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: 1
select_type: SIMPLE
table: options
partitions: NULL
type: const
possible_keys: PRIMARY
key: PRIMARY
key_len: 122
ref: const
rows: 1
filtered: 100.00
Extra: 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: 1
select_type: SIMPLE
table: options
partitions: NULL
type: const
possible_keys: PRIMARY
key: PRIMARY
key_len: 122
ref: const
rows: 1
filtered: 100.00
Extra: NULL
1 row in set, 1 warning (0.03 sec)