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--+--------+---------------+-------+---------+-------+------+-------+ | 1 | PRIMARY |
| system | NULL | NULL | NULL | NULL | 1 | NULL | | 2 | DERIVED | t1 | const | ename | ename | 23 | const | 1 | NULL | +----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+ 2 rows in set (0.00 sec) 7、type=NULL MySQL不用访问表或者索引就可以直接得到结果 (root@localhost) [sakila]> explain select sysdate(); +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used | +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ 1 row in set (0.00 sec)
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