[FEAT MERGE] implement mysql dblink and read consistency

Co-authored-by: xianyu-w <707512433@qq.com>
Co-authored-by: sdc <njucssdc@gmail.com>
Co-authored-by: seuwebber <webber_code@163.com>

src/sql/engine/expr/ob_expr_current_scn.h

@@ -0,0 +1,40 @@
+/**
+ * Copyright (c) 2021 OceanBase
+ * OceanBase CE is licensed under Mulan PubL v2.
+ * You can use this software according to the terms and conditions of the Mulan PubL v2.
+ * You may obtain a copy of Mulan PubL v2 at:
+ *          http://license.coscl.org.cn/MulanPubL-2.0
+ * THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+ * EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO NON-INFRINGEMENT,
+ * MERCHANTABILITY OR FIT FOR A PARTICULAR PURPOSE.
+ * See the Mulan PubL v2 for more details.
+ */
+
+#ifndef OCEANBASE_SQL_OB_EXPR_CURRENT_SCN_H_
+#define OCEANBASE_SQL_OB_EXPR_CURRENT_SCN_H_
+
+#include "sql/engine/expr/ob_expr_operator.h"
+
+namespace oceanbase
+{
+namespace sql
+{
+class ObExprCurrentScn : public ObFuncExprOperator
+{
+public:
+  explicit  ObExprCurrentScn(common::ObIAllocator &alloc);
+  virtual ~ObExprCurrentScn();
+  virtual int calc_result_type0(ObExprResType &type, common::ObExprTypeCtx &type_ctx) const;
+  static int eval_current_scn(const ObExpr &expr, ObEvalCtx &ctx, ObDatum &expr_datum);
+  virtual int cg_expr(ObExprCGCtx &op_cg_ctx,
+                      const ObRawExpr &raw_expr,
+                      ObExpr &rt_expr) const override;
+
+private:
+  // disallow copy
+  DISALLOW_COPY_AND_ASSIGN(ObExprCurrentScn);
+};
+
+} //sql
+} //oceanbase
+#endif //OCEANBASE_SQL_OB_EXPR_CURRENT_SCN_H_