Fix incorrect result of max aggregation fun with collation ci (pingca…

…p#7002) (pingcap#7016)

close pingcap#6807

dbms/src/Core/SortCursor.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <Columns/ColumnNullable.h>
 #include <Columns/ColumnString.h>
 #include <Columns/IColumn.h>
 #include <Common/typeid_cast.h>
@@ -72,7 +73,7 @@ struct SortCursorImpl
 
             sort_columns.push_back(block.safeGetByPosition(column_number).column.get());
 
-            need_collation[j] = desc[j].collator != nullptr && typeid_cast<const ColumnString *>(sort_columns.back()); /// TODO Nullable(String)
+            need_collation[j] = desc[j].collator != nullptr && typeid_cast<const ColumnString *>(std::get<0>(removeNullable(sort_columns.back())));
             has_collation |= need_collation[j];
         }
 

tests/tidb-ci/new_collation_fullstack/issue_6807.test

@@ -0,0 +1,39 @@
+# Copyright 2023 PingCAP, Ltd.
+#
+# Licensed 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.
+
+# Preparation.
+mysql> drop table if exists test.t1
+mysql> CREATE TABLE test.t1 (a varchar(20) COLLATE utf8mb4_general_ci DEFAULT NULL,id int(11) NOT NULL,PRIMARY KEY (id) /*T![clustered_index] CLUSTERED */) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
+mysql> insert into test.t1(a ,id) values('jlsf',1),(null,2),('YmkS',3),('0',4);
+mysql> drop table if exists test.t2
+mysql> CREATE TABLE test.t2 (a varchar(20) COLLATE utf8mb4_general_ci DEFAULT NULL,b char(20) COLLATE utf8mb4_general_ci DEFAULT NULL,id int(11) NOT NULL,PRIMARY KEY (id) /*T![clustered_index] CLUSTERED */) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
+mysql> insert into test.t2(a,b,id) values('yMKs','jlsf',1),('yyds','YmkS',2),('cc',4,3),(null,null,4),('abc',null,5),(null,'df',6);
+
+mysql> alter table test.t1 set tiflash replica 1
+mysql> alter table test.t2 set tiflash replica 1
+
+func> wait_table test t1
+func> wait_table test t2
+
+
+mysql> use test; set @@tidb_isolation_read_engines='tiflash'; set @@tidb_enforce_mpp=1; select max(a) from t1 where a in ( select b from t2 where a>b);
++--------+
+| max(a) |
++--------+
+| YmkS   |
++--------+
+
+# Clean up.
+# mysql> drop table if exists test.t1
+# mysql> drop table if exists test.t2