[Fix](Variant) fix variant cast to jsonb into wrong NULL values (#50180)

1. implement cast to jsonb in `ConvertImplGenericFromVariant`
2. implement `DataTypeObjectSerDe::serialize_column_to_json`

Author: eldenmoon

be/src/vec/data_types/serde/data_type_object_serde.cpp

@@ -87,6 +87,12 @@ Status DataTypeObjectSerDe::write_column_to_mysql(const IColumn& column,
     return _write_column_to_mysql(column, row_buffer, row_idx, col_const, options);
 }
 
+Status DataTypeObjectSerDe::serialize_column_to_json(const IColumn& column, int64_t start_idx,
+                                                     int64_t end_idx, BufferWritable& bw,
+                                                     FormatOptions& options) const {
+    SERIALIZE_COLUMN_TO_JSON();
+}
+
 void DataTypeObjectSerDe::write_one_cell_to_jsonb(const IColumn& column, JsonbWriter& result,
                                                   Arena* mem_pool, int32_t col_id,
                                                   int64_t row_num) const {

be/src/vec/data_types/serde/data_type_object_serde.h

@@ -42,9 +42,8 @@ class DataTypeObjectSerDe : public DataTypeSerDe {
                                       FormatOptions& options) const override;
 
     Status serialize_column_to_json(const IColumn& column, int64_t start_idx, int64_t end_idx,
-                                    BufferWritable& bw, FormatOptions& options) const override {
-        return Status::NotSupported("serialize_column_to_json with type [{}]", column.get_name());
-    }
+                                    BufferWritable& bw, FormatOptions& options) const override;
+
     Status deserialize_one_cell_from_json(IColumn& column, Slice& slice,
                                           const FormatOptions& options) const override {
         return Status::NotSupported("deserialize_one_cell_from_text with type " +

be/src/vec/functions/function_cast.h

@@ -1793,7 +1793,8 @@ class FunctionCast final : public IFunctionBase {
                     variant.is_scalar_variant() ||
                     (!variant.is_null_root() &&
                      !WhichDataType(remove_nullable(variant.get_root_type())).is_nothing() &&
-                     !WhichDataType(data_type_to).is_string());
+                     !WhichDataType(data_type_to).is_string() &&
+                     !WhichDataType(data_type_to).is_json());
             if (is_root_valuable) {
                 ColumnPtr nested = variant.get_root();
                 auto nested_from_type = variant.get_root_type();
@@ -1825,13 +1826,19 @@ class FunctionCast final : public IFunctionBase {
                     // TODO not found root cause, a tmp fix
                     col_to->assume_mutable()->insert_many_defaults(input_rows_count);
                     col_to = make_nullable(col_to, true);
+                } else if (WhichDataType(data_type_to).is_string()) {
+                    // serialize to string
+                    return ConvertImplGenericToString::execute2(context, block, arguments, result,
+                                                                input_rows_count);
+                } else if (WhichDataType(data_type_to).is_json()) {
+                    // serialize to json by parsing
+                    return ConvertImplGenericToJsonb::execute(context, block, arguments, result,
+                                                              input_rows_count);
                 } else if (!data_type_to->is_nullable() &&
                            !WhichDataType(data_type_to).is_string()) {
+                    // other types
                     col_to->assume_mutable()->insert_many_defaults(input_rows_count);
                     col_to = make_nullable(col_to, true);
-                } else if (WhichDataType(data_type_to).is_string()) {
-                    return ConvertImplGenericToString::execute2(context, block, arguments, result,
-                                                                input_rows_count);
                 } else {
                     assert_cast<ColumnNullable&>(*col_to->assume_mutable())
                             .insert_many_defaults(input_rows_count);

regression-test/data/variant_p0/cast.out

@@ -0,0 +1,25 @@
+-- This file is automatically generated. You should know what you did if you want to edit this
+-- !sql1 --
+{"a":1}
+
+-- !sql2 --
+int
+
+-- !sql3 --
+{"aaa":1}
+
+-- !sql4 --
+[1]
+
+-- !sql5 --
+123
+
+-- !sql6 --
+{"aaa":1}
+
+-- !sql7 --
+[1]
+
+-- !sql8 --
+123
+

regression-test/suites/variant_p0/cast.groovy

@@ -0,0 +1,60 @@
+// 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.
+
+import org.codehaus.groovy.runtime.IOGroovyMethods
+
+suite("test_variant_cast", "p0") {
+    qt_sql1 """select cast(cast('{"a" : 1}' as variant) as jsonb);"""
+    qt_sql2 """select json_type(cast(cast('{"a" : 1}' as variant) as jsonb), "\$.a");"""
+    sql "DROP TABLE IF EXISTS var_cast"
+    sql """
+        CREATE TABLE `var_cast` (
+            `k` int NULL,
+            `var` variant NULL
+        ) ENGINE=OLAP
+        UNIQUE KEY(`k`)
+        DISTRIBUTED BY HASH(k) BUCKETS 1
+        PROPERTIES (
+        "replication_allocation" = "tag.location.default: 1"
+        );
+    """
+    sql """insert into var_cast values (1, '{"aaa" : 1}')"""
+    qt_sql3 "select cast(var as json) from var_cast"
+    sql """insert into var_cast values (1, '[1]')"""
+    qt_sql4 "select cast(var as json) from var_cast"
+    sql """insert into var_cast values (1, '123')"""
+    qt_sql5 "select cast(var as json) from var_cast"
+
+    sql "DROP TABLE IF EXISTS var_not_null_cast"
+    sql """
+        CREATE TABLE `var_not_null_cast` (
+            `k` int NULL,
+            `var` variant NOT NULL
+        ) ENGINE=OLAP
+        UNIQUE KEY(`k`)
+        DISTRIBUTED BY HASH(k) BUCKETS 1
+        PROPERTIES (
+        "replication_allocation" = "tag.location.default: 1"
+        );
+    """
+    sql """insert into var_not_null_cast values (1, '{"aaa" : 1}')"""
+    qt_sql6 "select cast(var as json) from var_not_null_cast"
+    sql """insert into var_not_null_cast values (1, '[1]')"""
+    qt_sql7 "select cast(var as json) from var_not_null_cast"
+    sql """insert into var_not_null_cast values (1, '123')"""
+    qt_sql8 "select cast(var as json) from var_not_null_cast"
+}