MySQL使用索引的幾種方式

之前的blog有介紹令人迷糊的extra中信息，本文我們重點來看看當執行計劃使用索引的時候

extra中幾種顯示的場景

版本:Mysql 5.6.14

測試表結構如下:

CREATE TABLE `snapshot` (

`id` int(11) NOT NULL AUTO_INCREMENT,

`snap_id` int(11) DEFAULT NULL,

`name` varchar(500) DEFAULT NULL,

`value` int(11) DEFAULT NULL,

PRIMARY KEY (`id`),

KEY `snap_id_ix` (`snap_id`)

) ENGINE=InnoDB? DEFAULT CHARSET=utf8

在snap_id上創建了索引，然后插入一些測試數據。

1，snap_id為等于的操作

mysql> explain extended select * from snapshot where snap_id=10;

+----+-------------+----------+------+---------------+------------+---------+-------+------+----------+-------+

| id | select_type | table??? | type | possible_keys | key??????? | key_len | ref?? | rows | filtered | Extra |

+----+-------------+----------+------+---------------+------------+---------+-------+------+----------+-------+

|? 1 | SIMPLE????? | snapshot | ref? | snap_id_ix??? | snap_id_ix | 5?????? | const |??? 1 |?? 100.00 | NULL? |

+----+-------------+----------+------+---------------+------------+---------+-------+------+----------+-------+

1 row in set, 1 warning (0.00 sec)

這個雖然使用了索引，但是extra中沒有任何信息，顯示為空

2,使用索引條件，先掃描索引，然后再通過索引掃描表

mysql>? explain extended select *? from snapshot where snap_id<100;

+----+-------------+----------+-------+---------------+------------+---------+------+------+----------+-----------------------+

| id | select_type | table??? | type? | possible_keys | key??????? | key_len | ref? | rows | filtered | Extra???????????????? |

+----+-------------+----------+-------+---------------+------------+---------+------+------+----------+-----------------------+

|? 1 | SIMPLE????? | snapshot | range | snap_id_ix??? | snap_id_ix | 5?????? | NULL |??? 1 |?? 100.00 | Using index condition |

+----+-------------+----------+-------+---------------+------------+---------+------+------+----------+-----------------------+

1 row in set, 1 warning (0.00 sec)

通過索引條件，然后掃描表數據，extra是Using index condition

3,使用cover index，只查索引中的列，直接從索引中返回數據就可以了

mysql>? explain extended select snap_id from snapshot;

+----+-------------+----------+-------+---------------+------------+---------+------+------+----------+-------------+

| id | select_type | table??? | type? | possible_keys | key??????? | key_len | ref? | rows | filtered | Extra?????? |

+----+-------------+----------+-------+---------------+------------+---------+------+------+----------+-------------+

|? 1 | SIMPLE????? | snapshot | index | NULL????????? | snap_id_ix | 5?????? | NULL | 6415 |?? 100.00 | Using index |

+----+-------------+----------+-------+---------------+------------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

mysql>? explain extended select snap_id from snapshot where snap_id=10;

+----+-------------+----------+------+---------------+------------+---------+-------+------+----------+-------------+

| id | select_type | table??? | type | possible_keys | key??????? | key_len | ref?? | rows | filtered | Extra?????? |

+----+-------------+----------+------+---------------+------------+---------+-------+------+----------+-------------+

|? 1 | SIMPLE????? | snapshot | ref? | snap_id_ix??? | snap_id_ix | 5?????? | const |??? 1 |?? 100.00 | Using index |

+----+-------------+----------+------+---------------+------------+---------+-------+------+----------+-------------+

1 row in set, 1 warning (0.01 sec)

mysql>? explain extended select snap_id from snapshot where snap_id>10;

+----+-------------+----------+-------+---------------+------------+---------+------+------+----------+--------------------------+

| id | select_type | table??? | type? | possible_keys | key??????? | key_len | ref? | rows | filtered | Extra??????????????????? |

+----+-------------+----------+-------+---------------+------------+---------+------+------+----------+--------------------------+

|? 1 | SIMPLE????? | snapshot | range | snap_id_ix??? | snap_id_ix | 5?????? | NULL | 6367 |?? 100.00 | Using where; Using index |

+----+-------------+----------+-------+---------------+------------+---------+------+------+----------+--------------------------+

1 row in set, 1 warning (0.00 sec)

在extra出現的是Using index提示，表明只使用索引即可，不需要對表進行掃描操作.

4,?? 接下來我們看連接的操作，假設為snapshot作為連接的內表

mysql> explain

-> select *

-> from t1 join snapshot

-> on (t1.a=snapshot.snap_id)

-> ;

+----+-------------+----------+------+---------------+------------+---------+----------+------+-------+

| id | select_type | table??? | type | possible_keys | key??????? | key_len | ref????? | rows | Extra |

+----+-------------+----------+------+---------------+------------+---------+----------+------+-------+

|? 1 | SIMPLE????? | t1?????? | ALL? | PRIMARY?????? | NULL?????? | NULL??? | NULL???? |??? 3 | NULL? |

|? 1 | SIMPLE????? | snapshot | ref? | snap_id_ix??? | snap_id_ix | 5?????? | db1.t1.a |? 200 | NULL? |

+----+-------------+----------+------+---------------+------------+---------+----------+------+-------+

2 rows in set (0.00 sec)

通過執行計劃可以看出對snapshot也是索引來訪問的，外表傳過來的值，在內表很類似于snap_id=XX,所以和第1中的執行

計劃很類似.除ref之外，一個是來自于常量(const),而本例中是來自表t1的索引db.t1.a.

總結

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