[root@HE1 ~]# MySQL -uroot -p
Enter password: 
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.6.25-log MySQL Community Server (GPL)
Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> select version();
+------------+
| version() |
+------------+
| 5.6.25-log |
+------------+
1 row in set (0.26 sec)
  
mysql> explain (select id from helei order by id) union all (select id from t where id=0 order by id);
+----+--------------+------------+-------+---------------+--------+---------+------+------+-----------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref | rows | Extra      |
+----+--------------+------------+-------+---------------+--------+---------+------+------+-----------------+
| 1 | PRIMARY   | helei   | index | NULL     | idx_c1 | 4    | NULL | 5219 | Using index   |
| 2 | UNION    | t     | ALL  | NULL     | NULL  | NULL  | NULL |  1 | Using where   |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL     | NULL  | NULL  | NULL | NULL | Using temporary |
+----+--------------+------------+-------+---------------+--------+---------+------+------+-----------------+
3 rows in set (0.00 sec)

[root@HE1 ~]# mysql -uroot -p
Enter password: 
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 8
Server version: 5.7.15-log MySQL Community Server (GPL)
Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.15-log |
+------------+
1 row in set (0.00 sec)、
mysql> explain (select id from helei order by id) union all (select id from t where id=0 order by id);
+----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref | rows | filtered | Extra    |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+-------------+
| 1 | PRIMARY   | helei | NULL    | index | NULL     | idx_c1 | 4    | NULL | 5212 |  100.00 | Using index |
| 2 | UNION    | t   | NULL    | ALL  | NULL     | NULL  | NULL  | NULL |  1 |  100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

[root@HE3 ~]# /usr/local/mariadb/bin/mysql -uroot -S /tmp/mariadb.sock 
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 7
Server version: 10.1.16-MariaDB MariaDB Server
Copyright (c) 2000, 2016, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]>
MariaDB [helei]> explain (select id from helei order by id) union all (select id from t where id=0 order by id);
+------+-------------+-------+-------+---------------+--------+---------+------+------+-------------+
| id  | select_type | table | type | possible_keys | key  | key_len | ref | rows | Extra    |
+------+-------------+-------+-------+---------------+--------+---------+------+------+-------------+
|  1 | PRIMARY   | helei | index | NULL     | idx_c1 | 4    | NULL | 5198 | Using index |
|  2 | UNION    | t   | ALL  | NULL     | NULL  | NULL  | NULL |  1 | Using where |
+------+-------------+-------+-------+---------------+--------+---------+------+------+-------------+
2 rows in set (0.00 sec)