Fix that coalesce mistakenly removed nullable from the result column (#3394)

dbms/src/Functions/FunctionsNull.cpp

@@ -213,7 +213,7 @@ void FunctionCoalesce::executeImpl(Block & block, const ColumnNumbers & argument
     ColumnPtr res = std::move(temp_block.getByPosition(result).column);
 
     /// if last argument is not nullable, result should be also not nullable
-    if (!block.getByPosition(multi_if_args.back()).column->isColumnNullable() && res->isColumnNullable())
+    if (!block.getByPosition(filtered_args.back()).type->isNullable() && res->isColumnNullable())
         res = static_cast<const ColumnNullable &>(*res).getNestedColumnPtr();
 
     block.getByPosition(result).column = std::move(res);

dbms/src/Functions/tests/gtest_coalesce.cpp

@@ -0,0 +1,34 @@
+#include <Interpreters/Context.h>
+#include <TestUtils/FunctionTestUtils.h>
+#include <TestUtils/TiFlashTestBasic.h>
+
+#include <string>
+#include <vector>
+
+namespace DB::tests
+{
+class CoalesceTest : public DB::tests::FunctionTest
+{
+};
+
+TEST_F(CoalesceTest, testOnlyNull)
+try
+{
+    const String & func_name = "coalesce";
+
+    ASSERT_COLUMN_EQ(
+        createColumn<Nullable<String>>({"a"}),
+        executeFunction(
+            func_name,
+            createColumn<Nullable<String>>({"a"}),
+            createConstColumn<Nullable<String>>(1, std::nullopt)));
+
+    ASSERT_COLUMN_EQ(
+        createColumn<Nullable<String>>({"a"}),
+        executeFunction(
+            func_name,
+            createColumn<Nullable<String>>({"a"}),
+            createOnlyNullColumn(1)));
+}
+CATCH
+} // namespace DB::tests

tests/fullstack-test/expr/coalesce_pushdown.test

@@ -5,9 +5,11 @@ mysql> INSERT INTO test.test_tb(id,char_,enum_,longtext_,mediumtext_,set_,text_,
 
 mysql> analyze table test.test_tb
 mysql> alter table test.test_tb set tiflash replica 1
-func> wait_table test test_tb
-
-
+mysql> drop table if exists test.fix_3388
+mysql> create table if not exists test.fix_3388(a varchar(10))
+mysql> alter table test.fix_3388 set tiflash replica 1
+mysql> insert into test.fix_3388 values ('a')
+func> wait_table test test_tb fix_3388
 
 # start checking
 mysql> select /*+ read_from_storage(tiflash[test.test_tb]) */ id from test.test_tb where char_ = coalesce(null, char_);
@@ -335,3 +337,11 @@ mysql> select /*+ read_from_storage(tiflash[test.test_tb]) */ id from test.test_
 |  2 |
 +----+
 
+# fix 3388
+mysql> set tidb_enforce_mpp=1; select count(*) from test.fix_3388 where coalesce(a, null) = "a"
++----------+
+| count(*) |
++----------+
+|        1 |
++----------+
+