前言：当用Explain查看SQL的执行计划时，里面有列显示了 key_len 的值，根据这个值可以判断索引的长度，在组合索引里面可以更清楚的了解到了哪部分字段使用到了索引。下面演示中，表结构的合理性这边暂且不说，只是证明一下索引长度的计算方法。目前大部分博文是字符类型的索引长度计算方法，下面列举几个类型的索引长度计算方法

1、整数类型

(dg1)root@127.0.0.1 [mytest]> desc table_key;

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

| Field   | Type        | Null | Key | Default | Extra |

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

| id      | int()     | NO   |     | NULL    |       |

| sid     | bigint()  | NO   |     |        |       |

| name    | char()    | YES  |     | NULL    |       |

| age     | tinyint()  | YES  |     | NULL    |       |

| sex     | tinyint()  | NO   |     | NULL    |       |

| address | varchar() | YES  | MUL | NULL    |       |

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

 rows in set (0.01 sec)

(dg1)root@127.0.0.1 [mytest]>create index age_index on table_key (age);

来看看tinyint类型的索引长度，在NOT NULL 和 NULL 的时候 分别是1和2,tinyint字段长度为1，因为NULL 需要额外一个字节标记为空

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where age=;

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

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

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

|   | SIMPLE      | table_key | ref  | age_index     | age_index | 1       | const |     |   100.00 | NULL  |

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

 row in set,  warning (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify age  tinyint();

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where age=;

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

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

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

|   | SIMPLE      | table_key | ref  | age_index     | age_index | 2       | const |     |   100.00 | NULL  |

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

 row in set,  warning (0.00 sec)

(dg1)root@127.0.0.1 [mytest]>

看看bigint类型的索引长度，同样是 NOT NULL 和 NULL值的时候，分别是8和9,聪明的你应该知道了，bigint长度为8。

(dg1)root@127.0.0.1 [mytest]> alter table table_key add key sid_index (sid);

Query OK,  rows affected (0.04 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=;

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

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

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

|   | SIMPLE      | table_key | ref  | sid_index     | sid_index |        | const |     | NULL  |

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

 row in set (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid bigint();

Query OK,  rows affected (0.08 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=;

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

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

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

|   | SIMPLE      | table_key | ref  | sid_index     | sid_index |        | const |     | NULL  |

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

 row in set (0.00 sec)

看看smallint类型索引长度，同样是 NOT NULL 和 NULL值的时候，分别是2和3 smallint长度为2，允许为空需要一个字节标记
(dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid smallint not null default ;

Query OK,  rows affected (0.04 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=;

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

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

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

|   | SIMPLE      | table_key | ref  | sid_index     | sid_index |        | const |     | NULL  |

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

 row in set (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid smallint ;

Query OK,  rows affected (0.07 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=;

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

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

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

|   | SIMPLE      | table_key | ref  | sid_index     | sid_index |        | const |     | NULL  |

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

 row in set (0.00 sec)

看看mediumint类型索引长度，同样是 NOT NULL 和 NULL值的时候，分别是3和4

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid mediumint NOT NULL;

Query OK,  rows affected (0.06 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=;

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

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

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

|   | SIMPLE      | table_key | ref  | sid_index     | sid_index |        | const |     | NULL  |

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

 row in set (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 

（dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid mediumint ;

Query OK, 0 rows affected (0.06 sec)

Records: 0  Duplicates: 0  Warnings: 0

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=6;

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

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

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

|  1 | SIMPLE      | table_key | ref  | sid_index     | sid_index | 4       | const |    1 | NULL  |

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

1 row in set (0.00 sec)

(dg1)root@127.0.0.1 [mytest]>

 整数类型索引长度跟字段长度有关，如果允许为空，需要额外一个字节去标记为空

2.浮点数类型

表结构
CREATE TABLE `table_key1` (

`id`  int NOT NULL AUTO_INCREMENT ,

`c1`  float NOT NULL ,

`c2`  double NOT NULL ,

`c3`  decimal NOT NULL ,

`c4`  date NOT NULL ,

`c5`  timestamp NOT NULL ON UPDATE CURRENT_TIMESTAMP ,

`c6`  datetime NOT NULL ,

PRIMARY KEY (`id`)

)
看看float类型的索引长度，NOT NULL和NULL的时候，分别是4和5

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c1 = '3.22';

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

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

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

|   | SIMPLE      | table_key1 | ref  | c1_index      | c1_index |        | const |     | Using index condition |

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

 row in set (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c1 float;

Query OK,  rows affected (0.05 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c1 = '3.22';

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

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

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

|   | SIMPLE      | table_key1 | ref  | c1_index      | c1_index |        | const |     | Using index condition |

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

 row in set (0.00 sec)

看看double类型的索引长度，NOT NULL和NULL的时候，分别是8和9

(dg1)root@127.0.0.1 [mytest]> alter table table_key1 add key c2_index (c2);

Query OK,  rows affected (0.04 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c2 = '3.22';

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

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

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

|   | SIMPLE      | table_key1 | ref  | c2_index      | c2_index |        | const |     | NULL  |

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

 row in set (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c2 double;

Query OK,  rows affected (0.03 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c2 = '3.22';

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

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

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

|   | SIMPLE      | table_key1 | ref  | c2_index      | c2_index |        | const |     | NULL  |

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

 row in set (0.00 sec)

(dg1)root@127.0.0.1 [mytest]>

3、看看时间类型

看看date类型的索引长度，在NOT NULL和NULL的时候，分别是3和4

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c4 = '2015-05-06';

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

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

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

|   | SIMPLE      | table_key1 | ref  | c4_index      | c4_index |        | const |     | Using index condition |

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

 row in set,  warnings (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 

(dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c4 date;

Query OK,  rows affected (0.09 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c4 = '2015-05-06';

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

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

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

|   | SIMPLE      | table_key1 | ref  | c4_index      | c4_index |        | const |     | Using index condition |

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

 row in set,  warnings (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 

在timestamp类型的时候索引长度，在NOT NULL 和 NULL的时候，分别是4和5

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c5 = '2015-05-06 11:23:21' ;

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

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

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

|   | SIMPLE      | table_key1 | ref  | c5_index      | c5_index |        | const |     | Using index condition |

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

 row in set,  warnings (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 

dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c5 timestamp ON UPDATE CURRENT_TIMESTAMP ;

Query OK,  rows affected (0.06 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> 

(dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c5 = '2015-05-06 110:23:21';

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

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

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

|   | SIMPLE      | table_key1 | ref  | c5_index      | c5_index |        | const |     | Using index condition |

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

 row in set,  warnings (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 

##############################在大家认识里datetime是八个字节的长度，下面就来看看，是不是真的这样

(dg1)root@localhost [mytest]> alter table table_key1 modify c6 datetime not null;

Query OK,  rows affected (0.03 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@localhost [mytest]> desc select * from table_key1 where c6 = '2015-05-06 11:10:36';

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

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

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

|   | SIMPLE      | table_key1 | ref  | c6_index      | c6_index |        | const |     | NULL  |

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

 row in set (0.00 sec)

(dg1)root@localhost [mytest]> alter table table_key1 modify c6 datetime  null;

Query OK,  rows affected (0.06 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@localhost [mytest]> desc select * from table_key1 where c6 = '2015-05-06 11:10:36';

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

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

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

|   | SIMPLE      | table_key1 | ref  | c6_index      | c6_index |        | const |     | NULL  |

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

 row in set (0.00 sec)

颠覆了我们认识，datetime不是8个字节么，下面来看一下MySQL的版本，没错MySQL5.6是datetime长度是5个字节

(dg1)root@localhost [mytest]> \s

--------------

mysql  Ver 14.14 Distrib 5.6., for linux-glibc2. (x86_64) using  EditLine wrapper

Connection id:

Current database:    mytest

Current user:        root@localhost

SSL:            Not in use

Current pager:        stdout

Using outfile:        ''

Using delimiter:    ;

Server version:        5.6.-log MySQL Community Server (GPL)

Protocol version:

Connection:        Localhost via UNIX socket

Server characterset:    gbk

Db     characterset:    gbk

Client characterset:    gbk

Conn.  characterset:    gbk

UNIX socket:        /opt/app/mysql/mysql3307.socket

Uptime:             min  sec

Threads:   Questions:   Slow queries:   Opens:   Flush tables:   Open tables:   Queries per second avg: 0.066

--------------

(dg1)root@localhost [mytest]> 

那么真的是这样么，
(dg1)root@localhost [mytest]>alter table table_key1 change c6 c6 datetime(6) not null ;
(dg1)root@localhost [mytest]>explain select * from table_key1 where c6='2015-05-06 11:13:12.000000';

　　+----+-------------+------------+------------+------+---------------+----------+---------+-------+------+----------+-------+
　　| id | select_type | table      | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra |
　　+----+-------------+------------+------------+------+---------------+----------+---------+-------+------+----------+-------+
　　| 1  | SIMPLE      | table_key1 | NULL       | ref  | c6_index      | c6_index | 8       | const | 1    | 100.00   | NULL  |
　　+----+-------------+------------+------------+------+---------------+----------+---------+-------+------+----------+-------+
　　1 row in set, 1 warning (0.00 sec)

小结：在MySQL5.6版本，是否还得使用timestamp类型应该是仁者见仁智者见智的问题了，datetime不存毫秒时候五个字节，因为官方文档说了，5.6.4以上版本，时间和毫秒分开存储了。timestamp范围比较窄（1970-2037年），不排除后续版本会修改其范围值

4.字符类型

表结构，字符集是UTF8

(dg1)root@127.0.0.1 [mytest]> desc table_key;

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

| Field   | Type        | Null | Key | Default | Extra |

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

| id      | int()     | NO   |     | NULL    |       |

| sid     | bigint()  | NO   |     |        |       |

| name    | char()    | YES  |     | NULL    |       |

| age     | tinyint()  | YES  |     | NULL    |       |

| sex     | tinyint()  | NO   |     | NULL    |       |

| address | varchar() | YES  | MUL | NULL    |       |

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

 rows in set (0.01 sec)

看看定长字符类型char的索引长度，在NOT NULL 和NULL中分别为10*3和10*+

(dg1)root@127.0.0.1 [mytest]> alter table table_key add index name_index (name);

Query OK,  rows affected (0.01 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name='zhangsan';

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

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

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

|   | SIMPLE      | table_key | ref  | name_index    | name_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify name char();

Query OK,  rows affected (0.05 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name='zhangsan';

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

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

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

|   | SIMPLE      | table_key | ref  | name_index    | name_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.00 sec)

看看变长长字符类型varchar的索引长度，在NOT NULL 和NULL中分别为10*+2和10*++

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address='shanghai';

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

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

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

|   | SIMPLE      | table_key | ref  | address_index | address_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.01 sec)

(dg1)root@127.0.0.1 [mytest]> 

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify address varchar();

Query OK,  rows affected (0.10 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address='shanghai';

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

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

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

|   | SIMPLE      | table_key | ref  | address_index | address_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 
修改字符集为GBK，再来看看

(dg1)root@127.0.0.1 [mytest]> ALTER TABLE `table_key` DEFAULT CHARACTER SET=gbk COLLATE=gbk_chinese_ci;

(dg1)root@127.0.0.1 [mytest]> ALTER database `mytest` DEFAULT CHARACTER SET=gbk COLLATE=gbk_chinese_ci;

(dg1)root@127.0.0.1 [mytest]> show global variables like '%char%';

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

| Variable_name            | Value                          |

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

| character_set_client     | gbk                            |

| character_set_connection | gbk                            |

| character_set_database   | gbk                            |

| character_set_filesystem | binary                         |

| character_set_results    | gbk                            |

| character_set_server     | gbk                            |

| character_set_system     | utf8                           |

| character_sets_dir       | /opt/app/mysql/share/charsets/ |

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

 rows in set (0.00 sec)

奇怪了，为什么还是10*++1呢，是因为字段的字符集还没修改过来

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address='shanghai';

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

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

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

|   | SIMPLE      | table_key | ref  | address_index | address_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.01 sec)

(dg1)root@127.0.0.1 [mytest]> ALTER TABLE `table_key` MODIFY COLUMN `name`  char() CHARACTER SET gbk COLLATE gbk_chinese_ci NULL DEFAULT NULL , DEFAULT CHARACTER SET=gbk COLLATE=gbk_chinese_ci;

Query OK,  rows affected (0.04 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> ALTER TABLE `table_key` MODIFY COLUMN `address`  char() CHARACTER SET gbk COLLATE gbk_chinese_ci NULL DEFAULT NULL , DEFAULT CHARACTER SET=gbk COLLATE=gbk_chinese_ci;

Query OK,  rows affected (0.03 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address='shanghai';

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

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

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

|   | SIMPLE      | table_key | ref  | address_index | address_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.00 sec)

(dg1)root@127.0.0.1 [mytest]>

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify address varchar();

Query OK,  rows affected (0.04 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address='shanghai';

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

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

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

|   | SIMPLE      | table_key | ref  | address_index | address_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.01 sec)

(dg1)root@127.0.0.1 [mytest]> 

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name='zhangsan';

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

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

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

|   | SIMPLE      | table_key | ref  | name_index    | name_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify name char();

Query OK,  rows affected (0.00 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name='zhangsan';

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

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

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

|   | SIMPLE      | table_key | ref  | name_index    | name_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.01 sec)
来看看复合索引的key_len,（刚才测试GBK字符集，字符集转换成GBK了）
(dg1)root@127.0.0.1 [mytest]> desc table_key;

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

| Field   | Type        | Null | Key | Default | Extra |

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

| id      | int(11)     | NO   |     | NULL    |       |

| sid     | bigint(20)  | NO   |     | 0       |       |

| name    | char(10)    | NO   |     |         |       |

| age     | tinyint(4)  | YES  |     | NULL    |       |

| sex     | tinyint(4)  | NO   |     | NULL    |       |

| address | varchar(10) | NO   | MUL |         |       |

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

6 rows in set (0.01 sec)
(dg1)root@127.0.0.1 [mytest]> alter table table_key drop index name_index;

Query OK,  rows affected (0.01 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> alter table table_key drop index address_index;

Query OK,  rows affected (0.01 sec)

Records:   Duplicates:   Warnings:

(dg1)root@127.0.0.1 [mytest]> alter table table_key add index name_address_index (name,address);

Query OK,  rows affected (0.02 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address='shanghai' and name='zhangsan';

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

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

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

|   | SIMPLE      | table_key | ref  | name_address_index | name_address_index |       | const,const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.00 sec)

看看复合索引的长度，因为能全部使用到组合索引，所以是：2*（10）+2*（20）+2=42,下面将name字段允许为空，再来看看

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify name char(10);

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name='zhangsan' and address='shanghai';

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

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

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

|   | SIMPLE      | table_key | ref  | name_address_index | name_address_index |       | const,const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.00 sec)
看看复合索引的长度，因为能全部使用到组合索引，所以是：2*（10）+1+2*（20）+2=43
(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name='zhangsan';

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

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

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

|   | SIMPLE      | table_key | ref  | name_address_index | name_address_index |       | const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.00 sec)

那么我们来看看部分使用复合索引：2*（10）+1,将address设置为允许为空，再来看看

(dg1)root@127.0.0.1 [mytest]> alter table table_key modify address varchar();

Query OK,  rows affected (0.04 sec)

Records:   Duplicates:   Warnings: 

(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name='zhangsan' and address='shanghai';

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

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

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

|   | SIMPLE      | table_key | ref  | name_address_index | name_address_index |       | const,const |     |   100.00 | Using index condition |

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

 row in set,  warning (0.00 sec)

(dg1)root@127.0.0.1 [mytest]> 
这时候key_len=2*（10）+1+2*（10）+2+1=44

总结：

1.整数类型，浮点数类型，时间类型的索引长度

NOT NULL=字段本身的字段长度

NULL=字段本身的字段长度+1，因为需要有是否为空的标记，这个标记需要占用1个字节

datetime类型在5.6中字段长度是5个字节

2.字符类型

varchr(n)变长字段且允许NULL    =  n * ( utf8=,gbk=,latin1=)+(NULL)+

varchr(n)变长字段且不允许NULL  =  n * ( utf8=,gbk=,latin1=)+

char(n)固定字段且允许NULL      =  n * ( utf8=,gbk=,latin1=)+(NULL)

char(n)固定字段且允许NULL      =  n * ( utf8=,gbk=,latin1=)

变长字段需要额外的2个字节（VARCHAR值保存时只保存需要的字符数，另加一个字节来记录长度(如果列声明的长度超过255，则使用两个字节)，所以VARCAHR索引长度计算时候要加2），固定长度字段不需要额外的字节。而null都需要1个字节的额外空间,所以索引字段最好不要为NULL，因为NULL让统计更加复杂，并且需要额外的存储空间。这个结论在此得到了证实，复合索引有最左前缀的特性，如果复合索引能全部使用上，则是复合索引字段的索引长度之和，这也可以用来判定复合索引是否部分使用，还是全部使用。

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