MySQL补充

优化特例补充

mysql> explain select * from customer order by store_id ;                                      
+----+-------------+----------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table    | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+----------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | customer | ALL  | NULL          | NULL | NULL    | NULL |  671 | Using filesort |
+----+-------------+----------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

这里优化器为什么不使用store_id列上的辅助索引进行排序呢？
当通过辅助索引来查找数据时，innodb存储引擎会遍历辅助索引并通过叶节点的指针获得指向主键索引的主键; 然后通过主键索引来找到一个完整的行记录。举例来说，如果在一棵高度为3的辅助索引树中查找数据，那么需要对这个辅助索引树进行3次IO找到指定主键，如果聚集索引树的高度同样为3，那么还需要对聚集索引树进行3次查找，最终找到一个完整的行数所在的页，因此一共需要6次逻辑IO访问以得到最终的一个数据页。
总结:数据库觉得进行全表扫描 比使用这个索引更快! 事实也是如此!

mysql> explain select payment_date , sum(amount) from payment group by payment_date;                  
+----+-------------+---------+------+---------------+------+---------+------+-------+---------------------------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows  | Extra                           |
+----+-------------+---------+------+---------------+------+---------+------+-------+---------------------------------+
|  1 | SIMPLE      | payment | ALL  | NULL          | NULL | NULL    | NULL | 15422 | Using temporary; Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+-------+---------------------------------+
1 row in set (0.00 sec)

可以看见使用了Filesort，还使用了内存临时表，这条SQL严重影响性能，所以需要优化：
首先禁止排序，ORDER BY NULL

mysql> explain select payment_date , sum(amount) from payment group by payment_date order by null;    
+----+-------------+---------+------+---------------+------+---------+------+-------+-----------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows  | Extra           |
+----+-------------+---------+------+---------------+------+---------+------+-------+-----------------+
|  1 | SIMPLE      | payment | ALL  | NULL          | NULL | NULL    | NULL | 15422 | Using temporary |
+----+-------------+---------+------+---------------+------+---------+------+-------+-----------------+
1 row in set (0.00 sec)

可以看见已经没有使用Filesort，但是还是使用了内存临时表，这是我们可以创建一个复合索引来优化性能

mysql> alter table payment add key idx_pal (payment_date,amount,last_update);                                     
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select payment_date , sum(amount) from payment group by payment_date;              
+----+-------------+---------+-------+---------------+---------+---------+------+-------+-------------+
| id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows  | Extra       |
+----+-------------+---------+-------+---------------+---------+---------+------+-------+-------------+
|  1 | SIMPLE      | payment | index | NULL          | idx_pal | 15      | NULL | 15422 | Using index |
+----+-------------+---------+-------+---------------+---------+---------+------+-------+-------------+
1 row in set (0.01 sec)

Explain语句中重点字段理解：