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(11) | NO | | NULL | | | sid | bigint(20) | NO | | 0 | | | name | char(10) | YES | | NULL | | | age | tinyint(4) | YES | | NULL | | | sex | tinyint(4) | NO | | NULL | | | address | varchar(10) | 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=38; +----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+ | 1 | SIMPLE | table_key | ref | age_index | age_index | 1 | const | 1 | 100.00 | NULL | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+ row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]> alter table table_key modify age tinyint(4); (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where age=38; +----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+ | 1 | SIMPLE | table_key | ref | age_index | age_index | 2 | const | 1 | 100.00 | NULL | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+ row in set, 1 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, 0 rows affected (0.04 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 | 8 | const | 1 | 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(20); Query OK, 0 rows affected (0.08 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 | 9 | const | 1 | 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 0; Query OK, 9 rows affected (0.04 sec) Records: 9 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 | 2 | const | 1 | NULL | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ row in set (0.00 sec) (dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid smallint ; Query OK, 0 rows affected (0.07 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 | 3 | const | 1 | 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, 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 | 3 | const | 1 | 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 | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ 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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c1_index | c1_index | 4 | const | 8 | 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, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c1_index | c1_index | 5 | const | 8 | 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, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ | 1 | SIMPLE | table_key1 | ref | c2_index | c2_index | 8 | const | 1 | NULL | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ row in set (0.00 sec) (dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c2 double; Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ | 1 | SIMPLE | table_key1 | ref | c2_index | c2_index | 9 | const | 1 | 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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c4_index | c4_index | 3 | const | 4 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ row in set, 3 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, 0 rows affected (0.09 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c4_index | c4_index | 4 | const | 4 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ row in set, 3 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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c5_index | c5_index | 4 | const | 5 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ row in set, 3 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, 0 rows affected (0.06 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c5_index | c5_index | 5 | const | 5 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ row in set, 3 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, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ | 1 | SIMPLE | table_key1 | ref | c6_index | c6_index | 5 | const | 1 | NULL | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ row in set (0.00 sec) (dg1)root@localhost [mytest]> alter table table_key1 modify c6 datetime null; Query OK, 0 rows affected (0.06 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ | 1 | SIMPLE | table_key1 | ref | c6_index | c6_index | 6 | const | 1 | 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 5.6.22, for linux-glibc2.5 (x86_64) using EditLine wrapper Connection id: 3 Current database: mytest Current user: root@localhost SSL: Not in use Current pager: stdout Using outfile: '' Using delimiter: ; Server version: 5.6.22-log MySQL Community Server (GPL) Protocol version: 10 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: 4 min 47 sec Threads: 1 Questions: 19 Slow queries: 0 Opens: 75 Flush tables: 1 Open tables: 64 Queries per second avg: 0.066 -------------- (dg1)root@localhost [mytest]> 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是五个字节，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(11) | NO | | NULL | | | sid | bigint(20) | NO | | 0 | | | name | char(10) | YES | | NULL | | | age | tinyint(4) | YES | | NULL | | | sex | tinyint(4) | NO | | NULL | | | address | varchar(10) | 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*3+1 (dg1)root@127.0.0.1 [mytest]> alter table table_key add index name_index (name); Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_index | name_index | 30 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ row in set, 1 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(10); Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_index | name_index | 31 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ row in set, 1 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*3+2和10*3+2+1 (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 | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | address_index | address_index | 32 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ row in set, 1 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(10); Query OK, 0 rows affected (0.10 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | address_index | address_index | 33 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]> 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 | | | +---------+-------------+------+-----+---------+-------+ rows in set (0.01 sec) (dg1)root@127.0.0.1 [mytest]> alter table table_key drop index name_index; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> alter table table_key drop index address_index; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> alter table table_key add index name_address_index (name,address); Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_address_index | name_address_index | 42 | const,const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ row in set, 1 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 | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_address_index | name_address_index | 43 | const,const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ row in set, 1 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 | +----+-------------+-----------+------+--------------------+--------------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_address_index | name_address_index | 21 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+--------------------+--------------------+---------+-------+------+----------+-----------------------+ row in set, 1 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(10); Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0 (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 | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_address_index | name_address_index | 44 | const,const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ row in set, 1 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=3,gbk=2,latin1=1)+1(NULL)+2 varchr(n)变长字段且不允许NULL = n * ( utf8=3,gbk=2,latin1=1)+2 char(n)固定字段且允许NULL = n * ( utf8=3,gbk=2,latin1=1)+1(NULL) char(n)固定字段且允许NULL = n * ( utf8=3,gbk=2,latin1=1)
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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