mysql>  explain select t1.* from t2 ,t1 where t2.id=t1.id and t2.id<3;\+----+-------------+-------+------+---------------+------+---------+------+--------+----------------------------------------------------+| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows   | Extra                                              |+----+-------------+-------+------+---------------+------+---------+------+--------+----------------------------------------------------+|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    134 | Using where                                        ||  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL | 392625 | Using where; Using join buffer (Block Nested Loop) |+----+-------------+-------+------+---------------+------+---------+------+--------+----------------------------------------------------+2 rows in set (0.00 sec)mysql> select count(*) from t2;+----------+| count(*) |+----------+|      134 |+----------+1 row in set (0.00 sec)mysql> select count(*) from t2 where id<3;+----------+| count(*) |+----------+|      128 |+----------+1 row in set (0.00 sec)驱动表 一般返回较少的记录:mysql> explain select t1.* from (select * from t2 where id<3) t2 ,t1 where t2.id=t1.id;+----+-------------+------------+------+---------------+-------------+---------+--------------+--------+-------------+| id | select_type | table      | type | possible_keys | key         | key_len | ref          | rows   | Extra       |+----+-------------+------------+------+---------------+-------------+---------+--------------+--------+-------------+|  1 | PRIMARY     | t1         | ALL  | NULL          | NULL        | NULL    | NULL         | 392625 | Using where ||  1 | PRIMARY     | 
    
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        | 5       | DEVOPS.t1.id |     10 | NULL        ||  2 | DERIVED     | t2         | ALL  | NULL          | NULL        | NULL    | NULL         |    134 | Using where |+----+-------------+------------+------+---------------+-------------+---------+--------------+--------+-------------+3 rows in set (0.00 sec)是预估的需要扫描的行数不是返回的行数这就是两者的区别Oracle 是预估返回的行数MySQL 是预估扫描的行数,显示需要扫描的行数mysql> analyze table t2;+-----------+---------+----------+----------+| Table     | Op      | Msg_type | Msg_text |+-----------+---------+----------+----------+| DEVOPS.t2 | analyze | status   | OK       |+-----------+---------+----------+----------+1 row in set (0.17 sec)mysql>  explain select t1.* from t2 ,t1 where t2.id=t1.id and t2.id=3;+----+-------------+-------+------+---------------+------+---------+------+--------+----------------------------------------------------+| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows   | Extra                                              |+----+-------------+-------+------+---------------+------+---------+------+--------+----------------------------------------------------+|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |  16192 | Using where                                        ||  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL | 392625 | Using where; Using join buffer (Block Nested Loop) |+----+-------------+-------+------+---------------+------+---------+------+--------+----------------------------------------------------+2 rows in set (0.00 sec)mysql> select count(*) from t2;+----------+| count(*) |+----------+|    16390 |+----------+1 row in set (0.01 sec)mysql> select count(*) from t2 where id=3;+----------+| count(*) |+----------+|        6 |+----------+1 row in set (0.01 sec)