Mysql 和Oracle rows 区别

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     | <derived2> | ref  | <auto_key0>   | <auto_key0> | 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)