MYSQL EXPLAIN 中的KEY_LEN的说明

Posted 2020-09-28

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对于explain extended 查看执行计划里面的一些信息作为一个DBA还是必须掌握的。

参考博文：http://www.cnblogs.com/xuanzhi201111/p/4554769.html

环境： mysql5.6.36

默认字符集： utf8

一、前置回顾：

1、数值型的字段长度

字段类型长度 UNSIGNED SIGNED有符号型适用场合

tinyint: 1bytes 2^8-1 0-255 -128~127 小整数值

smallint: 2bytes 2^16-1 0-65535 -32768~32767 大整数值

mediumint: 3bytes 2^24-1 0-16777215 -8388608~8388607 大整数值

int: 4bytes 2^32-1 0-4294967295 -2147483648-2147483647 大整数值(最大只能存10位数字)

bigint: 8bytes 2^64-1 大整数值

float 4bytes 单精度浮点型

double 8bytes 双精度浮点型

2、char和varchar型的字段长度

char和varchar是日常使用最多的字符类型。char（N）用于保存固定长度的字符串，长度最大为255，比指定长度大的值将被截短，而比指定长度小的值将会用空格进行填补。

varchar（N）用于保存可以变长的字符串，长度最大为65535，只存储字符串实际实际需要的长度（它会增加一个额外字节来存储字符串本身的长度），varchar使用额外的1~2字节来存储值的的长度，如果列的最大长度小于或者等于255，则用1字节，否则用2字节。

char和varchar跟字符编码也有密切的联系，latin1占用1个字节，gbk占用2个字节，utf8占用3个字节。（不同字符编码占用的存储空间不同）

3、日期&时间类型的字段长度

DataType Storage Required Before MySQL 5.6.4 Storage Requiredas of MySQL 5.6.4

YEAR 1byte 1byte

DATE 3bytes 3bytes

TIME 3bytes 3 bytes + fractional seconds storage

DATETIME 8bytes 5 bytes + fractional seconds storage

TIMESTAMP 4bytes 4 bytes + fractional seconds storage

二、创建测试用表：

> use test;

> CREATE TABLE `t1` (

`a`int(11) NULL,

`aa` int(11) NOT NULL,

`b`char(10) NULL,

`bb` char(10) NOT NULL,

`c`varchar(20) NULL,

`cc` varchar(20) NOT NULL,

`d`time NULL,

`dd` time NOT NULL,

`e`timestamp NULL,

`ee` timestamp NOT NULL,

`f`datetime NULL,

`ff` datetime NOT NULL,

KEY`index_a` (`a`),

KEY`index_aa` (`aa`),

KEY`index_b` (`b`),

KEY`index_bb` (`bb`),

KEY`index_c` (`c`),

KEY`index_cc` (`cc`),

KEY`index_d` (`d`),

KEY`index_dd` (`dd`),

KEY`index_e` (`e`),

KEY`index_ee` (`ee`),

KEY`index_f` (`f`),

KEY`index_ff` (`ff`)

) ENGINE=InnoDB CHARSET=utf8 ;

上面创建了一张奇怪的表，基本上覆盖了常见的字段类型了，每个字段都加了索引。

> insert into t1 values(1,11,‘2‘,‘22‘,‘3‘,‘33‘,‘12:23:12‘,‘02:11:45‘,‘2017-02-12 01:00:12‘,‘2015-04-2121:09:12‘,‘2015-12-21 21:09:12‘,‘2021-04-21 01:45:56‘) ;

> insert into t1 values(12,131,‘62‘,‘262‘,‘38‘,‘332‘,‘22:23:12‘,‘02:18:45‘,‘2017-12-1201:34:12‘,‘2005-04-21 21:09:12‘,‘2065-12-21 21:09:12‘,‘1949-10-01 01:45:56‘) ;

> explain extended select * from t1where a=12;

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

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

| 1| SIMPLE | t1 | ref | index_a | index_a | 5 | const | 1 | 100.00 | NULL |

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

4（int占4bytes）+ 1 (1byte用来标记是否为null)

> explain extended select * from t1where aa=12;

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

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

| 1| SIMPLE | t1 | ref | index_aa | index_aa | 4 | const | 1 | 100.00 | NULL |

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

4（int占4bytes）

> explain extended select * from t1where b=‘abc‘;

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

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

| 1| SIMPLE | t1 | ref | index_b | index_b | 31 | const | 1 | 100.00 | Using index condition |

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

10*3 （char每个字符在utf字符集下占3bytes）+ 1 (1byte用来标记是否为null)

> explain extended select * from t1where bb=‘stfdg4‘;

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

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

| 1| SIMPLE | t1 | ref | index_bb | index_bb |30 | const | 1 | 100.00 | Using index condition |

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

10*3 （char每个字符在utf字符集下占3bytes）

> explain extended select * from t1where c=‘hfdg‘;

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

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

| 1| SIMPLE | t1 | ref | index_c | index_c | 63 | const | 1 | 100.00 | Using index condition |

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

20*3（varchar每个字符在utf字符集下占3bytes） + 2 (2bytes用来记录varchar这种变长字符的长度) + 1 (1byte用来标记是否为null)

> explain extended select * from t1where cc=‘fgd‘;

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

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

| 1| SIMPLE | t1 | ref | index_cc | index_cc | 62 | const | 1 | 100.00 | Using index condition |

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

20*3（varchar每个字符在utf字符集下占3bytes） + 2 (2bytes用来记录varchar这种变长字符的长度)

> explain extended select * from t1where d=‘01:02:02‘;

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

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

| 1| SIMPLE | t1 | ref | index_d | index_d | 4 | const | 1 | 100.00 | Using index condition |

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

3 (time 类型占据3bytes) + 1 (1byte用来标记是否为null)

> explain extended select * from t1where dd=‘01:02:02‘;

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

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

| 1| SIMPLE | t1 | ref | index_dd | index_dd |3 | const | 1 | 100.00 | Using index condition |

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

3 (time 类型占据3bytes)

> explain extended select * from t1where e=‘2017-02-11 01:02:02‘;

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

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

| 1| SIMPLE | t1 | ref | index_e | index_e | 5 | const | 1 | 100.00 | NULL |

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

4 (timestamp 类型占据4bytes) + 1 (1byte用来标记是否为null)

> explain extended select * from t1where ee=‘2017-02-11 01:02:02‘;

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

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

| 1| SIMPLE | t1 | ref | index_ee | index_ee | 4 | const | 1 | 100.00 | NULL |

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

4 (timestamp 类型占据4bytes)

> explain extended select * from t1where f=‘2017-02-11 01:02:02‘;

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

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

| 1| SIMPLE | t1 | ref | index_f | index_f | 6 | const | 1 | 100.00 | NULL |

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

5 (datetime 类型占据5bytes) + 1 (1byte用来标记是否为null)

> explain extended select * from t1where ff=‘2017-02-11 01:02:02‘;

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

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

| 1| SIMPLE | t1 | ref | index_ff | index_ff |5 | const | 1 | 100.00 | NULL |

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

5 (datetime 类型占据5bytes)

联合索引的key_len的计算：

先去掉上面的2个单列索引，防止执行计划跑偏，然后加上一个联合索引：

alter table t1 drop index index_c;

alter table t1 drop index index_d;

alter table t1 add index c_d (c,d);

> explain extended select * from t1where c=‘aaa‘ and d=‘12:21:11‘;

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

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

| 1| SIMPLE | t1 | ref | c_d | c_d | 67 | const,const | 1 | 100.00 | Using index condition |

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

`c`列 varchar(20)DEFAULT NULL,

`d`列 timeDEFAULT NULL,

key_len = （20*3+2+1）+（3+1）= 67

总结1：

char和varchar类型key_len计算公式:

varchr(N)变长字段且允许NULL = N *( character set：utf8=3,gbk=2,latin1=1)+1(NULL)+2(变长字段)

varchr(N)变长字段且不允许NULL = N * ( character set：utf8=3,gbk=2,latin1=1)+2(变长字段)

char(N)固定字段且允许NULL = N* ( character set：utf8=3,gbk=2,latin1=1)+1(NULL)

char(N)固定字段且允许NULL = N* ( character set：utf8=3,gbk=2,latin1=1)

数值数据的key_len计算公式：

TINYINT允许NULL = 1 +1(NULL)

TINYINT不允许NULL = 1

SMALLINT允许为NULL =2+1(NULL)

SMALLINT不允许为NULL = 2

INT允许为NULL =4+1(NULL)

INT不允许为NULL = 4

BIGINT允许为NULL = 8+1(NULL)

BIGINT不允许为NULL = 8

日期时间型的key_len计算：（针对mysql5.5及之前版本）

DATETIME允许为NULL= 8 + 1(NULL)

DATETIME不允许为NULL = 8

TIMESTAMP允许为NULL = 4+ 1(NULL)

TIMESTAMP不允许为NULL = 4

日期时间型的key_len计算：（针对mysql5.6.4及之后的版本）

DATETIME允许为NULL= 5 + 1(NULL)

DATETIME不允许为NULL = 5

TIMESTAMP允许为NULL = 4+ 1(NULL)

TIMESTAMP不允许为NULL = 4

总结2：

1、从上面几个测试可以看出，假如我们建表时候使用varchar(20),varchar(255) 虽然实际存放的内容长度是一样的。但是这2种情况下索引占据的长度是不一样的。

MySQL建立索引时如果没有限制索引的大小，索引长度会默认采用的该字段的长度，

也就是说varchar(20)和varchar(255)对应的索引长度分别为20*3(utf-8)(+2+1),255*3(utf-8)(+2+1)，

其中"+2"用来存储长度信息，“+1”用来标记是否为空。

加载索引信息时用varchar(255)类型会占用更多的内存。

2、另外，我们可以看到如果建表的时候，字段设置为DEFAULT NULL，会导致索引的长度增加1bytes，此外，NULL列会导致索引失效。

补充：

查看test库中的行数量、数据大小、索引大小等统计情况：

> SELECTCONCAT(table_schema,‘.‘,table_name) AS ‘Table Name‘,CONCAT(ROUND(table_rows/1000000,4),‘M‘) AS ‘Number of Rows‘,

CONCAT(ROUND(data_length/(1024*1024*1024),4),‘G‘)AS ‘Data Size‘,

CONCAT(ROUND(index_length/(1024*1024*1024),4),‘G‘)AS ‘Index Size‘,

CONCAT(ROUND((data_length+index_length)/(1024*1024*1024),4),‘G‘)AS‘Total‘

FROMinformation_schema.TABLES

WHEREtable_schema LIKE ‘test‘;

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

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

| test.a | 0.0164M | 0.0015G | 0.0020G | 0.0034G |

| test.b | 0.0165M | 0.0015G | 0.0000G | 0.0015G |

| test.c | 0.3594M | 0.0210G | 0.0000G | 0.0210G |

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

3 rows in set (0.00 sec)

本文出自 “一只菜鸡的笔记” 博客，请务必保留此出处http://lee90.blog.51cto.com/10414478/1947850

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