[bug]string pad functions should always be nullable (apache#11140)

* string pad functions should always be nullable

be/src/vec/functions/function_string.h

@@ -769,7 +769,7 @@ class FunctionStringPad : public IFunction {
     size_t get_number_of_arguments() const override { return 3; }
 
     DataTypePtr get_return_type_impl(const DataTypes& arguments) const override {
-        return std::make_shared<DataTypeString>();
+        return make_nullable(std::make_shared<DataTypeString>());
     }
     bool use_default_implementation_for_nulls() const override { return true; }
     bool use_default_implementation_for_constants() const override { return true; }

gensrc/script/doris_builtins_functions.py

@@ -1010,10 +1010,10 @@
         '15FunctionContextERKNS1_9StringValERKNS1_6IntValE', '', '', 'vec', ''],
     [['lpad'], 'VARCHAR', ['VARCHAR', 'INT', 'VARCHAR'],
             '_ZN5doris15StringFunctions4lpadEPN9doris_udf'
-            '15FunctionContextERKNS1_9StringValERKNS1_6IntValES6_', '', '', 'vec', ''],
+            '15FunctionContextERKNS1_9StringValERKNS1_6IntValES6_', '', '', 'vec', 'ALWAYS_NULLABLE'],
     [['rpad'], 'VARCHAR', ['VARCHAR', 'INT', 'VARCHAR'],
             '_ZN5doris15StringFunctions4rpadEPN9doris_udf'
-            '15FunctionContextERKNS1_9StringValERKNS1_6IntValES6_', '', '', 'vec', ''],
+            '15FunctionContextERKNS1_9StringValERKNS1_6IntValES6_', '', '', 'vec', 'ALWAYS_NULLABLE'],
     [['append_trailing_char_if_absent'], 'VARCHAR', ['VARCHAR', 'VARCHAR'],
             '_ZN5doris15StringFunctions30append_trailing_char_if_absentEPN9doris_udf15FunctionContextERKNS1_9StringValES6_',
             '', '', 'vec', 'ALWAYS_NULLABLE'],

regression-test/data/correctness/test_string_pad_function.out

@@ -0,0 +1,9 @@
+-- This file is automatically generated. You should know what you did if you want to edit this
+-- !select_lpad --
+\N
+10:00
+
+-- !select_rpad --
+\N
+10:00
+

regression-test/suites/correctness/test_string_pad_function.groovy

@@ -0,0 +1,46 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+suite("test_string_pad_function") {
+    sql """
+        drop table if exists table_pad;
+    """
+
+    sql """
+        create table table_pad (
+        a int not null,
+        b varchar(10) not null
+        )
+        ENGINE=OLAP
+        distributed by hash(a)
+        properties(
+        'replication_num' = '1'
+        );
+    """
+
+    sql """
+        insert into table_pad values(1,'100000'), (2,'200000');
+    """
+
+    qt_select_lpad """
+        select CASE WHEN table_pad.a = 1 THEN CONCAT(LPAD(b, 2, 0), ':00') END result from table_pad order by result;
+    """
+
+    qt_select_rpad """
+        select CASE WHEN table_pad.a = 1 THEN CONCAT(RPAD(b, 2, 0), ':00') END result from table_pad order by result;
+    """
+}