【Mysql优化】索引覆盖

索引覆盖

　　是指如果查询的列恰好是索引的一部分,那么查询只需要在索引文件上进行,不需要回行到磁盘再找数据.这种查询速度非常快,称为”索引覆盖”,比平时的查询少一次到磁盘读数据的操作。(索引正好覆盖到查询的数据)

例如下面:

mysql> use exam9;
Database changed
mysql> desc options;
+----------------+---------------+------+-----+---------+-------+
| Field          | Type          | Null | Key | Default | Extra |
+----------------+---------------+------+-----+---------+-------+
| optionId       | varchar(40)   | NO   | PRI | NULL    |       |
| questionId     | varchar(40)   | YES  | MUL | NULL    |       |
| optionContent  | varchar(2000) | YES  |     | NULL    |       |
| optionWithTag  | varchar(2000) | YES  |     | NULL    |       |
| optionSequence | varchar(2)    | YES  |     | NULL    |       |
| isAnswer       | varchar(2)    | YES  |     | NULL    |       |
| description    | varchar(300)  | YES  |     | NULL    |       |
+----------------+---------------+------+-----+---------+-------+
7 rows in set (0.00 sec)mysql> reset query cache;      #清空缓存
Query OK, 0 rows affected (0.00 sec)mysql> set profiling=on;　　　　#打开profiling功能
Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> select optioncontent from options where optionid='000406aa1b89461d8cfd85f
b0e5d9e01';
+------------------------+
| optioncontent          |
+------------------------+
| 基础工程完毕进行回填后 |
+------------------------+
1 row in set (0.03 sec)mysql> select optionid from options where optionid='000406aa1b89461d8cfd85fb0e5d
9e01';
+----------------------------------+
| optionid                         |
+----------------------------------+
| 000406aa1b89461d8cfd85fb0e5d9e01 |
+----------------------------------+
1 row in set (0.03 sec)mysql> show profiles;　　#显示概要信息
+----------+------------+-------------------------------------------------------
------------------------------+
| Query_ID | Duration   | Query|
+----------+------------+-------------------------------------------------------
------------------------------+
|        1 | 0.03480675 | select optioncontent from options where optionid='0004
06aa1b89461d8cfd85fb0e5d9e01' |
|        2 | 0.03624525 | select optionid from options where optionid='000406aa1
b89461d8cfd85fb0e5d9e01'      |
+----------+------------+-------------------------------------------------------
------------------------------+
2 rows in set, 1 warning (0.00 sec)mysql> explainselect optionid from options where optionid='000406aa1b89461d8cfd8
5fb0e5d9e01'\G
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near 'expla
inselect optionid from options where optionid='000406aa1b89461d8cfd85fb0e5d' at
line 1
mysql> explain select optionid from options where optionid='000406aa1b89461d8cfd
85fb0e5d9e01'\G    #explain分析语句
*************************** 1. row ***************************id: 1select_type: SIMPLEtable: optionspartitions: NULLtype: const
possible_keys: PRIMARYkey: PRIMARYkey_len: 122ref: constrows: 1filtered: 100.00Extra: Using index　　#表示索引覆盖
1 row in set, 1 warning (0.03 sec)mysql> explain select optioncontent from options where optionid='000406aa1b89461
d8cfd85fb0e5d9e01'\G　　　　#分析语句
*************************** 1. row ***************************id: 1select_type: SIMPLEtable: optionspartitions: NULLtype: const
possible_keys: PRIMARYkey: PRIMARYkey_len: 122ref: constrows: 1filtered: 100.00Extra: NULL
1 row in set, 1 warning (0.03 sec)