[FEAT MERGE] implement dynamic sampling

src/sql/optimizer/ob_optimizer.h

@@ -215,6 +215,7 @@ namespace sql
                              const ObColumnRefRawExpr &column_expr,
                              int64_t flag);
     int check_whether_contain_nested_sql(const ObDMLStmt &stmt);
+    int check_force_default_stat();
     int calc_link_stmt_count(const ObDMLStmt &stmt, int64_t &count);
   private:
     ObOptimizerContext &ctx_;