,14 @@ findTypeReceiveFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1845,7 +1845,7 @@ findTypeSendFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1868,7 +1868,7 @@ findTypeTypmodinFunction(List *procname)
 	 */
 	argList[0] = CSTRINGARRAYOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1895,7 +1895,7 @@ findTypeTypmodoutFunction(List *procname)
 	 */
 	argList[0] = INT4OID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1922,7 +1922,7 @@ findTypeAnalyzeFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1998,7 +1998,7 @@ findRangeCanonicalFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, true, false);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
@@ -2040,7 +2040,7 @@ findRangeSubtypeDiffFunction(List *procname, Oid subtype)
 	argList[0] = subtype;
 	argList[1] = subtype;
 
-	procOid = LookupFuncName(procname, 2, argList, true);
+	procOid = LookupFuncName(procname, 2, argList, true, false);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index c6ce1011e2..e5cc88b245 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -933,7 +933,7 @@ transformRangeTableSample(ParseState *pstate, RangeTableSample *rts)
 	 */
 	funcargtypes[0] = INTERNALOID;
 
-	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, true);
+	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, true, false);
 
 	/* we want error to complain about no-such-method, not no-such-function */
 	if (!OidIsValid(handlerOid))
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 5222231b51..c2c7a9cacb 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -2032,17 +2032,21 @@ func_signature_string(List *funcname, int nargs,
  * namespace search path.
  *
  * If the function is not found, we return InvalidOid if noError is true,
- * else raise an error.
+ * else raise an error.  missing_ok is similar to noError except we still
+ * raise an error for an ambigious function, regardless of what missing_ok is
+ * set to.
  */
 Oid
-LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
+LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError,
+			   bool missing_ok)
 {
 	FuncCandidateList clist;
 
 	/* Passing NULL for argtypes is no longer allowed */
 	Assert(argtypes);
 
-	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false, noError);
+	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false,
+								  noError || missing_ok);
 
 	/*
 	 * If no arguments were specified, the name must yield a unique candidate.
@@ -2065,7 +2069,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		}
 		else
 		{
-			if (!noError)
+			if (!noError && !missing_ok)
 				ereport(ERROR,
 						(errcode(ERRCODE_UNDEFINED_FUNCTION),
 						 errmsg("could not find a function named \"%s\"",
@@ -2080,7 +2084,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		clist = clist->next;
 	}
 
-	if (!noError)
+	if (!noError && !missing_ok)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
 				 errmsg("function %s does not exist",
@@ -2135,10 +2139,11 @@ LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 
 	/*
 	 * When looking for a function or routine, we pass noError through to
-	 * LookupFuncName and let it make any error messages.  Otherwise, we make
-	 * our own errors for the aggregate and procedure cases.
+	 * LookupFuncName's missing_ok parameter and let it make any error
+	 * messages.  Otherwise, we make our own errors for the aggregate and
+	 * procedure cases.
 	 */
-	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids,
+	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids, false,
 						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? noError : true);
 
 	if (objtype == OBJECT_FUNCTION)
diff --git a/src/include/parser/parse_func.h b/src/include/parser/parse_func.h
index ff1477e64f..4e8f7af0f3 100644
--- a/src/include/parser/parse_func.h
+++ b/src/include/parser/parse_func.h
@@ -63,7 +63,7 @@ extern const char *func_signature_string(List *funcname, int nargs,
 					  List *argnames, const Oid *argtypes);
 
 extern Oid LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
-			   bool noError);
+			   bool noError, bool missing_ok);
 extern Oid LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
 				   bool noError);
 
diff --git a/src/pl/tcl/pltcl.c b/src/pl/tcl/pltcl.c
index bfbf62305c..05a4b2af16 100644
--- a/src/pl/tcl/pltcl.c
+++ b/src/pl/tcl/pltcl.c
@@ -616,7 +616,7 @@ call_pltcl_start_proc(Oid prolang, bool pltrusted)
 
 	/* Parse possibly-qualified identifier and look up the function */
 	namelist = stringToQualifiedNameList(start_proc);
-	procOid = LookupFuncName(namelist, 0, fargtypes, false);
+	procOid = LookupFuncName(namelist, 0, fargtypes, false, false);
 
 	/* Current user must have permission to call function */
 	aclresult = pg_proc_aclcheck(procOid, GetUserId(), ACL_EXECUTE);
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index b80c5ed2d8..dd4e6c0f6d 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -138,6 +138,16 @@ DROP FUNCTION test_function_exists(int, text, int[]);
 ERROR:  function test_function_exists(integer, text, integer[]) does not exist
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 NOTICE:  function test_function_exists(pg_catalog.int4,text,pg_catalog.int4[]) does not exist, skipping
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+ERROR:  function name "test_ambiguous_funcname" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 ERROR:  aggregate test_aggregate_exists(*) does not exist
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index c1d30bc4a5..ad4ef76460 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -153,6 +153,16 @@ DROP FUNCTION IF EXISTS test_function_exists();
 DROP FUNCTION test_function_exists(int, text, int[]);
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 DROP AGGREGATE IF EXISTS test_aggregate_exists(*);

diff --git a/src/backend/catalog/aclchk.c b/src/backend/catalog/aclchk.c
index 11ddce2a8b..f92f2f0a25 100644
--- a/src/backend/catalog/aclchk.c
+++ b/src/backend/catalog/aclchk.c
@@ -681,7 +681,8 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				ObjectWithArgs *func = (ObjectWithArgs *) lfirst(cell);
 				Oid			funcid;
 
-				funcid = LookupFuncWithArgs(OBJECT_FUNCTION, func, false);
+				funcid = LookupFuncWithArgs(OBJECT_FUNCTION, func,
+											FUNCLOOKUP_NORMAL);
 				objects = lappend_oid(objects, funcid);
 			}
 			break;
@@ -725,7 +726,8 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				ObjectWithArgs *func = (ObjectWithArgs *) lfirst(cell);
 				Oid			procid;
 
-				procid = LookupFuncWithArgs(OBJECT_PROCEDURE, func, false);
+				procid = LookupFuncWithArgs(OBJECT_PROCEDURE, func,
+											FUNCLOOKUP_NORMAL);
 				objects = lappend_oid(objects, procid);
 			}
 			break;
@@ -735,7 +737,8 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				ObjectWithArgs *func = (ObjectWithArgs *) lfirst(cell);
 				Oid			routid;
 
-				routid = LookupFuncWithArgs(OBJECT_ROUTINE, func, false);
+				routid = LookupFuncWithArgs(OBJECT_ROUTINE, func,
+											FUNCLOOKUP_NORMAL);
 				objects = lappend_oid(objects, routid);
 			}
 			break;
diff --git a/src/backend/catalog/objectaddress.c b/src/backend/catalog/objectaddress.c
index ab3ec7e356..2f6fd892bf 100644
--- a/src/backend/catalog/objectaddress.c
+++ b/src/backend/catalog/objectaddress.c
@@ -926,7 +926,9 @@ get_object_address(ObjectType objtype, Node *object,
 			case OBJECT_PROCEDURE:
 			case OBJECT_ROUTINE:
 				address.classId = ProcedureRelationId;
-				address.objectId = LookupFuncWithArgs(objtype, castNode(ObjectWithArgs, object), missing_ok);
+				address.objectId = LookupFuncWithArgs(objtype, castNode(ObjectWithArgs, object),
+													  missing_ok ? FUNCLOOKUP_ERRIFAMBIGUOUS :
+													  FUNCLOOKUP_NORMAL);
 				address.objectSubId = 0;
 				break;
 			case OBJECT_OPERATOR:
diff --git a/src/backend/commands/amcmds.c b/src/backend/commands/amcmds.c
index c84507b5d0..db27616f32 100644
--- a/src/backend/commands/amcmds.c
+++ b/src/backend/commands/amcmds.c
@@ -254,7 +254,8 @@ lookup_index_am_handler_func(List *handler_name, char amtype)
 				 errmsg("handler function is not specified")));
 
 	/* handlers have one argument of type internal */
-	handlerOid = LookupFuncName(handler_name, 1, funcargtypes, false);
+	handlerOid = LookupFuncName(handler_name, 1, funcargtypes,
+								FUNCLOOKUP_NORMAL);
 
 	/* check that handler has the correct return type */
 	switch (amtype)
diff --git a/src/backend/commands/conversioncmds.c b/src/backend/commands/conversioncmds.c
index 5afe86712f..1329c58d27 100644
--- a/src/backend/commands/conversioncmds.c
+++ b/src/backend/commands/conversioncmds.c
@@ -77,7 +77,7 @@ CreateConversionCommand(CreateConversionStmt *stmt)
 	 * a qualified name.
 	 */
 	funcoid = LookupFuncName(func_name, sizeof(funcargs) / sizeof(Oid),
-							 funcargs, false);
+							 funcargs, FUNCLOOKUP_NORMAL);
 
 	/* Check it returns VOID, else it's probably the wrong function */
 	if (get_func_rettype(funcoid) != VOIDOID)
diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index adb77d8f69..8e922e6c2a 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -238,7 +238,7 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 						stmt->trigname)));
 
 	/* Find and validate the trigger function. */
-	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, FUNCLOOKUP_NORMAL);
 	funcrettype = get_func_rettype(funcoid);
 	if (funcrettype != EVTTRIGGEROID)
 		ereport(ERROR,
diff --git a/src/backend/commands/foreigncmds.c b/src/backend/commands/foreigncmds.c
index 413ce3fcb6..9cb4dadfc4 100644
--- a/src/backend/commands/foreigncmds.c
+++ b/src/backend/commands/foreigncmds.c
@@ -484,7 +484,8 @@ lookup_fdw_handler_func(DefElem *handler)
 		return InvalidOid;
 
 	/* handlers have no arguments */
-	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false);
+	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes,
+								FUNCLOOKUP_NORMAL);
 
 	/* check that handler has correct return type */
 	if (get_func_rettype(handlerOid) != FDW_HANDLEROID)
@@ -511,7 +512,8 @@ lookup_fdw_validator_func(DefElem *validator)
 	funcargtypes[0] = TEXTARRAYOID;
 	funcargtypes[1] = OIDOID;
 
-	return LookupFuncName((List *) validator->arg, 2, funcargtypes, false);
+	return LookupFuncName((List *) validator->arg, 2, funcargtypes,
+						  FUNCLOOKUP_NORMAL);
 	/* validator's return value is ignored, so we don't check the type */
 }
 
diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c
index 4f62e48d98..c3a350d6ed 100644
--- a/src/backend/commands/functioncmds.c
+++ b/src/backend/commands/functioncmds.c
@@ -658,7 +658,7 @@ interpret_func_support(DefElem *defel)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procName, 1, argList, true);
+	procOid = LookupFuncName(procName, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1251,7 +1251,8 @@ AlterFunction(ParseState *pstate, AlterFunctionStmt *stmt)
 
 	rel = table_open(ProcedureRelationId, RowExclusiveLock);
 
-	funcOid = LookupFuncWithArgs(stmt->objtype, stmt->func, false);
+	funcOid = LookupFuncWithArgs(stmt->objtype, stmt->func,
+								 FUNCLOOKUP_NORMAL);
 
 	ObjectAddressSet(address, ProcedureRelationId, funcOid);
 
@@ -1567,7 +1568,8 @@ CreateCast(CreateCastStmt *stmt)
 	{
 		Form_pg_proc procstruct;
 
-		funcid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->func, false);
+		funcid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->func,
+									FUNCLOOKUP_NORMAL);
 
 		tuple = SearchSysCache1(PROCOID, ObjectIdGetDatum(funcid));
 		if (!HeapTupleIsValid(tuple))
@@ -1942,7 +1944,8 @@ CreateTransform(CreateTransformStmt *stmt)
 	 */
 	if (stmt->fromsql)
 	{
-		fromsqlfuncid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->fromsql, false);
+		fromsqlfuncid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->fromsql,
+										   FUNCLOOKUP_NORMAL);
 
 		if (!pg_proc_ownercheck(fromsqlfuncid, GetUserId()))
 			aclcheck_error(ACLCHECK_NOT_OWNER, OBJECT_FUNCTION, NameListToString(stmt->fromsql->objname));
diff --git a/src/backend/commands/opclasscmds.c b/src/backend/commands/opclasscmds.c
index 5d73848a8b..34312e5e72 100644
--- a/src/backend/commands/opclasscmds.c
+++ b/src/backend/commands/opclasscmds.c
@@ -529,7 +529,8 @@ DefineOpClass(CreateOpClassStmt *stmt)
 							 errmsg("invalid function number %d,"
 									" must be between 1 and %d",
 									item->number, maxProcNumber)));
-				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name, false);
+				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name,
+											 FUNCLOOKUP_NORMAL);
 #ifdef NOT_USED
 				/* XXX this is unnecessary given the superuser check above */
 				/* Caller must own function */
@@ -908,7 +909,8 @@ AlterOpFamilyAdd(AlterOpFamilyStmt *stmt, Oid amoid, Oid opfamilyoid,
 							 errmsg("invalid function number %d,"
 									" must be between 1 and %d",
 									item->number, maxProcNumber)));
-				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name, false);
+				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name,
+											 FUNCLOOKUP_NORMAL);
 #ifdef NOT_USED
 				/* XXX this is unnecessary given the superuser check above */
 				/* Caller must own function */
diff --git a/src/backend/commands/operatorcmds.c b/src/backend/commands/operatorcmds.c
index 7b98e4b910..6c1bc5208e 100644
--- a/src/backend/commands/operatorcmds.c
+++ b/src/backend/commands/operatorcmds.c
@@ -208,7 +208,8 @@ DefineOperator(List *names, List *parameters)
 		typeId[1] = typeId2;
 		nargs = 2;
 	}
-	functionOid = LookupFuncName(functionName, nargs, typeId, false);
+	functionOid = LookupFuncName(functionName, nargs, typeId,
+								 FUNCLOOKUP_NORMAL);
 
 	/*
 	 * We require EXECUTE rights for the function.  This isn't strictly
@@ -271,7 +272,8 @@ ValidateRestrictionEstimator(List *restrictionName)
 	typeId[2] = INTERNALOID;	/* args list */
 	typeId[3] = INT4OID;		/* varRelid */
 
-	restrictionOid = LookupFuncName(restrictionName, 4, typeId, false);
+	restrictionOid = LookupFuncName(restrictionName, 4, typeId,
+									FUNCLOOKUP_NORMAL);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(restrictionOid) != FLOAT8OID)
@@ -312,12 +314,12 @@ ValidateJoinEstimator(List *joinName)
 	 * arguments, but we still allow the old 4-argument form. Try the
 	 * preferred form first.
 	 */
-	joinOid = LookupFuncName(joinName, 5, typeId, true);
+	joinOid = LookupFuncName(joinName, 5, typeId, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 4, typeId, true);
+		joinOid = LookupFuncName(joinName, 4, typeId, FUNCLOOKUP_NOERROR);
 	/* If not found, reference the 5-argument signature in error msg */
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 5, typeId, false);
+		joinOid = LookupFuncName(joinName, 5, typeId, FUNCLOOKUP_NORMAL);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(joinOid) != FLOAT8OID)
diff --git a/src/backend/commands/proclang.c b/src/backend/commands/proclang.c
index 59c4e8dfd0..e94ca419be 100644
--- a/src/backend/commands/proclang.c
+++ b/src/backend/commands/proclang.c
@@ -106,7 +106,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * return type.
 		 */
 		funcname = SystemFuncName(pltemplate->tmplhandler);
-		handlerOid = LookupFuncName(funcname, 0, funcargtypes, true);
+		handlerOid = LookupFuncName(funcname, 0, funcargtypes,
+									FUNCLOOKUP_NOERROR);
 		if (OidIsValid(handlerOid))
 		{
 			funcrettype = get_func_rettype(handlerOid);
@@ -155,7 +156,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplinline);
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			inlineOid = LookupFuncName(funcname, 1, funcargtypes,
+									   FUNCLOOKUP_NOERROR);
 			if (!OidIsValid(inlineOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplinline,
@@ -198,7 +200,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplvalidator);
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			valOid = LookupFuncName(funcname, 1, funcargtypes,
+									FUNCLOOKUP_NOERROR);
 			if (!OidIsValid(valOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplvalidator,
@@ -264,7 +267,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * Lookup the PL handler function and check that it is of the expected
 		 * return type
 		 */
-		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes, false);
+		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes,
+									FUNCLOOKUP_NORMAL);
 		funcrettype = get_func_rettype(handlerOid);
 		if (funcrettype != LANGUAGE_HANDLEROID)
 		{
@@ -293,7 +297,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plinline)
 		{
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes, false);
+			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes,
+									   FUNCLOOKUP_NORMAL);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
@@ -303,7 +308,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plvalidator)
 		{
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes, false);
+			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes,
+									FUNCLOOKUP_NORMAL);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index 0b245a613e..68fb839524 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -667,7 +667,8 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 	 * Find and validate the trigger function.
 	 */
 	if (!OidIsValid(funcoid))
-		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes,
+								 FUNCLOOKUP_NORMAL);
 	if (!isInternal)
 	{
 		aclresult = pg_proc_aclcheck(funcoid, GetUserId(), ACL_EXECUTE);
diff --git a/src/backend/commands/tsearchcmds.c b/src/backend/commands/tsearchcmds.c
index 8e5eec22b5..30d3635598 100644
--- a/src/backend/commands/tsearchcmds.c
+++ b/src/backend/commands/tsearchcmds.c
@@ -108,7 +108,7 @@ get_ts_parser_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, FUNCLOOKUP_NORMAL);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
@@ -679,7 +679,7 @@ get_ts_template_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, FUNCLOOKUP_NORMAL);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
diff --git a/src/backend/commands/typecmds.c b/src/backend/commands/typecmds.c
index 448926db12..fe7a5c1017 100644
--- a/src/backend/commands/typecmds.c
+++ b/src/backend/commands/typecmds.c
@@ -1700,28 +1700,28 @@ findTypeInputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = CSTRINGOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 
 	if (!OidIsValid(procOid))
 	{
 		argList[1] = OIDOID;
 		argList[2] = INT4OID;
 
-		procOid = LookupFuncName(procname, 3, argList, true);
+		procOid = LookupFuncName(procname, 3, argList, FUNCLOOKUP_NOERROR);
 	}
 
 	if (OidIsValid(procOid))
@@ -1766,14 +1766,14 @@ findTypeOutputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 
 	if (OidIsValid(procOid))
 	{
@@ -1815,14 +1815,14 @@ findTypeReceiveFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1845,7 +1845,7 @@ findTypeSendFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1868,7 +1868,7 @@ findTypeTypmodinFunction(List *procname)
 	 */
 	argList[0] = CSTRINGARRAYOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1895,7 +1895,7 @@ findTypeTypmodoutFunction(List *procname)
 	 */
 	argList[0] = INT4OID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1922,7 +1922,7 @@ findTypeAnalyzeFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1998,7 +1998,7 @@ findRangeCanonicalFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
@@ -2040,7 +2040,7 @@ findRangeSubtypeDiffFunction(List *procname, Oid subtype)
 	argList[0] = subtype;
 	argList[1] = subtype;
 
-	procOid = LookupFuncName(procname, 2, argList, true);
+	procOid = LookupFuncName(procname, 2, argList, FUNCLOOKUP_NOERROR);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index c6ce1011e2..35fc73b493 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -933,7 +933,7 @@ transformRangeTableSample(ParseState *pstate, RangeTableSample *rts)
 	 */
 	funcargtypes[0] = INTERNALOID;
 
-	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, true);
+	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, FUNCLOOKUP_NOERROR);
 
 	/* we want error to complain about no-such-method, not no-such-function */
 	if (!OidIsValid(handlerOid))
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 5222231b51..4192ae9b2d 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -2031,18 +2031,25 @@ func_signature_string(List *funcname, int nargs,
  * If the function name is not schema-qualified, it is sought in the current
  * namespace search path.
  *
- * If the function is not found, we return InvalidOid if noError is true,
- * else raise an error.
+ * If the function is not found the behavior depends on the value of
+ * 'checktype'.  When it's set to FUNCLOOKUP_NORMAL, we report errors for each
+ * of the possible error situations.  When set to FUNCLOOKUP_ERRIFAMBIGUOUS,
+ * the only error we raise is for an ambiguous function definition, otherwise
+ * if set to FUNCLOOKUP_NOERROR we don't raise any errors.  In each of the
+ * cases we skip reporting an error, we return InvalidOid to indicate to the
+ * caller that the lookup has failed.
  */
 Oid
-LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
+LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
+			   FuncLookupCheckType checktype)
 {
 	FuncCandidateList clist;
 
 	/* Passing NULL for argtypes is no longer allowed */
 	Assert(argtypes);
 
-	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false, noError);
+	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false,
+								  (checktype != FUNCLOOKUP_NORMAL));
 
 	/*
 	 * If no arguments were specified, the name must yield a unique candidate.
@@ -2053,7 +2060,11 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		{
 			if (clist->next)
 			{
-				if (!noError)
+				/*
+				 * Raise this error on FUNCLOOKUP_NORMAL and
+				 * FUNCLOOKUP_ERRIFAMBIGUOUS
+				 */
+				if (checktype != FUNCLOOKUP_NOERROR)
 					ereport(ERROR,
 							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
 							 errmsg("function name \"%s\" is not unique",
@@ -2065,7 +2076,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		}
 		else
 		{
-			if (!noError)
+			if (checktype == FUNCLOOKUP_NORMAL)
 				ereport(ERROR,
 						(errcode(ERRCODE_UNDEFINED_FUNCTION),
 						 errmsg("could not find a function named \"%s\"",
@@ -2080,7 +2091,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		clist = clist->next;
 	}
 
-	if (!noError)
+	if (checktype == FUNCLOOKUP_NORMAL)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
 				 errmsg("function %s does not exist",
@@ -2102,10 +2113,12 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
  * function.
  */
 Oid
-LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
+LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
+				   FuncLookupCheckType checktype)
 {
 	Oid			argoids[FUNC_MAX_ARGS];
 	int			argcount;
+	bool		noError = (checktype != FUNCLOOKUP_NORMAL);
 	int			i;
 	ListCell   *args_item;
 	Oid			oid;
@@ -2134,12 +2147,14 @@ LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 	}
 
 	/*
-	 * When looking for a function or routine, we pass noError through to
-	 * LookupFuncName and let it make any error messages.  Otherwise, we make
+	 * When looking for a function or routine, we pass checktype through to
+	 * LookupFuncNameand let it make any error messages.  Otherwise, we make
 	 * our own errors for the aggregate and procedure cases.
 	 */
-	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids,
-						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? noError : true);
+	oid = LookupFuncName(func->objname,
+						 func->args_unspecified ? -1 : argcount,
+						 argoids,
+						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? checktype : FUNCLOOKUP_NOERROR);
 
 	if (objtype == OBJECT_FUNCTION)
 	{
diff --git a/src/include/parser/parse_func.h b/src/include/parser/parse_func.h
index ff1477e64f..d279183f17 100644
--- a/src/include/parser/parse_func.h
+++ b/src/include/parser/parse_func.h
@@ -30,6 +30,13 @@ typedef enum
 	FUNCDETAIL_COERCION			/* it's a type coercion request */
 } FuncDetailCode;
 
+/* Error checking types for LookupFuncName and LookupFuncWithArgs */
+typedef enum
+{
+	FUNCLOOKUP_NORMAL,			/* Full error checking */
+	FUNCLOOKUP_NOERROR,			/* Don't raise error for lookup failure */
+	FUNCLOOKUP_ERRIFAMBIGUOUS	/* Only raise ambiguous function errors */
+} FuncLookupCheckType;
 
 extern Node *ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 				  Node *last_srf, FuncCall *fn, bool proc_call,
@@ -63,9 +70,9 @@ extern const char *func_signature_string(List *funcname, int nargs,
 					  List *argnames, const Oid *argtypes);
 
 extern Oid LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
-			   bool noError);
+			   FuncLookupCheckType checktype);
 extern Oid LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
-				   bool noError);
+				   FuncLookupCheckType checktype);
 
 extern void check_srf_call_placement(ParseState *pstate, Node *last_srf,
 						 int location);
diff --git a/src/pl/tcl/pltcl.c b/src/pl/tcl/pltcl.c
index bfbf62305c..f075d2589a 100644
--- a/src/pl/tcl/pltcl.c
+++ b/src/pl/tcl/pltcl.c
@@ -616,7 +616,7 @@ call_pltcl_start_proc(Oid prolang, bool pltrusted)
 
 	/* Parse possibly-qualified identifier and look up the function */
 	namelist = stringToQualifiedNameList(start_proc);
-	procOid = LookupFuncName(namelist, 0, fargtypes, false);
+	procOid = LookupFuncName(namelist, 0, fargtypes, FUNCLOOKUP_NORMAL);
 
 	/* Current user must have permission to call function */
 	aclresult = pg_proc_aclcheck(procOid, GetUserId(), ACL_EXECUTE);
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index b80c5ed2d8..dd4e6c0f6d 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -138,6 +138,16 @@ DROP FUNCTION test_function_exists(int, text, int[]);
 ERROR:  function test_function_exists(integer, text, integer[]) does not exist
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 NOTICE:  function test_function_exists(pg_catalog.int4,text,pg_catalog.int4[]) does not exist, skipping
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+ERROR:  function name "test_ambiguous_funcname" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 ERROR:  aggregate test_aggregate_exists(*) does not exist
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index c1d30bc4a5..ad4ef76460 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -153,6 +153,16 @@ DROP FUNCTION IF EXISTS test_function_exists();
 DROP FUNCTION test_function_exists(int, text, int[]);
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 DROP AGGREGATE IF EXISTS test_aggregate_exists(*);

diff --git a/src/backend/catalog/aclchk.c b/src/backend/catalog/aclchk.c
index 11ddce2a8b..f92f2f0a25 100644
--- a/src/backend/catalog/aclchk.c
+++ b/src/backend/catalog/aclchk.c
@@ -681,7 +681,8 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				ObjectWithArgs *func = (ObjectWithArgs *) lfirst(cell);
 				Oid			funcid;
 
-				funcid = LookupFuncWithArgs(OBJECT_FUNCTION, func, false);
+				funcid = LookupFuncWithArgs(OBJECT_FUNCTION, func,
+											FUNCLOOKUP_NORMAL);
 				objects = lappend_oid(objects, funcid);
 			}
 			break;
@@ -725,7 +726,8 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				ObjectWithArgs *func = (ObjectWithArgs *) lfirst(cell);
 				Oid			procid;
 
-				procid = LookupFuncWithArgs(OBJECT_PROCEDURE, func, false);
+				procid = LookupFuncWithArgs(OBJECT_PROCEDURE, func,
+											FUNCLOOKUP_NORMAL);
 				objects = lappend_oid(objects, procid);
 			}
 			break;
@@ -735,7 +737,8 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				ObjectWithArgs *func = (ObjectWithArgs *) lfirst(cell);
 				Oid			routid;
 
-				routid = LookupFuncWithArgs(OBJECT_ROUTINE, func, false);
+				routid = LookupFuncWithArgs(OBJECT_ROUTINE, func,
+											FUNCLOOKUP_NORMAL);
 				objects = lappend_oid(objects, routid);
 			}
 			break;
diff --git a/src/backend/catalog/objectaddress.c b/src/backend/catalog/objectaddress.c
index ab3ec7e356..2f6fd892bf 100644
--- a/src/backend/catalog/objectaddress.c
+++ b/src/backend/catalog/objectaddress.c
@@ -926,7 +926,9 @@ get_object_address(ObjectType objtype, Node *object,
 			case OBJECT_PROCEDURE:
 			case OBJECT_ROUTINE:
 				address.classId = ProcedureRelationId;
-				address.objectId = LookupFuncWithArgs(objtype, castNode(ObjectWithArgs, object), missing_ok);
+				address.objectId = LookupFuncWithArgs(objtype, castNode(ObjectWithArgs, object),
+													  missing_ok ? FUNCLOOKUP_ERRIFAMBIGUOUS :
+													  FUNCLOOKUP_NORMAL);
 				address.objectSubId = 0;
 				break;
 			case OBJECT_OPERATOR:
diff --git a/src/backend/commands/amcmds.c b/src/backend/commands/amcmds.c
index c84507b5d0..db27616f32 100644
--- a/src/backend/commands/amcmds.c
+++ b/src/backend/commands/amcmds.c
@@ -254,7 +254,8 @@ lookup_index_am_handler_func(List *handler_name, char amtype)
 				 errmsg("handler function is not specified")));
 
 	/* handlers have one argument of type internal */
-	handlerOid = LookupFuncName(handler_name, 1, funcargtypes, false);
+	handlerOid = LookupFuncName(handler_name, 1, funcargtypes,
+								FUNCLOOKUP_NORMAL);
 
 	/* check that handler has the correct return type */
 	switch (amtype)
diff --git a/src/backend/commands/conversioncmds.c b/src/backend/commands/conversioncmds.c
index 5afe86712f..1329c58d27 100644
--- a/src/backend/commands/conversioncmds.c
+++ b/src/backend/commands/conversioncmds.c
@@ -77,7 +77,7 @@ CreateConversionCommand(CreateConversionStmt *stmt)
 	 * a qualified name.
 	 */
 	funcoid = LookupFuncName(func_name, sizeof(funcargs) / sizeof(Oid),
-							 funcargs, false);
+							 funcargs, FUNCLOOKUP_NORMAL);
 
 	/* Check it returns VOID, else it's probably the wrong function */
 	if (get_func_rettype(funcoid) != VOIDOID)
diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index adb77d8f69..8e922e6c2a 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -238,7 +238,7 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 						stmt->trigname)));
 
 	/* Find and validate the trigger function. */
-	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, FUNCLOOKUP_NORMAL);
 	funcrettype = get_func_rettype(funcoid);
 	if (funcrettype != EVTTRIGGEROID)
 		ereport(ERROR,
diff --git a/src/backend/commands/foreigncmds.c b/src/backend/commands/foreigncmds.c
index 413ce3fcb6..9cb4dadfc4 100644
--- a/src/backend/commands/foreigncmds.c
+++ b/src/backend/commands/foreigncmds.c
@@ -484,7 +484,8 @@ lookup_fdw_handler_func(DefElem *handler)
 		return InvalidOid;
 
 	/* handlers have no arguments */
-	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false);
+	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes,
+								FUNCLOOKUP_NORMAL);
 
 	/* check that handler has correct return type */
 	if (get_func_rettype(handlerOid) != FDW_HANDLEROID)
@@ -511,7 +512,8 @@ lookup_fdw_validator_func(DefElem *validator)
 	funcargtypes[0] = TEXTARRAYOID;
 	funcargtypes[1] = OIDOID;
 
-	return LookupFuncName((List *) validator->arg, 2, funcargtypes, false);
+	return LookupFuncName((List *) validator->arg, 2, funcargtypes,
+						  FUNCLOOKUP_NORMAL);
 	/* validator's return value is ignored, so we don't check the type */
 }
 
diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c
index 4f62e48d98..c3a350d6ed 100644
--- a/src/backend/commands/functioncmds.c
+++ b/src/backend/commands/functioncmds.c
@@ -658,7 +658,7 @@ interpret_func_support(DefElem *defel)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procName, 1, argList, true);
+	procOid = LookupFuncName(procName, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1251,7 +1251,8 @@ AlterFunction(ParseState *pstate, AlterFunctionStmt *stmt)
 
 	rel = table_open(ProcedureRelationId, RowExclusiveLock);
 
-	funcOid = LookupFuncWithArgs(stmt->objtype, stmt->func, false);
+	funcOid = LookupFuncWithArgs(stmt->objtype, stmt->func,
+								 FUNCLOOKUP_NORMAL);
 
 	ObjectAddressSet(address, ProcedureRelationId, funcOid);
 
@@ -1567,7 +1568,8 @@ CreateCast(CreateCastStmt *stmt)
 	{
 		Form_pg_proc procstruct;
 
-		funcid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->func, false);
+		funcid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->func,
+									FUNCLOOKUP_NORMAL);
 
 		tuple = SearchSysCache1(PROCOID, ObjectIdGetDatum(funcid));
 		if (!HeapTupleIsValid(tuple))
@@ -1942,7 +1944,8 @@ CreateTransform(CreateTransformStmt *stmt)
 	 */
 	if (stmt->fromsql)
 	{
-		fromsqlfuncid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->fromsql, false);
+		fromsqlfuncid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->fromsql,
+										   FUNCLOOKUP_NORMAL);
 
 		if (!pg_proc_ownercheck(fromsqlfuncid, GetUserId()))
 			aclcheck_error(ACLCHECK_NOT_OWNER, OBJECT_FUNCTION, NameListToString(stmt->fromsql->objname));
diff --git a/src/backend/commands/opclasscmds.c b/src/backend/commands/opclasscmds.c
index 5d73848a8b..34312e5e72 100644
--- a/src/backend/commands/opclasscmds.c
+++ b/src/backend/commands/opclasscmds.c
@@ -529,7 +529,8 @@ DefineOpClass(CreateOpClassStmt *stmt)
 							 errmsg("invalid function number %d,"
 									" must be between 1 and %d",
 									item->number, maxProcNumber)));
-				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name, false);
+				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name,
+											 FUNCLOOKUP_NORMAL);
 #ifdef NOT_USED
 				/* XXX this is unnecessary given the superuser check above */
 				/* Caller must own function */
@@ -908,7 +909,8 @@ AlterOpFamilyAdd(AlterOpFamilyStmt *stmt, Oid amoid, Oid opfamilyoid,
 							 errmsg("invalid function number %d,"
 									" must be between 1 and %d",
 									item->number, maxProcNumber)));
-				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name, false);
+				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name,
+											 FUNCLOOKUP_NORMAL);
 #ifdef NOT_USED
 				/* XXX this is unnecessary given the superuser check above */
 				/* Caller must own function */
diff --git a/src/backend/commands/operatorcmds.c b/src/backend/commands/operatorcmds.c
index 7b98e4b910..6c1bc5208e 100644
--- a/src/backend/commands/operatorcmds.c
+++ b/src/backend/commands/operatorcmds.c
@@ -208,7 +208,8 @@ DefineOperator(List *names, List *parameters)
 		typeId[1] = typeId2;
 		nargs = 2;
 	}
-	functionOid = LookupFuncName(functionName, nargs, typeId, false);
+	functionOid = LookupFuncName(functionName, nargs, typeId,
+								 FUNCLOOKUP_NORMAL);
 
 	/*
 	 * We require EXECUTE rights for the function.  This isn't strictly
@@ -271,7 +272,8 @@ ValidateRestrictionEstimator(List *restrictionName)
 	typeId[2] = INTERNALOID;	/* args list */
 	typeId[3] = INT4OID;		/* varRelid */
 
-	restrictionOid = LookupFuncName(restrictionName, 4, typeId, false);
+	restrictionOid = LookupFuncName(restrictionName, 4, typeId,
+									FUNCLOOKUP_NORMAL);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(restrictionOid) != FLOAT8OID)
@@ -312,12 +314,12 @@ ValidateJoinEstimator(List *joinName)
 	 * arguments, but we still allow the old 4-argument form. Try the
 	 * preferred form first.
 	 */
-	joinOid = LookupFuncName(joinName, 5, typeId, true);
+	joinOid = LookupFuncName(joinName, 5, typeId, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 4, typeId, true);
+		joinOid = LookupFuncName(joinName, 4, typeId, FUNCLOOKUP_NOERROR);
 	/* If not found, reference the 5-argument signature in error msg */
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 5, typeId, false);
+		joinOid = LookupFuncName(joinName, 5, typeId, FUNCLOOKUP_NORMAL);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(joinOid) != FLOAT8OID)
diff --git a/src/backend/commands/proclang.c b/src/backend/commands/proclang.c
index 59c4e8dfd0..e94ca419be 100644
--- a/src/backend/commands/proclang.c
+++ b/src/backend/commands/proclang.c
@@ -106,7 +106,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * return type.
 		 */
 		funcname = SystemFuncName(pltemplate->tmplhandler);
-		handlerOid = LookupFuncName(funcname, 0, funcargtypes, true);
+		handlerOid = LookupFuncName(funcname, 0, funcargtypes,
+									FUNCLOOKUP_NOERROR);
 		if (OidIsValid(handlerOid))
 		{
 			funcrettype = get_func_rettype(handlerOid);
@@ -155,7 +156,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplinline);
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			inlineOid = LookupFuncName(funcname, 1, funcargtypes,
+									   FUNCLOOKUP_NOERROR);
 			if (!OidIsValid(inlineOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplinline,
@@ -198,7 +200,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplvalidator);
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			valOid = LookupFuncName(funcname, 1, funcargtypes,
+									FUNCLOOKUP_NOERROR);
 			if (!OidIsValid(valOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplvalidator,
@@ -264,7 +267,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * Lookup the PL handler function and check that it is of the expected
 		 * return type
 		 */
-		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes, false);
+		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes,
+									FUNCLOOKUP_NORMAL);
 		funcrettype = get_func_rettype(handlerOid);
 		if (funcrettype != LANGUAGE_HANDLEROID)
 		{
@@ -293,7 +297,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plinline)
 		{
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes, false);
+			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes,
+									   FUNCLOOKUP_NORMAL);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
@@ -303,7 +308,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plvalidator)
 		{
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes, false);
+			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes,
+									FUNCLOOKUP_NORMAL);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index 409bee24f8..60798722a1 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -667,7 +667,8 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 	 * Find and validate the trigger function.
 	 */
 	if (!OidIsValid(funcoid))
-		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes,
+								 FUNCLOOKUP_NORMAL);
 	if (!isInternal)
 	{
 		aclresult = pg_proc_aclcheck(funcoid, GetUserId(), ACL_EXECUTE);
diff --git a/src/backend/commands/tsearchcmds.c b/src/backend/commands/tsearchcmds.c
index 8e5eec22b5..30d3635598 100644
--- a/src/backend/commands/tsearchcmds.c
+++ b/src/backend/commands/tsearchcmds.c
@@ -108,7 +108,7 @@ get_ts_parser_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, FUNCLOOKUP_NORMAL);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
@@ -679,7 +679,7 @@ get_ts_template_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, FUNCLOOKUP_NORMAL);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
diff --git a/src/backend/commands/typecmds.c b/src/backend/commands/typecmds.c
index 448926db12..fe7a5c1017 100644
--- a/src/backend/commands/typecmds.c
+++ b/src/backend/commands/typecmds.c
@@ -1700,28 +1700,28 @@ findTypeInputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = CSTRINGOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 
 	if (!OidIsValid(procOid))
 	{
 		argList[1] = OIDOID;
 		argList[2] = INT4OID;
 
-		procOid = LookupFuncName(procname, 3, argList, true);
+		procOid = LookupFuncName(procname, 3, argList, FUNCLOOKUP_NOERROR);
 	}
 
 	if (OidIsValid(procOid))
@@ -1766,14 +1766,14 @@ findTypeOutputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 
 	if (OidIsValid(procOid))
 	{
@@ -1815,14 +1815,14 @@ findTypeReceiveFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1845,7 +1845,7 @@ findTypeSendFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1868,7 +1868,7 @@ findTypeTypmodinFunction(List *procname)
 	 */
 	argList[0] = CSTRINGARRAYOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1895,7 +1895,7 @@ findTypeTypmodoutFunction(List *procname)
 	 */
 	argList[0] = INT4OID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1922,7 +1922,7 @@ findTypeAnalyzeFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1998,7 +1998,7 @@ findRangeCanonicalFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
@@ -2040,7 +2040,7 @@ findRangeSubtypeDiffFunction(List *procname, Oid subtype)
 	argList[0] = subtype;
 	argList[1] = subtype;
 
-	procOid = LookupFuncName(procname, 2, argList, true);
+	procOid = LookupFuncName(procname, 2, argList, FUNCLOOKUP_NOERROR);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index c6ce1011e2..35fc73b493 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -933,7 +933,7 @@ transformRangeTableSample(ParseState *pstate, RangeTableSample *rts)
 	 */
 	funcargtypes[0] = INTERNALOID;
 
-	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, true);
+	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, FUNCLOOKUP_NOERROR);
 
 	/* we want error to complain about no-such-method, not no-such-function */
 	if (!OidIsValid(handlerOid))
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 5222231b51..907b2d7062 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -2031,18 +2031,25 @@ func_signature_string(List *funcname, int nargs,
  * If the function name is not schema-qualified, it is sought in the current
  * namespace search path.
  *
- * If the function is not found, we return InvalidOid if noError is true,
- * else raise an error.
+ * If the function is not found the behavior depends on the value of
+ * 'checktype'.  When it's set to FUNCLOOKUP_NORMAL, we report errors for each
+ * of the possible error situations.  When set to FUNCLOOKUP_ERRIFAMBIGUOUS,
+ * the only error we raise is for an ambiguous function definition, otherwise
+ * if set to FUNCLOOKUP_NOERROR we don't raise any errors.  In each of the
+ * cases we skip reporting an error, we return InvalidOid to indicate to the
+ * caller that the lookup has failed.
  */
 Oid
-LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
+LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
+			   FuncLookupCheckType checktype)
 {
 	FuncCandidateList clist;
 
 	/* Passing NULL for argtypes is no longer allowed */
 	Assert(argtypes);
 
-	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false, noError);
+	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false,
+								  (checktype != FUNCLOOKUP_NORMAL));
 
 	/*
 	 * If no arguments were specified, the name must yield a unique candidate.
@@ -2053,7 +2060,11 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		{
 			if (clist->next)
 			{
-				if (!noError)
+				/*
+				 * Raise this error on FUNCLOOKUP_NORMAL and
+				 * FUNCLOOKUP_ERRIFAMBIGUOUS
+				 */
+				if (checktype != FUNCLOOKUP_NOERROR)
 					ereport(ERROR,
 							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
 							 errmsg("function name \"%s\" is not unique",
@@ -2065,7 +2076,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		}
 		else
 		{
-			if (!noError)
+			if (checktype == FUNCLOOKUP_NORMAL)
 				ereport(ERROR,
 						(errcode(ERRCODE_UNDEFINED_FUNCTION),
 						 errmsg("could not find a function named \"%s\"",
@@ -2080,7 +2091,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		clist = clist->next;
 	}
 
-	if (!noError)
+	if (checktype == FUNCLOOKUP_NORMAL)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
 				 errmsg("function %s does not exist",
@@ -2102,10 +2113,12 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
  * function.
  */
 Oid
-LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
+LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
+				   FuncLookupCheckType checktype)
 {
 	Oid			argoids[FUNC_MAX_ARGS];
 	int			argcount;
+	bool		noError = (checktype != FUNCLOOKUP_NORMAL);
 	int			i;
 	ListCell   *args_item;
 	Oid			oid;
@@ -2134,12 +2147,14 @@ LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 	}
 
 	/*
-	 * When looking for a function or routine, we pass noError through to
+	 * When looking for a function or routine, we pass checktype through to
 	 * LookupFuncName and let it make any error messages.  Otherwise, we make
 	 * our own errors for the aggregate and procedure cases.
 	 */
-	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids,
-						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? noError : true);
+	oid = LookupFuncName(func->objname,
+						 func->args_unspecified ? -1 : argcount,
+						 argoids,
+						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? checktype : FUNCLOOKUP_NOERROR);
 
 	if (objtype == OBJECT_FUNCTION)
 	{
diff --git a/src/include/parser/parse_func.h b/src/include/parser/parse_func.h
index ff1477e64f..d279183f17 100644
--- a/src/include/parser/parse_func.h
+++ b/src/include/parser/parse_func.h
@@ -30,6 +30,13 @@ typedef enum
 	FUNCDETAIL_COERCION			/* it's a type coercion request */
 } FuncDetailCode;
 
+/* Error checking types for LookupFuncName and LookupFuncWithArgs */
+typedef enum
+{
+	FUNCLOOKUP_NORMAL,			/* Full error checking */
+	FUNCLOOKUP_NOERROR,			/* Don't raise error for lookup failure */
+	FUNCLOOKUP_ERRIFAMBIGUOUS	/* Only raise ambiguous function errors */
+} FuncLookupCheckType;
 
 extern Node *ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 				  Node *last_srf, FuncCall *fn, bool proc_call,
@@ -63,9 +70,9 @@ extern const char *func_signature_string(List *funcname, int nargs,
 					  List *argnames, const Oid *argtypes);
 
 extern Oid LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
-			   bool noError);
+			   FuncLookupCheckType checktype);
 extern Oid LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
-				   bool noError);
+				   FuncLookupCheckType checktype);
 
 extern void check_srf_call_placement(ParseState *pstate, Node *last_srf,
 						 int location);
diff --git a/src/pl/tcl/pltcl.c b/src/pl/tcl/pltcl.c
index 76c9afc339..1e651203da 100644
--- a/src/pl/tcl/pltcl.c
+++ b/src/pl/tcl/pltcl.c
@@ -616,7 +616,7 @@ call_pltcl_start_proc(Oid prolang, bool pltrusted)
 
 	/* Parse possibly-qualified identifier and look up the function */
 	namelist = stringToQualifiedNameList(start_proc);
-	procOid = LookupFuncName(namelist, 0, fargtypes, false);
+	procOid = LookupFuncName(namelist, 0, fargtypes, FUNCLOOKUP_NORMAL);
 
 	/* Current user must have permission to call function */
 	aclresult = pg_proc_aclcheck(procOid, GetUserId(), ACL_EXECUTE);
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index b80c5ed2d8..dd4e6c0f6d 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -138,6 +138,16 @@ DROP FUNCTION test_function_exists(int, text, int[]);
 ERROR:  function test_function_exists(integer, text, integer[]) does not exist
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 NOTICE:  function test_function_exists(pg_catalog.int4,text,pg_catalog.int4[]) does not exist, skipping
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+ERROR:  function name "test_ambiguous_funcname" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 ERROR:  aggregate test_aggregate_exists(*) does not exist
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index c1d30bc4a5..ad4ef76460 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -153,6 +153,16 @@ DROP FUNCTION IF EXISTS test_function_exists();
 DROP FUNCTION test_function_exists(int, text, int[]);
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 DROP AGGREGATE IF EXISTS test_aggregate_exists(*);

diff --git a/src/backend/catalog/aclchk.c b/src/backend/catalog/aclchk.c
index 11ddce2a8b..f92f2f0a25 100644
--- a/src/backend/catalog/aclchk.c
+++ b/src/backend/catalog/aclchk.c
@@ -681,7 +681,8 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				ObjectWithArgs *func = (ObjectWithArgs *) lfirst(cell);
 				Oid			funcid;
 
-				funcid = LookupFuncWithArgs(OBJECT_FUNCTION, func, false);
+				funcid = LookupFuncWithArgs(OBJECT_FUNCTION, func,
+											FUNCLOOKUP_NORMAL);
 				objects = lappend_oid(objects, funcid);
 			}
 			break;
@@ -725,7 +726,8 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				ObjectWithArgs *func = (ObjectWithArgs *) lfirst(cell);
 				Oid			procid;
 
-				procid = LookupFuncWithArgs(OBJECT_PROCEDURE, func, false);
+				procid = LookupFuncWithArgs(OBJECT_PROCEDURE, func,
+											FUNCLOOKUP_NORMAL);
 				objects = lappend_oid(objects, procid);
 			}
 			break;
@@ -735,7 +737,8 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				ObjectWithArgs *func = (ObjectWithArgs *) lfirst(cell);
 				Oid			routid;
 
-				routid = LookupFuncWithArgs(OBJECT_ROUTINE, func, false);
+				routid = LookupFuncWithArgs(OBJECT_ROUTINE, func,
+											FUNCLOOKUP_NORMAL);
 				objects = lappend_oid(objects, routid);
 			}
 			break;
diff --git a/src/backend/catalog/objectaddress.c b/src/backend/catalog/objectaddress.c
index ab3ec7e356..2f6fd892bf 100644
--- a/src/backend/catalog/objectaddress.c
+++ b/src/backend/catalog/objectaddress.c
@@ -926,7 +926,9 @@ get_object_address(ObjectType objtype, Node *object,
 			case OBJECT_PROCEDURE:
 			case OBJECT_ROUTINE:
 				address.classId = ProcedureRelationId;
-				address.objectId = LookupFuncWithArgs(objtype, castNode(ObjectWithArgs, object), missing_ok);
+				address.objectId = LookupFuncWithArgs(objtype, castNode(ObjectWithArgs, object),
+													  missing_ok ? FUNCLOOKUP_ERRIFAMBIGUOUS :
+													  FUNCLOOKUP_NORMAL);
 				address.objectSubId = 0;
 				break;
 			case OBJECT_OPERATOR:
diff --git a/src/backend/commands/amcmds.c b/src/backend/commands/amcmds.c
index c84507b5d0..db27616f32 100644
--- a/src/backend/commands/amcmds.c
+++ b/src/backend/commands/amcmds.c
@@ -254,7 +254,8 @@ lookup_index_am_handler_func(List *handler_name, char amtype)
 				 errmsg("handler function is not specified")));
 
 	/* handlers have one argument of type internal */
-	handlerOid = LookupFuncName(handler_name, 1, funcargtypes, false);
+	handlerOid = LookupFuncName(handler_name, 1, funcargtypes,
+								FUNCLOOKUP_NORMAL);
 
 	/* check that handler has the correct return type */
 	switch (amtype)
diff --git a/src/backend/commands/conversioncmds.c b/src/backend/commands/conversioncmds.c
index 5afe86712f..1329c58d27 100644
--- a/src/backend/commands/conversioncmds.c
+++ b/src/backend/commands/conversioncmds.c
@@ -77,7 +77,7 @@ CreateConversionCommand(CreateConversionStmt *stmt)
 	 * a qualified name.
 	 */
 	funcoid = LookupFuncName(func_name, sizeof(funcargs) / sizeof(Oid),
-							 funcargs, false);
+							 funcargs, FUNCLOOKUP_NORMAL);
 
 	/* Check it returns VOID, else it's probably the wrong function */
 	if (get_func_rettype(funcoid) != VOIDOID)
diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index adb77d8f69..8e922e6c2a 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -238,7 +238,7 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 						stmt->trigname)));
 
 	/* Find and validate the trigger function. */
-	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, FUNCLOOKUP_NORMAL);
 	funcrettype = get_func_rettype(funcoid);
 	if (funcrettype != EVTTRIGGEROID)
 		ereport(ERROR,
diff --git a/src/backend/commands/foreigncmds.c b/src/backend/commands/foreigncmds.c
index 413ce3fcb6..9cb4dadfc4 100644
--- a/src/backend/commands/foreigncmds.c
+++ b/src/backend/commands/foreigncmds.c
@@ -484,7 +484,8 @@ lookup_fdw_handler_func(DefElem *handler)
 		return InvalidOid;
 
 	/* handlers have no arguments */
-	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false);
+	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes,
+								FUNCLOOKUP_NORMAL);
 
 	/* check that handler has correct return type */
 	if (get_func_rettype(handlerOid) != FDW_HANDLEROID)
@@ -511,7 +512,8 @@ lookup_fdw_validator_func(DefElem *validator)
 	funcargtypes[0] = TEXTARRAYOID;
 	funcargtypes[1] = OIDOID;
 
-	return LookupFuncName((List *) validator->arg, 2, funcargtypes, false);
+	return LookupFuncName((List *) validator->arg, 2, funcargtypes,
+						  FUNCLOOKUP_NORMAL);
 	/* validator's return value is ignored, so we don't check the type */
 }
 
diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c
index 4f62e48d98..76a4212e05 100644
--- a/src/backend/commands/functioncmds.c
+++ b/src/backend/commands/functioncmds.c
@@ -658,7 +658,7 @@ interpret_func_support(DefElem *defel)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procName, 1, argList, true);
+	procOid = LookupFuncName(procName, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1251,7 +1251,8 @@ AlterFunction(ParseState *pstate, AlterFunctionStmt *stmt)
 
 	rel = table_open(ProcedureRelationId, RowExclusiveLock);
 
-	funcOid = LookupFuncWithArgs(stmt->objtype, stmt->func, false);
+	funcOid = LookupFuncWithArgs(stmt->objtype, stmt->func,
+								 FUNCLOOKUP_NORMAL);
 
 	ObjectAddressSet(address, ProcedureRelationId, funcOid);
 
@@ -1567,7 +1568,8 @@ CreateCast(CreateCastStmt *stmt)
 	{
 		Form_pg_proc procstruct;
 
-		funcid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->func, false);
+		funcid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->func,
+									FUNCLOOKUP_NORMAL);
 
 		tuple = SearchSysCache1(PROCOID, ObjectIdGetDatum(funcid));
 		if (!HeapTupleIsValid(tuple))
@@ -1942,7 +1944,8 @@ CreateTransform(CreateTransformStmt *stmt)
 	 */
 	if (stmt->fromsql)
 	{
-		fromsqlfuncid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->fromsql, false);
+		fromsqlfuncid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->fromsql,
+										   FUNCLOOKUP_NORMAL);
 
 		if (!pg_proc_ownercheck(fromsqlfuncid, GetUserId()))
 			aclcheck_error(ACLCHECK_NOT_OWNER, OBJECT_FUNCTION, NameListToString(stmt->fromsql->objname));
@@ -1968,7 +1971,8 @@ CreateTransform(CreateTransformStmt *stmt)
 
 	if (stmt->tosql)
 	{
-		tosqlfuncid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->tosql, false);
+		tosqlfuncid = LookupFuncWithArgs(OBJECT_FUNCTION, stmt->tosql,
+										 FUNCLOOKUP_NORMAL);
 
 		if (!pg_proc_ownercheck(tosqlfuncid, GetUserId()))
 			aclcheck_error(ACLCHECK_NOT_OWNER, OBJECT_FUNCTION, NameListToString(stmt->tosql->objname));
diff --git a/src/backend/commands/opclasscmds.c b/src/backend/commands/opclasscmds.c
index 5d73848a8b..34312e5e72 100644
--- a/src/backend/commands/opclasscmds.c
+++ b/src/backend/commands/opclasscmds.c
@@ -529,7 +529,8 @@ DefineOpClass(CreateOpClassStmt *stmt)
 							 errmsg("invalid function number %d,"
 									" must be between 1 and %d",
 									item->number, maxProcNumber)));
-				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name, false);
+				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name,
+											 FUNCLOOKUP_NORMAL);
 #ifdef NOT_USED
 				/* XXX this is unnecessary given the superuser check above */
 				/* Caller must own function */
@@ -908,7 +909,8 @@ AlterOpFamilyAdd(AlterOpFamilyStmt *stmt, Oid amoid, Oid opfamilyoid,
 							 errmsg("invalid function number %d,"
 									" must be between 1 and %d",
 									item->number, maxProcNumber)));
-				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name, false);
+				funcOid = LookupFuncWithArgs(OBJECT_FUNCTION, item->name,
+											 FUNCLOOKUP_NORMAL);
 #ifdef NOT_USED
 				/* XXX this is unnecessary given the superuser check above */
 				/* Caller must own function */
diff --git a/src/backend/commands/operatorcmds.c b/src/backend/commands/operatorcmds.c
index 7b98e4b910..6c1bc5208e 100644
--- a/src/backend/commands/operatorcmds.c
+++ b/src/backend/commands/operatorcmds.c
@@ -208,7 +208,8 @@ DefineOperator(List *names, List *parameters)
 		typeId[1] = typeId2;
 		nargs = 2;
 	}
-	functionOid = LookupFuncName(functionName, nargs, typeId, false);
+	functionOid = LookupFuncName(functionName, nargs, typeId,
+								 FUNCLOOKUP_NORMAL);
 
 	/*
 	 * We require EXECUTE rights for the function.  This isn't strictly
@@ -271,7 +272,8 @@ ValidateRestrictionEstimator(List *restrictionName)
 	typeId[2] = INTERNALOID;	/* args list */
 	typeId[3] = INT4OID;		/* varRelid */
 
-	restrictionOid = LookupFuncName(restrictionName, 4, typeId, false);
+	restrictionOid = LookupFuncName(restrictionName, 4, typeId,
+									FUNCLOOKUP_NORMAL);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(restrictionOid) != FLOAT8OID)
@@ -312,12 +314,12 @@ ValidateJoinEstimator(List *joinName)
 	 * arguments, but we still allow the old 4-argument form. Try the
 	 * preferred form first.
 	 */
-	joinOid = LookupFuncName(joinName, 5, typeId, true);
+	joinOid = LookupFuncName(joinName, 5, typeId, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 4, typeId, true);
+		joinOid = LookupFuncName(joinName, 4, typeId, FUNCLOOKUP_NOERROR);
 	/* If not found, reference the 5-argument signature in error msg */
 	if (!OidIsValid(joinOid))
-		joinOid = LookupFuncName(joinName, 5, typeId, false);
+		joinOid = LookupFuncName(joinName, 5, typeId, FUNCLOOKUP_NORMAL);
 
 	/* estimators must return float8 */
 	if (get_func_rettype(joinOid) != FLOAT8OID)
diff --git a/src/backend/commands/proclang.c b/src/backend/commands/proclang.c
index 59c4e8dfd0..e94ca419be 100644
--- a/src/backend/commands/proclang.c
+++ b/src/backend/commands/proclang.c
@@ -106,7 +106,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * return type.
 		 */
 		funcname = SystemFuncName(pltemplate->tmplhandler);
-		handlerOid = LookupFuncName(funcname, 0, funcargtypes, true);
+		handlerOid = LookupFuncName(funcname, 0, funcargtypes,
+									FUNCLOOKUP_NOERROR);
 		if (OidIsValid(handlerOid))
 		{
 			funcrettype = get_func_rettype(handlerOid);
@@ -155,7 +156,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplinline);
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			inlineOid = LookupFuncName(funcname, 1, funcargtypes,
+									   FUNCLOOKUP_NOERROR);
 			if (!OidIsValid(inlineOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplinline,
@@ -198,7 +200,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		{
 			funcname = SystemFuncName(pltemplate->tmplvalidator);
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(funcname, 1, funcargtypes, true);
+			valOid = LookupFuncName(funcname, 1, funcargtypes,
+									FUNCLOOKUP_NOERROR);
 			if (!OidIsValid(valOid))
 			{
 				tmpAddr = ProcedureCreate(pltemplate->tmplvalidator,
@@ -264,7 +267,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		 * Lookup the PL handler function and check that it is of the expected
 		 * return type
 		 */
-		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes, false);
+		handlerOid = LookupFuncName(stmt->plhandler, 0, funcargtypes,
+									FUNCLOOKUP_NORMAL);
 		funcrettype = get_func_rettype(handlerOid);
 		if (funcrettype != LANGUAGE_HANDLEROID)
 		{
@@ -293,7 +297,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plinline)
 		{
 			funcargtypes[0] = INTERNALOID;
-			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes, false);
+			inlineOid = LookupFuncName(stmt->plinline, 1, funcargtypes,
+									   FUNCLOOKUP_NORMAL);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
@@ -303,7 +308,8 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 		if (stmt->plvalidator)
 		{
 			funcargtypes[0] = OIDOID;
-			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes, false);
+			valOid = LookupFuncName(stmt->plvalidator, 1, funcargtypes,
+									FUNCLOOKUP_NORMAL);
 			/* return value is ignored, so we don't check the type */
 		}
 		else
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index 409bee24f8..60798722a1 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -667,7 +667,8 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 	 * Find and validate the trigger function.
 	 */
 	if (!OidIsValid(funcoid))
-		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
+		funcoid = LookupFuncName(stmt->funcname, 0, fargtypes,
+								 FUNCLOOKUP_NORMAL);
 	if (!isInternal)
 	{
 		aclresult = pg_proc_aclcheck(funcoid, GetUserId(), ACL_EXECUTE);
diff --git a/src/backend/commands/tsearchcmds.c b/src/backend/commands/tsearchcmds.c
index 8e5eec22b5..30d3635598 100644
--- a/src/backend/commands/tsearchcmds.c
+++ b/src/backend/commands/tsearchcmds.c
@@ -108,7 +108,7 @@ get_ts_parser_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, FUNCLOOKUP_NORMAL);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
@@ -679,7 +679,7 @@ get_ts_template_func(DefElem *defel, int attnum)
 			nargs = 0;			/* keep compiler quiet */
 	}
 
-	procOid = LookupFuncName(funcName, nargs, typeId, false);
+	procOid = LookupFuncName(funcName, nargs, typeId, FUNCLOOKUP_NORMAL);
 	if (get_func_rettype(procOid) != retTypeId)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
diff --git a/src/backend/commands/typecmds.c b/src/backend/commands/typecmds.c
index 448926db12..fe7a5c1017 100644
--- a/src/backend/commands/typecmds.c
+++ b/src/backend/commands/typecmds.c
@@ -1700,28 +1700,28 @@ findTypeInputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = CSTRINGOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 
 	if (!OidIsValid(procOid))
 	{
 		argList[1] = OIDOID;
 		argList[2] = INT4OID;
 
-		procOid = LookupFuncName(procname, 3, argList, true);
+		procOid = LookupFuncName(procname, 3, argList, FUNCLOOKUP_NOERROR);
 	}
 
 	if (OidIsValid(procOid))
@@ -1766,14 +1766,14 @@ findTypeOutputFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	/* No luck, try it with OPAQUE */
 	argList[0] = OPAQUEOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 
 	if (OidIsValid(procOid))
 	{
@@ -1815,14 +1815,14 @@ findTypeReceiveFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
 	argList[1] = OIDOID;
 	argList[2] = INT4OID;
 
-	procOid = LookupFuncName(procname, 3, argList, true);
+	procOid = LookupFuncName(procname, 3, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1845,7 +1845,7 @@ findTypeSendFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (OidIsValid(procOid))
 		return procOid;
 
@@ -1868,7 +1868,7 @@ findTypeTypmodinFunction(List *procname)
 	 */
 	argList[0] = CSTRINGARRAYOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1895,7 +1895,7 @@ findTypeTypmodoutFunction(List *procname)
 	 */
 	argList[0] = INT4OID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1922,7 +1922,7 @@ findTypeAnalyzeFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = INTERNALOID;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
@@ -1998,7 +1998,7 @@ findRangeCanonicalFunction(List *procname, Oid typeOid)
 	 */
 	argList[0] = typeOid;
 
-	procOid = LookupFuncName(procname, 1, argList, true);
+	procOid = LookupFuncName(procname, 1, argList, FUNCLOOKUP_NOERROR);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
@@ -2040,7 +2040,7 @@ findRangeSubtypeDiffFunction(List *procname, Oid subtype)
 	argList[0] = subtype;
 	argList[1] = subtype;
 
-	procOid = LookupFuncName(procname, 2, argList, true);
+	procOid = LookupFuncName(procname, 2, argList, FUNCLOOKUP_NOERROR);
 
 	if (!OidIsValid(procOid))
 		ereport(ERROR,
diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index c6ce1011e2..35fc73b493 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -933,7 +933,7 @@ transformRangeTableSample(ParseState *pstate, RangeTableSample *rts)
 	 */
 	funcargtypes[0] = INTERNALOID;
 
-	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, true);
+	handlerOid = LookupFuncName(rts->method, 1, funcargtypes, FUNCLOOKUP_NOERROR);
 
 	/* we want error to complain about no-such-method, not no-such-function */
 	if (!OidIsValid(handlerOid))
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 5222231b51..907b2d7062 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -2031,18 +2031,25 @@ func_signature_string(List *funcname, int nargs,
  * If the function name is not schema-qualified, it is sought in the current
  * namespace search path.
  *
- * If the function is not found, we return InvalidOid if noError is true,
- * else raise an error.
+ * If the function is not found the behavior depends on the value of
+ * 'checktype'.  When it's set to FUNCLOOKUP_NORMAL, we report errors for each
+ * of the possible error situations.  When set to FUNCLOOKUP_ERRIFAMBIGUOUS,
+ * the only error we raise is for an ambiguous function definition, otherwise
+ * if set to FUNCLOOKUP_NOERROR we don't raise any errors.  In each of the
+ * cases we skip reporting an error, we return InvalidOid to indicate to the
+ * caller that the lookup has failed.
  */
 Oid
-LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
+LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
+			   FuncLookupCheckType checktype)
 {
 	FuncCandidateList clist;
 
 	/* Passing NULL for argtypes is no longer allowed */
 	Assert(argtypes);
 
-	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false, noError);
+	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false,
+								  (checktype != FUNCLOOKUP_NORMAL));
 
 	/*
 	 * If no arguments were specified, the name must yield a unique candidate.
@@ -2053,7 +2060,11 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		{
 			if (clist->next)
 			{
-				if (!noError)
+				/*
+				 * Raise this error on FUNCLOOKUP_NORMAL and
+				 * FUNCLOOKUP_ERRIFAMBIGUOUS
+				 */
+				if (checktype != FUNCLOOKUP_NOERROR)
 					ereport(ERROR,
 							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
 							 errmsg("function name \"%s\" is not unique",
@@ -2065,7 +2076,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		}
 		else
 		{
-			if (!noError)
+			if (checktype == FUNCLOOKUP_NORMAL)
 				ereport(ERROR,
 						(errcode(ERRCODE_UNDEFINED_FUNCTION),
 						 errmsg("could not find a function named \"%s\"",
@@ -2080,7 +2091,7 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		clist = clist->next;
 	}
 
-	if (!noError)
+	if (checktype == FUNCLOOKUP_NORMAL)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_FUNCTION),
 				 errmsg("function %s does not exist",
@@ -2102,10 +2113,12 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
  * function.
  */
 Oid
-LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
+LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
+				   FuncLookupCheckType checktype)
 {
 	Oid			argoids[FUNC_MAX_ARGS];
 	int			argcount;
+	bool		noError = (checktype != FUNCLOOKUP_NORMAL);
 	int			i;
 	ListCell   *args_item;
 	Oid			oid;
@@ -2134,12 +2147,14 @@ LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 	}
 
 	/*
-	 * When looking for a function or routine, we pass noError through to
+	 * When looking for a function or routine, we pass checktype through to
 	 * LookupFuncName and let it make any error messages.  Otherwise, we make
 	 * our own errors for the aggregate and procedure cases.
 	 */
-	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids,
-						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? noError : true);
+	oid = LookupFuncName(func->objname,
+						 func->args_unspecified ? -1 : argcount,
+						 argoids,
+						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? checktype : FUNCLOOKUP_NOERROR);
 
 	if (objtype == OBJECT_FUNCTION)
 	{
diff --git a/src/include/parser/parse_func.h b/src/include/parser/parse_func.h
index ff1477e64f..d279183f17 100644
--- a/src/include/parser/parse_func.h
+++ b/src/include/parser/parse_func.h
@@ -30,6 +30,13 @@ typedef enum
 	FUNCDETAIL_COERCION			/* it's a type coercion request */
 } FuncDetailCode;
 
+/* Error checking types for LookupFuncName and LookupFuncWithArgs */
+typedef enum
+{
+	FUNCLOOKUP_NORMAL,			/* Full error checking */
+	FUNCLOOKUP_NOERROR,			/* Don't raise error for lookup failure */
+	FUNCLOOKUP_ERRIFAMBIGUOUS	/* Only raise ambiguous function errors */
+} FuncLookupCheckType;
 
 extern Node *ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 				  Node *last_srf, FuncCall *fn, bool proc_call,
@@ -63,9 +70,9 @@ extern const char *func_signature_string(List *funcname, int nargs,
 					  List *argnames, const Oid *argtypes);
 
 extern Oid LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
-			   bool noError);
+			   FuncLookupCheckType checktype);
 extern Oid LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
-				   bool noError);
+				   FuncLookupCheckType checktype);
 
 extern void check_srf_call_placement(ParseState *pstate, Node *last_srf,
 						 int location);
diff --git a/src/pl/tcl/pltcl.c b/src/pl/tcl/pltcl.c
index 76c9afc339..1e651203da 100644
--- a/src/pl/tcl/pltcl.c
+++ b/src/pl/tcl/pltcl.c
@@ -616,7 +616,7 @@ call_pltcl_start_proc(Oid prolang, bool pltrusted)
 
 	/* Parse possibly-qualified identifier and look up the function */
 	namelist = stringToQualifiedNameList(start_proc);
-	procOid = LookupFuncName(namelist, 0, fargtypes, false);
+	procOid = LookupFuncName(namelist, 0, fargtypes, FUNCLOOKUP_NORMAL);
 
 	/* Current user must have permission to call function */
 	aclresult = pg_proc_aclcheck(procOid, GetUserId(), ACL_EXECUTE);
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index b80c5ed2d8..dd4e6c0f6d 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -138,6 +138,16 @@ DROP FUNCTION test_function_exists(int, text, int[]);
 ERROR:  function test_function_exists(integer, text, integer[]) does not exist
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 NOTICE:  function test_function_exists(pg_catalog.int4,text,pg_catalog.int4[]) does not exist, skipping
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+ERROR:  function name "test_ambiguous_funcname" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 ERROR:  aggregate test_aggregate_exists(*) does not exist
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index c1d30bc4a5..ad4ef76460 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -153,6 +153,16 @@ DROP FUNCTION IF EXISTS test_function_exists();
 DROP FUNCTION test_function_exists(int, text, int[]);
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 DROP AGGREGATE IF EXISTS test_aggregate_exists(*);

diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 654ee80b27..86b0d9fba7 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -34,6 +34,11 @@
 #include "utils/lsyscache.h"
 #include "utils/syscache.h"
 
+typedef enum
+{
+	FUNCLOOKUP_NOSUCHFUNC,
+	FUNCLOOKUP_AMBIGUOUS
+} FuncLookupError;
 
 static void unify_hypothetical_args(ParseState *pstate,
 						List *fargs, int numAggregatedArgs,
@@ -41,6 +46,9 @@ static void unify_hypothetical_args(ParseState *pstate,
 static Oid	FuncNameAsType(List *funcname);
 static Node *ParseComplexProjection(ParseState *pstate, const char *funcname,
 					   Node *first_arg, int location);
+static Oid LookupFuncNameInternal(List *funcname, int nargs,
+					   const Oid *argtypes,
+					   bool noError, FuncLookupError *lookupError);
 
 
 /*
@@ -2022,20 +2030,19 @@ func_signature_string(List *funcname, int nargs,
 }
 
 /*
- * LookupFuncName
- *
- * 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.
+ * LookupFuncNameInternal
+ *		Lookup a function Oid from its name and arguments.
  *
- * If the function name is not schema-qualified, it is sought in the current
- * namespace search path.
+ * In an error situation, e.g. can't find the function then we return
+ * InvalidOid and set *lookupError to indicate what went wrong.
  *
- * If the function is not found, we return InvalidOid if noError is true,
- * else raise an error.
+ * Possible Errors:
+ *	FUNCLOOKUP_NOSUCHFUNC -- When we can't find a function of this name.
+ *	FUNCLOOKUP_AMBIGUOUS -- When nargs == -1 and more than one func matches.
  */
-Oid
-LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
+static Oid
+LookupFuncNameInternal(List *funcname, int nargs, const Oid *argtypes,
+					   bool noError, FuncLookupError *lookupError)
 {
 	FuncCandidateList clist;
 
@@ -2051,25 +2058,20 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 	{
 		if (clist)
 		{
+			/* If there is another match then it's an ambiguous error */
 			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.")));
+				*lookupError = FUNCLOOKUP_AMBIGUOUS;
+				return InvalidOid;
 			}
+			/* Otherwise return the match */
 			else
 				return clist->oid;
 		}
 		else
 		{
-			if (!noError)
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("could not find a function named \"%s\"",
-								NameListToString(funcname))));
+			*lookupError = FUNCLOOKUP_NOSUCHFUNC;
+			return InvalidOid;
 		}
 	}
 
@@ -2080,16 +2082,75 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		clist = clist->next;
 	}
 
-	if (!noError)
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_FUNCTION),
-				 errmsg("function %s does not exist",
-						func_signature_string(funcname, nargs,
-											  NIL, argtypes))));
-
+	*lookupError = FUNCLOOKUP_NOSUCHFUNC;
 	return InvalidOid;
 }
 
+/*
+ * LookupFuncName
+ *
+ * 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.
+ *
+ * If the function is not found, we return InvalidOid if noError is true,
+ * else raise an error.
+ *
+ * If nargs == -1 and multiple functions are found matching this function name
+ * then we raise an ambiguous function error regardless of what noError is set
+ * to.
+ */
+Oid
+LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
+{
+	FuncLookupError lookupError;
+	Oid		funcoid;
+
+	funcoid = LookupFuncNameInternal(funcname, nargs, argtypes, noError,
+									 &lookupError);
+
+	if (OidIsValid(funcoid))
+		return funcoid;
+
+	switch (lookupError)
+	{
+		case FUNCLOOKUP_AMBIGUOUS:
+			/*
+			 * Raise an error regardless of if noError is set if there are
+			 * multiple matching functions.
+			 */
+			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.")));
+			break;
+
+		case FUNCLOOKUP_NOSUCHFUNC:
+			/* Let the caller deal with it when noError is true */
+			if (noError)
+				return InvalidOid;
+
+			if (nargs == -1)
+				ereport(ERROR,
+						(errcode(ERRCODE_UNDEFINED_FUNCTION),
+						errmsg("could not find a function named \"%s\"",
+							   NameListToString(funcname))));
+			else
+				ereport(ERROR,
+						(errcode(ERRCODE_UNDEFINED_FUNCTION),
+						errmsg("function %s does not exist",
+							   func_signature_string(funcname, nargs,
+													 NIL, argtypes))));
+			break;
+	}
+
+	return InvalidOid;		/* Keep compiler quiet */
+}
+
 /*
  * LookupFuncWithArgs
  *
@@ -2104,8 +2165,10 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 Oid
 LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 {
+	FuncLookupError lookupError;
 	Oid			argoids[FUNC_MAX_ARGS];
 	int			argcount;
+	int			nargs;
 	int			i;
 	ListCell   *args_item;
 	Oid			oid;
@@ -2117,12 +2180,22 @@ LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 
 	argcount = list_length(func->objargs);
 	if (argcount > FUNC_MAX_ARGS)
-		ereport(ERROR,
+	{
+		if (objtype == OBJECT_PROCEDURE)
+			ereport(ERROR,
+					(errcode(ERRCODE_TOO_MANY_ARGUMENTS),
+					 errmsg_plural("procedure cannot have more than %d argument",
+								   "procedure cannot have more than %d arguments",
+								   FUNC_MAX_ARGS,
+								   FUNC_MAX_ARGS)));
+		else
+			ereport(ERROR,
 				(errcode(ERRCODE_TOO_MANY_ARGUMENTS),
 				 errmsg_plural("functions cannot have more than %d argument",
 							   "functions cannot have more than %d arguments",
 							   FUNC_MAX_ARGS,
 							   FUNC_MAX_ARGS)));
+	}
 
 	i = 0;
 	foreach(args_item, func->objargs)
@@ -2133,97 +2206,156 @@ LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 	}
 
 	/*
-	 * When looking for a function or routine, we pass noError through to
-	 * LookupFuncName and let it make any error messages.  Otherwise, we make
-	 * our own errors for the aggregate and procedure cases.
+	 * Set nargs for the LookupFuncName call. It expects -1 to mean no args
+	 * were specified.
 	 */
-	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids,
-						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? noError : true);
+	nargs = func->args_unspecified ? -1 : argcount;
 
-	if (objtype == OBJECT_FUNCTION)
-	{
-		/* Make sure it's a function, not a procedure */
-		if (oid && get_func_prokind(oid) == PROKIND_PROCEDURE)
-		{
-			if (noError)
-				return InvalidOid;
-			ereport(ERROR,
-					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-					 errmsg("%s is not a function",
-							func_signature_string(func->objname, argcount,
-												  NIL, argoids))));
-		}
-	}
-	else if (objtype == OBJECT_PROCEDURE)
+	oid = LookupFuncNameInternal(func->objname, nargs, argoids, noError,
+								 &lookupError);
+
+	if (OidIsValid(oid))
 	{
-		if (!OidIsValid(oid))
+		/*
+		 * Even if we found the function, perform validation that the objtype
+		 * matches the prokind of the found function.  For historical reasons
+		 * we allow the objtype of FUNCTION to mean other things, but we draw
+		 * the line if the object is a procedure.  This is a new enough
+		 * feature that this historical rule does not apply.
+		 */
+		switch (objtype)
 		{
-			if (noError)
-				return InvalidOid;
-			else if (func->args_unspecified)
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("could not find a procedure named \"%s\"",
-								NameListToString(func->objname))));
-			else
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("procedure %s does not exist",
-								func_signature_string(func->objname, argcount,
-													  NIL, argoids))));
-		}
+			case OBJECT_FUNCTION:
+				/* Only complain if it's a procedure. */
+				if (get_func_prokind(oid) == PROKIND_PROCEDURE)
+					ereport(ERROR,
+							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+									 errmsg("%s is not a function",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+				break;
 
-		/* Make sure it's a procedure */
-		if (get_func_prokind(oid) != PROKIND_PROCEDURE)
-		{
-			if (noError)
-				return InvalidOid;
-			ereport(ERROR,
-					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-					 errmsg("%s is not a procedure",
-							func_signature_string(func->objname, argcount,
-												  NIL, argoids))));
+			case OBJECT_PROCEDURE:
+				/* Reject found object is not a procedure */
+				if (get_func_prokind(oid) != PROKIND_PROCEDURE)
+					ereport(ERROR,
+							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+									 errmsg("%s is not a procedure",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+				break;
+
+			case OBJECT_AGGREGATE:
+				/* Reject found object is not an aggregate */
+				if (get_func_prokind(oid) != PROKIND_AGGREGATE)
+					ereport(ERROR,
+							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+									 errmsg("function %s is not an aggregate",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+				break;
+
+			default:
+				/* Don't complain for routines */
+				break;
 		}
+
+		return oid;		/* All good */
 	}
-	else if (objtype == OBJECT_AGGREGATE)
+
+	/* Deal with cases where the lookup failed */
+	else
 	{
-		if (!OidIsValid(oid))
+		switch (lookupError)
 		{
-			if (noError)
+			case FUNCLOOKUP_AMBIGUOUS:
+				switch (objtype)
+				{
+					case OBJECT_FUNCTION:
+						ereport(ERROR,
+							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
+								errmsg("function name \"%s\" is not unique",
+									NameListToString(func->objname)),
+								errhint("Specify the argument list to select the function unambiguously.")));
+						break;
+					case OBJECT_PROCEDURE:
+						ereport(ERROR,
+							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
+								errmsg("procedure name \"%s\" is not unique",
+									NameListToString(func->objname)),
+								errhint("Specify the argument list to select the procedure unambiguously.")));
+						break;
+					case OBJECT_AGGREGATE:
+						ereport(ERROR,
+							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
+								errmsg("aggregate name \"%s\" is not unique",
+									NameListToString(func->objname)),
+								errhint("Specify the argument list to select the aggregate unambiguously.")));
+						break;
+					default:
+						break;
+				}
 				return InvalidOid;
-			else if (func->args_unspecified)
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("could not find an aggregate named \"%s\"",
-								NameListToString(func->objname))));
-			else if (argcount == 0)
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("aggregate %s(*) does not exist",
-								NameListToString(func->objname))));
-			else
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("aggregate %s does not exist",
-								func_signature_string(func->objname, argcount,
-													  NIL, argoids))));
-		}
 
-		/* Make sure it's an aggregate */
-		if (get_func_prokind(oid) != PROKIND_AGGREGATE)
-		{
-			if (noError)
-				return InvalidOid;
-			/* we do not use the (*) notation for functions... */
-			ereport(ERROR,
-					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-					 errmsg("function %s is not an aggregate",
-							func_signature_string(func->objname, argcount,
-												  NIL, argoids))));
+			case FUNCLOOKUP_NOSUCHFUNC:
+
+				/* Suppress no such func errors when noError is true */
+				if (noError)
+					break;
+
+				switch (objtype)
+				{
+					case OBJECT_PROCEDURE:
+						if (func->args_unspecified)
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("could not find a procedure named \"%s\"",
+											NameListToString(func->objname))));
+						else
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("procedure %s does not exist",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+						break;
+
+					case OBJECT_AGGREGATE:
+						if (func->args_unspecified)
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("could not find an aggregate named \"%s\"",
+											NameListToString(func->objname))));
+						else if (argcount == 0)
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("aggregate %s(*) does not exist",
+											NameListToString(func->objname))));
+						else
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("aggregate %s does not exist",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+						break;
+
+					default:
+						/* FUNCTION and ROUTINE */
+						if (func->args_unspecified)
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("could not find a function named \"%s\"",
+											NameListToString(func->objname))));
+						else
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("function %s does not exist",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+						break;
+				}
 		}
+		return InvalidOid;
 	}
-
-	return oid;
 }
 
 /*
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index b80c5ed2d8..5290b1ca7d 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -138,6 +138,27 @@ DROP FUNCTION test_function_exists(int, text, int[]);
 ERROR:  function test_function_exists(integer, text, integer[]) does not exist
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 NOTICE:  function test_function_exists(pg_catalog.int4,text,pg_catalog.int4[]) does not exist, skipping
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+ERROR:  function name "test_ambiguous_funcname" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+-- Ensure we get an ambiguous function error for procedures too.
+CREATE PROCEDURE test_ambiguous_procname(int) as $$ begin end; $$ language plpgsql;
+CREATE PROCEDURE test_ambiguous_procname(text) as $$ begin end; $$ language plpgsql;
+DROP PROCEDURE IF EXISTS test_ambiguous_procname;
+ERROR:  procedure name "test_ambiguous_procname" is not unique
+HINT:  Specify the argument list to select the procedure unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_procname(int);
+ERROR:  test_ambiguous_procname(integer) is not a function
+DROP FUNCTION test_ambiguous_procname(text);
+ERROR:  test_ambiguous_procname(text) is not a function
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 ERROR:  aggregate test_aggregate_exists(*) does not exist
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index c1d30bc4a5..138a713b9e 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -153,6 +153,25 @@ DROP FUNCTION IF EXISTS test_function_exists();
 DROP FUNCTION test_function_exists(int, text, int[]);
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+
+-- Ensure we get an ambiguous function error for procedures too.
+CREATE PROCEDURE test_ambiguous_procname(int) as $$ begin end; $$ language plpgsql;
+CREATE PROCEDURE test_ambiguous_procname(text) as $$ begin end; $$ language plpgsql;
+DROP PROCEDURE IF EXISTS test_ambiguous_procname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_procname(int);
+DROP FUNCTION test_ambiguous_procname(text);
+
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 DROP AGGREGATE IF EXISTS test_aggregate_exists(*);

diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 654ee80b27..6899a9f033 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -34,6 +34,14 @@
 #include "utils/lsyscache.h"
 #include "utils/syscache.h"
 
+/*
+ * Error codes for when function lookup failures occur
+ */
+typedef enum
+{
+	FUNCLOOKUP_NOSUCHFUNC,
+	FUNCLOOKUP_AMBIGUOUS
+} FuncLookupError;
 
 static void unify_hypothetical_args(ParseState *pstate,
 						List *fargs, int numAggregatedArgs,
@@ -41,6 +49,9 @@ static void unify_hypothetical_args(ParseState *pstate,
 static Oid	FuncNameAsType(List *funcname);
 static Node *ParseComplexProjection(ParseState *pstate, const char *funcname,
 					   Node *first_arg, int location);
+static Oid LookupFuncNameInternal(List *funcname, int nargs,
+					   const Oid *argtypes,
+					   bool missing_ok, FuncLookupError *lookupError);
 
 
 /*
@@ -2022,27 +2033,27 @@ func_signature_string(List *funcname, int nargs,
 }
 
 /*
- * LookupFuncName
- *
- * 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.
+ * LookupFuncNameInternal
+ *		Lookup a function Oid from its name and arguments.
  *
- * If the function name is not schema-qualified, it is sought in the current
- * namespace search path.
+ * In an error situation, e.g. can't find the function, then we return
+ * InvalidOid and set *lookupError to indicate what went wrong.
  *
- * If the function is not found, we return InvalidOid if noError is true,
- * else raise an error.
+ * Possible Errors:
+ *	FUNCLOOKUP_NOSUCHFUNC -- When we can't find a function of this name.
+ *	FUNCLOOKUP_AMBIGUOUS -- When nargs == -1 and more than one func matches.
  */
-Oid
-LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
+static Oid
+LookupFuncNameInternal(List *funcname, int nargs, const Oid *argtypes,
+					   bool missing_ok, FuncLookupError *lookupError)
 {
 	FuncCandidateList clist;
 
 	/* Passing NULL for argtypes is no longer allowed */
 	Assert(argtypes);
 
-	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false, noError);
+	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false,
+								  missing_ok);
 
 	/*
 	 * If no arguments were specified, the name must yield a unique candidate.
@@ -2051,25 +2062,20 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 	{
 		if (clist)
 		{
+			/* If there is another match then it's an ambiguous error */
 			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.")));
+				*lookupError = FUNCLOOKUP_AMBIGUOUS;
+				return InvalidOid;
 			}
+			/* Otherwise return the match */
 			else
 				return clist->oid;
 		}
 		else
 		{
-			if (!noError)
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("could not find a function named \"%s\"",
-								NameListToString(funcname))));
+			*lookupError = FUNCLOOKUP_NOSUCHFUNC;
+			return InvalidOid;
 		}
 	}
 
@@ -2080,16 +2086,75 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 		clist = clist->next;
 	}
 
-	if (!noError)
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_FUNCTION),
-				 errmsg("function %s does not exist",
-						func_signature_string(funcname, nargs,
-											  NIL, argtypes))));
-
+	*lookupError = FUNCLOOKUP_NOSUCHFUNC;
 	return InvalidOid;
 }
 
+/*
+ * LookupFuncName
+ *
+ * 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.
+ *
+ * If the function is not found, we return InvalidOid if missing_ok is true,
+ * else raise an error.
+ *
+ * If nargs == -1 and multiple functions are found matching this function name
+ * then we raise an ambiguous function error regardless of what missing_ok is
+ * set to.
+ */
+Oid
+LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool missing_ok)
+{
+	FuncLookupError lookupError;
+	Oid		funcoid;
+
+	funcoid = LookupFuncNameInternal(funcname, nargs, argtypes, missing_ok,
+									 &lookupError);
+
+	if (OidIsValid(funcoid))
+		return funcoid;
+
+	switch (lookupError)
+	{
+		case FUNCLOOKUP_AMBIGUOUS:
+			/*
+			 * Raise an error regardless of if missing_ok is set if there are
+			 * multiple matching functions.
+			 */
+			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.")));
+			break;
+
+		case FUNCLOOKUP_NOSUCHFUNC:
+			/* Let the caller deal with it when missing_ok is true */
+			if (missing_ok)
+				return InvalidOid;
+
+			if (nargs == -1)
+				ereport(ERROR,
+						(errcode(ERRCODE_UNDEFINED_FUNCTION),
+						errmsg("could not find a function named \"%s\"",
+							   NameListToString(funcname))));
+			else
+				ereport(ERROR,
+						(errcode(ERRCODE_UNDEFINED_FUNCTION),
+						errmsg("function %s does not exist",
+							   func_signature_string(funcname, nargs,
+													 NIL, argtypes))));
+			break;
+	}
+
+	return InvalidOid;		/* Keep compiler quiet */
+}
+
 /*
  * LookupFuncWithArgs
  *
@@ -2100,12 +2165,18 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
  *
  * For historical reasons, we also accept aggregates when looking for a
  * function.
+ *
+ * When missing_ok is true we don't generate any error for missing objects and
+ * return InvalidOid.  Other types of errors can still be raised, regardless
+ * of the value of missing_ok.
  */
 Oid
-LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
+LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool missing_ok)
 {
+	FuncLookupError lookupError;
 	Oid			argoids[FUNC_MAX_ARGS];
 	int			argcount;
+	int			nargs;
 	int			i;
 	ListCell   *args_item;
 	Oid			oid;
@@ -2117,113 +2188,191 @@ LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func, bool noError)
 
 	argcount = list_length(func->objargs);
 	if (argcount > FUNC_MAX_ARGS)
-		ereport(ERROR,
+	{
+		if (objtype == OBJECT_PROCEDURE)
+			ereport(ERROR,
+					(errcode(ERRCODE_TOO_MANY_ARGUMENTS),
+					 errmsg_plural("procedure cannot have more than %d argument",
+								   "procedure cannot have more than %d arguments",
+								   FUNC_MAX_ARGS,
+								   FUNC_MAX_ARGS)));
+		else
+			ereport(ERROR,
 				(errcode(ERRCODE_TOO_MANY_ARGUMENTS),
 				 errmsg_plural("functions cannot have more than %d argument",
 							   "functions cannot have more than %d arguments",
 							   FUNC_MAX_ARGS,
 							   FUNC_MAX_ARGS)));
+	}
 
 	i = 0;
 	foreach(args_item, func->objargs)
 	{
 		TypeName   *t = (TypeName *) lfirst(args_item);
 
-		argoids[i++] = LookupTypeNameOid(NULL, t, noError);
+		argoids[i++] = LookupTypeNameOid(NULL, t, missing_ok);
 	}
 
 	/*
-	 * When looking for a function or routine, we pass noError through to
-	 * LookupFuncName and let it make any error messages.  Otherwise, we make
-	 * our own errors for the aggregate and procedure cases.
+	 * Set nargs for the LookupFuncName call. It expects -1 to mean no args
+	 * were specified.
 	 */
-	oid = LookupFuncName(func->objname, func->args_unspecified ? -1 : argcount, argoids,
-						 (objtype == OBJECT_FUNCTION || objtype == OBJECT_ROUTINE) ? noError : true);
+	nargs = func->args_unspecified ? -1 : argcount;
 
-	if (objtype == OBJECT_FUNCTION)
-	{
-		/* Make sure it's a function, not a procedure */
-		if (oid && get_func_prokind(oid) == PROKIND_PROCEDURE)
-		{
-			if (noError)
-				return InvalidOid;
-			ereport(ERROR,
-					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-					 errmsg("%s is not a function",
-							func_signature_string(func->objname, argcount,
-												  NIL, argoids))));
-		}
-	}
-	else if (objtype == OBJECT_PROCEDURE)
+	oid = LookupFuncNameInternal(func->objname, nargs, argoids, missing_ok,
+								 &lookupError);
+
+	if (OidIsValid(oid))
 	{
-		if (!OidIsValid(oid))
+		/*
+		 * Even if we found the function, perform validation that the objtype
+		 * matches the prokind of the found function.  For historical reasons
+		 * we allow the objtype of FUNCTION to mean other things, but we draw
+		 * the line if the object is a procedure.  This is a new enough
+		 * feature that this historical rule does not apply.
+		 */
+		switch (objtype)
 		{
-			if (noError)
-				return InvalidOid;
-			else if (func->args_unspecified)
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("could not find a procedure named \"%s\"",
-								NameListToString(func->objname))));
-			else
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("procedure %s does not exist",
-								func_signature_string(func->objname, argcount,
-													  NIL, argoids))));
-		}
+			case OBJECT_FUNCTION:
+				/* Only complain if it's a procedure. */
+				if (get_func_prokind(oid) == PROKIND_PROCEDURE)
+					ereport(ERROR,
+							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+									 errmsg("%s is not a function",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+				break;
 
-		/* Make sure it's a procedure */
-		if (get_func_prokind(oid) != PROKIND_PROCEDURE)
-		{
-			if (noError)
-				return InvalidOid;
-			ereport(ERROR,
-					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-					 errmsg("%s is not a procedure",
-							func_signature_string(func->objname, argcount,
-												  NIL, argoids))));
+			case OBJECT_PROCEDURE:
+				/* Reject found object is not a procedure */
+				if (get_func_prokind(oid) != PROKIND_PROCEDURE)
+					ereport(ERROR,
+							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+									 errmsg("%s is not a procedure",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+				break;
+
+			case OBJECT_AGGREGATE:
+				/* Reject found object is not an aggregate */
+				if (get_func_prokind(oid) != PROKIND_AGGREGATE)
+					ereport(ERROR,
+							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+									 errmsg("function %s is not an aggregate",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+				break;
+
+			default:
+				/* Don't complain for routines */
+				break;
 		}
+
+		return oid;		/* All good */
 	}
-	else if (objtype == OBJECT_AGGREGATE)
+
+	/* Deal with cases where the lookup failed */
+	else
 	{
-		if (!OidIsValid(oid))
+		switch (lookupError)
 		{
-			if (noError)
+			case FUNCLOOKUP_AMBIGUOUS:
+				switch (objtype)
+				{
+					case OBJECT_FUNCTION:
+						ereport(ERROR,
+							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
+								errmsg("function name \"%s\" is not unique",
+									NameListToString(func->objname)),
+								errhint("Specify the argument list to select the function unambiguously.")));
+						break;
+					case OBJECT_PROCEDURE:
+						ereport(ERROR,
+							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
+								errmsg("procedure name \"%s\" is not unique",
+									NameListToString(func->objname)),
+								errhint("Specify the argument list to select the procedure unambiguously.")));
+						break;
+					case OBJECT_AGGREGATE:
+						ereport(ERROR,
+							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
+								errmsg("aggregate name \"%s\" is not unique",
+									NameListToString(func->objname)),
+								errhint("Specify the argument list to select the aggregate unambiguously.")));
+						break;
+					case OBJECT_ROUTINE:
+						ereport(ERROR,
+							(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
+								errmsg("routine name \"%s\" is not unique",
+									NameListToString(func->objname)),
+								errhint("Specify the argument list to select the routine unambiguously.")));
+						break;
+
+					default:
+						Assert(false);		/* Disallowed by Assert above */
+						break;
+				}
 				return InvalidOid;
-			else if (func->args_unspecified)
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("could not find an aggregate named \"%s\"",
-								NameListToString(func->objname))));
-			else if (argcount == 0)
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("aggregate %s(*) does not exist",
-								NameListToString(func->objname))));
-			else
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_FUNCTION),
-						 errmsg("aggregate %s does not exist",
-								func_signature_string(func->objname, argcount,
-													  NIL, argoids))));
-		}
 
-		/* Make sure it's an aggregate */
-		if (get_func_prokind(oid) != PROKIND_AGGREGATE)
-		{
-			if (noError)
-				return InvalidOid;
-			/* we do not use the (*) notation for functions... */
-			ereport(ERROR,
-					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-					 errmsg("function %s is not an aggregate",
-							func_signature_string(func->objname, argcount,
-												  NIL, argoids))));
+			case FUNCLOOKUP_NOSUCHFUNC:
+
+				/* Suppress no such func errors when missing_ok is true */
+				if (missing_ok)
+					break;
+
+				switch (objtype)
+				{
+					case OBJECT_PROCEDURE:
+						if (func->args_unspecified)
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("could not find a procedure named \"%s\"",
+											NameListToString(func->objname))));
+						else
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("procedure %s does not exist",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+						break;
+
+					case OBJECT_AGGREGATE:
+						if (func->args_unspecified)
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("could not find an aggregate named \"%s\"",
+											NameListToString(func->objname))));
+						else if (argcount == 0)
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("aggregate %s(*) does not exist",
+											NameListToString(func->objname))));
+						else
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("aggregate %s does not exist",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+						break;
+
+					default:
+						/* FUNCTION and ROUTINE */
+						if (func->args_unspecified)
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("could not find a function named \"%s\"",
+											NameListToString(func->objname))));
+						else
+							ereport(ERROR,
+									(errcode(ERRCODE_UNDEFINED_FUNCTION),
+									 errmsg("function %s does not exist",
+											func_signature_string(func->objname, argcount,
+																  NIL, argoids))));
+						break;
+				}
 		}
+		return InvalidOid;
 	}
-
-	return oid;
 }
 
 /*
diff --git a/src/include/parser/parse_func.h b/src/include/parser/parse_func.h
index ff1477e64f..743e6668f0 100644
--- a/src/include/parser/parse_func.h
+++ b/src/include/parser/parse_func.h
@@ -63,9 +63,9 @@ extern const char *func_signature_string(List *funcname, int nargs,
 					  List *argnames, const Oid *argtypes);
 
 extern Oid LookupFuncName(List *funcname, int nargs, const Oid *argtypes,
-			   bool noError);
+			   bool missing_ok);
 extern Oid LookupFuncWithArgs(ObjectType objtype, ObjectWithArgs *func,
-				   bool noError);
+				   bool missing_ok);
 
 extern void check_srf_call_placement(ParseState *pstate, Node *last_srf,
 						 int location);
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index b80c5ed2d8..488678fa41 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -138,6 +138,31 @@ DROP FUNCTION test_function_exists(int, text, int[]);
 ERROR:  function test_function_exists(integer, text, integer[]) does not exist
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 NOTICE:  function test_function_exists(pg_catalog.int4,text,pg_catalog.int4[]) does not exist, skipping
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+ERROR:  function name "test_ambiguous_funcname" is not unique
+HINT:  Specify the argument list to select the function unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+-- Ensure we get an ambiguous function error for procedures too.
+CREATE PROCEDURE test_ambiguous_procname(int) as $$ begin end; $$ language plpgsql;
+CREATE PROCEDURE test_ambiguous_procname(text) as $$ begin end; $$ language plpgsql;
+DROP PROCEDURE IF EXISTS test_ambiguous_procname;
+ERROR:  procedure name "test_ambiguous_procname" is not unique
+HINT:  Specify the argument list to select the procedure unambiguously.
+-- Ensure we get a similar error if we use ROUTINE instead of PROCEDURE.
+DROP ROUTINE IF EXISTS test_ambiguous_procname;
+ERROR:  routine name "test_ambiguous_procname" is not unique
+HINT:  Specify the argument list to select the routine unambiguously.
+-- cleanup
+DROP FUNCTION test_ambiguous_procname(int);
+ERROR:  test_ambiguous_procname(integer) is not a function
+DROP FUNCTION test_ambiguous_procname(text);
+ERROR:  test_ambiguous_procname(text) is not a function
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 ERROR:  aggregate test_aggregate_exists(*) does not exist
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index c1d30bc4a5..451fa6ea35 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -153,6 +153,28 @@ DROP FUNCTION IF EXISTS test_function_exists();
 DROP FUNCTION test_function_exists(int, text, int[]);
 DROP FUNCTION IF EXISTS test_function_exists(int, text, int[]);
 
+-- Ensure we still receive an ambiguous function error when there are
+-- multiple matching functions.
+CREATE FUNCTION test_ambiguous_funcname(int) returns int as $$ select $1; $$ language sql;
+CREATE FUNCTION test_ambiguous_funcname(text) returns text as $$ select $1; $$ language sql;
+DROP FUNCTION IF EXISTS test_ambiguous_funcname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_funcname(int);
+DROP FUNCTION test_ambiguous_funcname(text);
+
+-- Ensure we get an ambiguous function error for procedures too.
+CREATE PROCEDURE test_ambiguous_procname(int) as $$ begin end; $$ language plpgsql;
+CREATE PROCEDURE test_ambiguous_procname(text) as $$ begin end; $$ language plpgsql;
+DROP PROCEDURE IF EXISTS test_ambiguous_procname;
+
+-- Ensure we get a similar error if we use ROUTINE instead of PROCEDURE.
+DROP ROUTINE IF EXISTS test_ambiguous_procname;
+
+-- cleanup
+DROP FUNCTION test_ambiguous_procname(int);
+DROP FUNCTION test_ambiguous_procname(text);
+
 -- aggregate
 DROP AGGREGATE test_aggregate_exists(*);
 DROP AGGREGATE IF EXISTS test_aggregate_exists(*);