mysql 索引相关知识

由where 1 =1 引发的思考

最近工作上被说了

说代码中不能用 where 1=1，当时觉得是应该可以用的，但是找不到什么理据，

而且mysql 语句优化这方面确实很薄弱

感觉自己mysql方面是知识还是不够哇

得好好研究研究

还有发现很多知识点光看的话根本记不住，也不深刻。还是得亲手实践下

so ~~~ 挫折越多进步越快

1、关于sql语句大小问题

1）mysql默认情况下没有设置 root用户密码

给mysql的root用户设置密码先

[ztao@localhost ~]$ mysqladmin -uroot password "root"

2）默认情况下sql语句是限制在1M以内

max_allowed_packet         表示sql最大为1M

mysql> show VARIABLES like '%max_allowed_packet%';

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

| Variable_name            | Value      |

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

| max_allowed_packet       | 1048576    |

| slave_max_allowed_packet | 1073741824 |

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

2 rows in set (0.00 sec)

这里最好改大，如果太小的话sql语句超过1M会导致sql语句执行失败

[root@localhost ztao]# vim /etc/my.cnf

添加红色的那一行

[mysqld]

datadir=/var/lib/mysql

socket=/var/lib/mysql/mysql.sock

user=mysql

# Disabling symbolic-links is recommended to prevent assorted security risks

symbolic-links=

max_allowed_packet = 160M

[mysqld_safe]

log-error=/var/log/mysqld.log

pid-file=/var/run/mysqld/mysqld.pid

4）

然后重启mysql 在查看下

160M = 160*1024*1024=167772160

mysql> show VARIABLES like '%max_allowed_packet%';

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

| Variable_name            | Value      |

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

| max_allowed_packet       | 167772160  |

| slave_max_allowed_packet | 1073741824 |

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

2 rows in set (0.00 sec)

这样能支持你一条sql语句在160M以内

2、在window下写的脚本文件放在linux上执行失败

在Windows下写了一个shell脚本，

上传到Linux下执行时经常会报这种错误：

[root@localhost test]# ./test.sh

-bash: ./test.sh: /bin/sh^M: bad interpreter: No such file or directory

主要原因是test.sh是我在windows下编辑然后上传到linux系统里执行的。

windows下编辑的.sh文件的格式为dos格式。

而linux只能执行格式为unix格式的脚本。

因为在dos/window下按一次回车键实际上输入的是“回车（CR)”和“换行（LF）”，

而Linux/unix下按一次回车键只输入“换行（LF）”，所以修改的sh文件在每行都会多了一个CR，所以Linux下运行时就会报错找不到命令。

我们可以通过vi编辑器来查看文件的format格式。步骤如下：

1.首先用vi命令打开文件

2.在vi命令模式中使用 :set ff 命令

可以看到改文件的格式为

fileformat=dos

3.修改文件format为unix

使用vi/vim修改文件format

命令：:set ff=unix

或者：:set fileformat=unix

然后:wq保存退出就可以了

3、通过脚本创建表并插入数据

1）通过shell创建表

#!/bin/sh

dbenv=dev

if [ "${dbenv}" = "dev" ]; then

    mysql_user=root

    mysql_pass=root

    mysql_host=

fi

if [ "${dbenv}" = "idc" ]; then

    mysql_user=root

    mysql_pass=root

    mysql_host="-h 10.10.10.10"

fi

mysql_cmd="mysql -u${mysql_user} -p${mysql_pass} ${mysql_host} --default-character-set=utf8"

${mysql_cmd}<<EOF

use test;

drop table if exists t_user;

create table t_user

(

    f_uin bigint() NOT NULL  auto_increment,

    f_name varchar() NOT NULL DEFAULT '',

    f_createtime datetime not null default '0000-00-00 00:00:00',

    f_status int() NOT NULL DEFAULT '',

    f_rangenum int() NOT NULL DEFAULT '',

    PRIMARY KEY (f_uin)

)ENGINE=InnoDB DEFAULT CHARSET=utf8;

EOF

echo "create table t_user ok"

2）通过python脚本来创建sql脚本

#!/usr/bin/python

#-*-coding:UTF-8-*-

import sys

def Insert(num):

        if(num<=10000000):

                str = "insert into t_user (f_name,f_createtime,f_status,f_rangenum) values "

                for i in range(num):

                        name = "zhantao_%d"%i

                        status = 1

                        rangenum = i

                        if(i==0):

                                strsql = "('%s',now(),%d,%d)"%(name,status,rangenum)

                        else:

                                strsql = ",('%s',now(),%d,%d)"%(name,status,rangenum)

                        str += strsql

                f = open("insert.sql","w")

                f.write(str)

                f.close

if __name__ == '__main__':

        #num = int(sys.argv[1])

        Insert(1000000)

可以看到创建出来的sql脚本一个就33兆如果前面没有修改sql语句最大长度，是肯定执行不了的

[ztao@localhost 0418]$ ll -alh

total 33M

drwxrwxr-x 2 ztao ztao 4.0K Apr 18 20:16 .

drwxrwxr-x 3 ztao ztao 4.0K Apr 18 20:02 ..

-rwxrwxr-x 1 ztao ztao 733 Apr 18 20:03 create_table.sh

-rwxrwxr-x 1 ztao ztao 538 Apr 18 20:10 insert.py

-rw-rw-r-- 1 ztao ztao 33M Apr 18 20:11 insert.sql

3）把数据导入到sql里面

这里面导入的五次，一次100w

因为主键是自增的所以重复插入不会产生冲突

mysql> use test

Reading table information for completion of table and column names

You can turn off this feature to get a quicker startup with -A

Database changed

mysql> source /home/ztao/Work/Test/0418/insert.sql

Query OK, 1000000 rows affected (11.77 sec)

Records: 1000000  Duplicates: 0  Warnings: 0

mysql> source /home/ztao/Work/Test/0418/insert.sql

Query OK, 1000000 rows affected (8.90 sec)

Records: 1000000  Duplicates: 0  Warnings: 0

mysql> source /home/ztao/Work/Test/0418/insert.sql

Query OK, 1000000 rows affected (8.43 sec)

Records: 1000000  Duplicates: 0  Warnings: 0

mysql> source /home/ztao/Work/Test/0418/insert.sql

Query OK, 1000000 rows affected (8.28 sec)

Records: 1000000  Duplicates: 0  Warnings: 0

mysql> source /home/ztao/Work/Test/0418/insert.sql

Query OK, 1000000 rows affected (7.87 sec)

Records: 1000000  Duplicates: 0  Warnings: 0

mysql> select count(*) from t_user;

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

| count(*) |

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

|  5000000 |

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

1 row in set (3.78 sec)

以下是实验的重点

还有以下测试情况不考虑联合索引

联合索引的情况复杂一点、下次再验证

1、我们先分析在没有索引的情况下。

mysql> select * from t_user where f_rangenum = 123568;

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

| f_uin   | f_name         | f_createtime        | f_status | f_rangenum |

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

|  123569 | zhantao_123568 | 2015-04-18 20:11:36 |        1 |     123568 |

| 1123569 | zhantao_123568 | 2015-04-18 20:11:54 |        1 |     123568 |

| 2123569 | zhantao_123568 | 2015-04-18 20:12:05 |        1 |     123568 |

| 3123569 | zhantao_123568 | 2015-04-18 20:12:15 |        1 |     123568 |

| 4123569 | zhantao_123568 | 2015-04-18 20:12:25 |        1 |     123568 |

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

5 rows in set (2.19 sec)

mysql> explain select * from t_user where f_rangenum = 123568;

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

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

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

|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL | 4989899 | Using where |

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

1 row in set (0.00 sec)

在没有索引的情况下执行了2秒多

explian sql语句发现几乎它的type类型是All 即效率最差的

rows 为mysql认为需要查看的条数。几乎是全表每条数据的扫了一边

效率极低

2、在f_rangenum上增加索引，分析sql语句

mysql> alter table t_user add index f_rangenum_index(f_rangenum);

Query OK, 5000000 rows affected (1 min 45.16 sec)

Records: 5000000  Duplicates: 0  Warnings: 0

数量量大了，添加索引很慢。。。。。。。。

mysql> select * from t_user where f_rangenum = 123199;

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

| f_uin   | f_name         | f_createtime        | f_status | f_rangenum |

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

|  123200 | zhantao_123199 | 2015-04-18 20:11:36 |        1 |     123199 |

| 1123200 | zhantao_123199 | 2015-04-18 20:11:54 |        1 |     123199 |

| 2123200 | zhantao_123199 | 2015-04-18 20:12:05 |        1 |     123199 |

| 3123200 | zhantao_123199 | 2015-04-18 20:12:15 |        1 |     123199 |

| 4123200 | zhantao_123199 | 2015-04-18 20:12:25 |        1 |     123199 |

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

5 rows in set (0.02 sec)

mysql> explain select * from t_user where f_rangenum = 123199;

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

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

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

|  1 | SIMPLE      | t_user | ref  | f_rangenum_index | f_rangenum_index | 4       | const |    5 |       |

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

1 row in set (0.00 sec)

在有索引的情况下执行效率非常高

看explain rows为5 只查了五条数据

它的type为ref 表示是根据key（f_rangenum_index ）常数123199来查询的

所以效果很好

3、where后面的过滤条件顺序是否有影响

mysql> select * from t_user where f_rangenum = 123199;

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

| f_uin   | f_name                  | f_createtime        | f_status | f_rangenum |

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

|  123200 | zhantao_123599001123200 | 2015-04-18 20:11:36 |        1 |     123199 |

| 1123200 | zhantao_123599002       | 2015-04-18 20:11:54 |        1 |     123199 |

| 2123200 | zhantao_123199          | 2015-04-18 20:12:05 |        1 |     123199 |

| 3123200 | zhantao_123199          | 2015-04-18 20:12:15 |        1 |     123199 |

| 4123200 | zhantao_123199          | 2015-04-18 20:12:25 |        1 |     123199 |

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

5 rows in set (0.00 sec)

mysql> select * from t_user where f_rangenum = 123199 and f_name = "zhantao_123599001123200";

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

| f_uin  | f_name                  | f_createtime        | f_status | f_rangenum |

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

| 123200 | zhantao_123599001123200 | 2015-04-18 20:11:36 |        1 |     123199 |

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

1 row in set (0.00 sec)

mysql> select * from t_user where  f_name = "zhantao_123599001123200" and  f_rangenum = 123199;

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

| f_uin  | f_name                  | f_createtime        | f_status | f_rangenum |

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

| 123200 | zhantao_123599001123200 | 2015-04-18 20:11:36 |        1 |     123199 |

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

1 row in set (0.00 sec)

mysql> explain select * from t_user where  f_name = "zhantao_123599001123200" and  f_rangenum = 123199;

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

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

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

|  1 | SIMPLE      | t_user | ref  | f_rangenum_index | f_rangenum_index | 4       | const |    5 | Using where |

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

1 row in set (0.00 sec)

mysql> explain select * from t_user where f_rangenum = 123199 and f_name = "zhantao_123599001123200";

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

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

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

|  1 | SIMPLE      | t_user | ref  | f_rangenum_index | f_rangenum_index | 4       | const |    5 | Using where |

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

1 row in set (0.00 sec)

select * from t_user where f_name = "zhantao_123599001123200" and f_rangenum = 123199;

select * from t_user where f_rangenum = 123199 and f_name = "zhantao_123599001123200";

两个语句一样

只是把where后面的字段顺序挑换了一下

一个先写设置了索引的过滤条件，另一个先写没有过滤索引的字段

我们可以发现它们执行的时间是一样的。

它们的explain的结果也一样

说明它们where后面的顺序不影响。

我想应该是mysql 的sql优化器会去先执行有索引的字段然后再查找非索引的字段

4、关于where 1=1 是否会导致全变扫描

废话不多说情况跟上面的一样

应该是mysql 的sql优化器会过滤掉1=1 所以根本不会导致全表扫描

mysql> explain select * from t_user where  1=1 and  f_rangenum = 123199;

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

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

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

|  1 | SIMPLE      | t_user | ref  | f_rangenum_index | f_rangenum_index | 4       | const |    5 |       |

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

1 row in set (0.00 sec)

mysql> explain select * from t_user where   f_rangenum = 123199;

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

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

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

|  1 | SIMPLE      | t_user | ref  | f_rangenum_index | f_rangenum_index | 4       | const |    5 |       |

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

1 row in set (0.00 sec)

mysql> select * from t_user where   f_rangenum = 123199;

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

| f_uin   | f_name                  | f_createtime        | f_status | f_rangenum |

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

|  123200 | zhantao_123599001123200 | 2015-04-18 20:11:36 |        1 |     123199 |

| 1123200 | zhantao_123599002       | 2015-04-18 20:11:54 |        1 |     123199 |

| 2123200 | zhantao_123199          | 2015-04-18 20:12:05 |        1 |     123199 |

| 3123200 | zhantao_123199          | 2015-04-18 20:12:15 |        1 |     123199 |

| 4123200 | zhantao_123199          | 2015-04-18 20:12:25 |        1 |     123199 |

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

5 rows in set (0.00 sec)

mysql> select * from t_user where 1=1 and  f_rangenum = 123199;

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

| f_uin   | f_name                  | f_createtime        | f_status | f_rangenum |

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

|  123200 | zhantao_123599001123200 | 2015-04-18 20:11:36 |        1 |     123199 |

| 1123200 | zhantao_123599002       | 2015-04-18 20:11:54 |        1 |     123199 |

| 2123200 | zhantao_123199          | 2015-04-18 20:12:05 |        1 |     123199 |

| 3123200 | zhantao_123199          | 2015-04-18 20:12:15 |        1 |     123199 |

| 4123200 | zhantao_123199          | 2015-04-18 20:12:25 |        1 |     123199 |

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

5 rows in set (0.00 sec)

总结是无论where后面的索引在什么位置都会使用索引

mysql 组合索引

1、组合索引

1）添加组合索引

mysql> alter table t_user_bak add index f_namerangenum_index(f_name,f_rangenum);

Query OK, 5000000 rows affected, 2 warnings (2 min 54.18 sec)

Records: 5000000  Duplicates: 0  Warnings: 0

mysql> show create table t_user_bak\G;

*************************** 1. row ***************************

       Table: t_user_bak

Create Table: CREATE TABLE `t_user_bak` (

  `f_uin` bigint(20) NOT NULL AUTO_INCREMENT,

  `f_name` varchar(256) NOT NULL DEFAULT '',

  `f_createtime` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',

  `f_status` int(11) NOT NULL DEFAULT '',

  `f_rangenum` int(11) NOT NULL DEFAULT '',

  PRIMARY KEY (`f_uin`),

  KEY `f_namerangenum_index` (`f_name`(255),`f_rangenum`)

) ENGINE=InnoDB AUTO_INCREMENT=5000001 DEFAULT CHARSET=utf8

1 row in set (0.00 sec)

2）

对于组合索引

我们看到只要是查询条件中有联合索引中最左边的那个索引字段

那么都会走索引

(3个或者多个联合索引道理是一样的 )

mysql> explain select * from t_user_bak where  f_rangenum= 155465 and f_name="zhantao_155465";

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

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

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

|  1 | SIMPLE      | t_user_bak | ref  | f_namerangenum_index | f_namerangenum_index | 771     | const,const |    5 | Using where |

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

1 row in set (0.00 sec)

mysql> explain select * from t_user_bak where   f_name="zhantao_155465" and  f_rangenum= 155465;

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

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

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

|  1 | SIMPLE      | t_user_bak | ref  | f_namerangenum_index | f_namerangenum_index | 771     | const,const |    5 | Using where |

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

1 row in set (0.00 sec）

mysql> explain select * from t_user_bak where   f_name="zhantao_155465" ;

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

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

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

|  1 | SIMPLE      | t_user_bak | ref  | f_namerangenum_index | f_namerangenum_index | 767     | const |    5 | Using where |

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

1 row in set (0.00 sec)

mysql> explain select * from t_user_bak where  1=1 and f_name="zhantao_155465" ;

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

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

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

|  1 | SIMPLE      | t_user_bak | ref  | f_namerangenum_index | f_namerangenum_index | 767     | const |    5 | Using where |

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

1 row in set (0.00 sec)

mysql> explain select * from t_user_bak where  f_createtime = '2015-04-18 21:19:35' and f_name="zhantao_155465" ;

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

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

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

|  1 | SIMPLE      | t_user_bak | ref  | f_namerangenum_index | f_namerangenum_index | 767     | const |    5 | Using where |

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

1 row in set (0.00 sec)

3）各个查询条件下的速度

mysql> show create table t_user_bak\G;

*************************** 1. row ***************************

       Table: t_user_bak

Create Table: CREATE TABLE `t_user_bak` (

  `f_uin` bigint(20) NOT NULL AUTO_INCREMENT,

  `f_name` varchar(256) NOT NULL DEFAULT '',

  `f_createtime` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',

  `f_status` int(11) NOT NULL DEFAULT '',

  `f_rangenum` int(11) NOT NULL DEFAULT '',

  PRIMARY KEY (`f_uin`),

  KEY `f_namecreatetimerangenum_index` (`f_name`(255),`f_createtime`,`f_rangenum`)

) ENGINE=InnoDB AUTO_INCREMENT=5000001 DEFAULT CHARSET=utf8

1 row in set (0.00 sec)

ERROR:

No query specified

mysql> show create table t_user\G;

*************************** 1. row ***************************

       Table: t_user

Create Table: CREATE TABLE `t_user` (

  `f_uin` bigint(20) NOT NULL AUTO_INCREMENT,

  `f_name` varchar(256) NOT NULL DEFAULT '',

  `f_createtime` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',

  `f_status` int(11) NOT NULL DEFAULT '',

  `f_rangenum` int(11) NOT NULL DEFAULT '',

  PRIMARY KEY (`f_uin`),

  KEY `f_name_index` (`f_name`(255)),

  KEY `f_rangenum_index` (`f_rangenum`)

) ENGINE=InnoDB AUTO_INCREMENT=6000001 DEFAULT CHARSET=utf8

1 row in set (0.00 sec)

如下图

可以看出来如果如果是联合索引只会在最左边的那个索引字段上显示 MUL

但是是单列索引每个索引上都有 MUL

这也就可以明白为什么在使用联合索引的时候一定要有最左边的那个索引字段

mysql> desc t_user_bak;

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

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

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

| f_uin        | bigint(20)   | NO   | PRI | NULL                | auto_increment |

| f_name       | varchar(256) | NO   | MUL |                     |                |

| f_createtime | datetime     | NO   |     | 0000-00-00 00:00:00 |                |

| f_status     | int(11)      | NO   |     | 0                   |                |

| f_rangenum   | int(11)      | NO   |     | 0                   |                |

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

5 rows in set (0.00 sec)

mysql> desc t_user;

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

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

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

| f_uin        | bigint(20)   | NO   | PRI | NULL                | auto_increment |

| f_name       | varchar(256) | NO   | MUL |                     |                |

| f_createtime | datetime     | NO   |     | 0000-00-00 00:00:00 |                |

| f_status     | int(11)      | NO   |     | 0                   |                |

| f_rangenum   | int(11)      | NO   | MUL | 0                   |                |

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

5 rows in set (0.00 sec)

可以看到单列索引和组合索引查询效果都是差不多的不知道为什么

对组合索引和在各个单列上建索引的区别

mysql>  select * from t_user where  1=1 and f_name = "zhantao_155465" and f_status !=2 and f_rangenum= 155465 ;

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

| f_uin   | f_name         | f_createtime        | f_status | f_rangenum |

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

|  155466 | zhantao_155465 | 2015-04-18 21:21:11 |        1 |     155465 |

| 1155466 | zhantao_155465 | 2015-04-18 21:21:20 |        1 |     155465 |

| 2155466 | zhantao_155465 | 2015-04-18 21:21:29 |        1 |     155465 |

| 3155466 | zhantao_155465 | 2015-04-18 21:21:38 |        1 |     155465 |

| 4155466 | zhantao_155465 | 2015-04-18 21:21:48 |        1 |     155465 |

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

5 rows in set (0.03 sec)

mysql>  select * from t_user_bak where  1=1 and f_name = "zhantao_155465" and f_status !=2 and f_rangenum= 155465 ;

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

| f_uin   | f_name         | f_createtime        | f_status | f_rangenum |

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

|  155466 | zhantao_155465 | 2015-04-18 21:19:35 |        1 |     155465 |

| 1155466 | zhantao_155465 | 2015-04-18 21:19:47 |        1 |     155465 |

| 2155466 | zhantao_155465 | 2015-04-18 21:20:10 |        1 |     155465 |

| 3155466 | zhantao_155465 | 2015-04-18 21:20:20 |        1 |     155465 |

| 4155466 | zhantao_155465 | 2015-04-18 21:20:37 |        1 |     155465 |

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

5 rows in set (0.03 sec)

下面是从网上找的分析

感觉说的很有道理

给我的感觉就是联合索引不用回表

有了上面这些数据，我们整理下吧。未存在缓存下的数据。

mysql 索引相关知识

看这个表，全表扫描最慢，我们可以理解，同时主键查询比覆盖所有扫描慢也还能接受，但是为什么主键扫描会比非主键扫描慢？

而且非主键查询需要消耗的1次查询的io+一次回表的查询IO，理论上是要比主键扫描慢，而出来的数据缺不是如此。

那么就仔细看下是个查询方式在各个主要阶段消耗的时间吧。

查询是否存在缓存，打开表及锁表这些操作时间是差不多，我们不会计入。具体还是看init，optimizing等环节消耗的时间。

mysql 索引相关知识

1.从这个表中，我们看到非主键索引和覆盖索引在准备时间上需要开销很多的时间，预估这两种查询方式都需要进行回表操作，所以花在准备上更多时间。

2.第二项optimizing上，可以清晰知道，覆盖索引话在优化上大量的时间，这样在二级索引上就无需回表。

3. Sendingdata，全表扫描慢就慢在这一项上，因为是加载所有的数据页，所以花费在这块上时间较大，其他三者都差不多。

4. 非主键查询话在freeingitems上时间最少，那么可以看出它在读取数据块的时候最少。

5.相比较主键查询和非主键查询，非主键查询在Init，statistics都远高于主键查询，只是在freeingitems开销时间比主键查询少。因为这里测试数据比较少，但是我们可以预见在大数据量的查询上，不走缓存的话，那么主键查询的速度是要快于非主键查询的，本次数据不过是太小体现不出差距而已。

6.在大多数情况下，全表扫描还是要慢于索引扫描的。

tips:

过程中的辅助命令：

1.清楚缓存

reset query cache ;

flush tables;

2.查看表的索引：

show index from tablename;

mysql> set profiling=1;

Query OK, 0 rows affected (0.00 sec)

mysql> show profiles;

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

| Query_ID | Duration   | Query                                                      |

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

|        1 | 0.00665000 | select * from t_user where f_rangenum in (1,2,3,4,5,6,7,8) |

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

1 row in set (0.00 sec)

mysql> show profile for query 1;

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

| Status             | Duration |

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

| starting           | 0.000153 |

| Opening tables     | 0.000023 |

| System lock        | 0.000006 |

| Table lock         | 0.000012 |

| init               | 0.000084 |

| optimizing         | 0.000014 |

| statistics         | 0.001265 |

| preparing          | 0.000018 |

| executing          | 0.000004 |

| Sending data       | 0.003164 |

| end                | 0.000010 |

| query end          | 0.000004 |

| freeing items      | 0.001880 |

| logging slow query | 0.000007 |

| cleaning up        | 0.000006 |

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

15 rows in set (0.00 sec)

最后关于mysql where in 是否启用索引

直接上explain结果一目了然

mysql> explain select * from t_user where f_rangenum in (select f_id from t_globe_num);

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

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

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

|  1 | PRIMARY            | t_user      | ALL             | NULL          | NULL    | NULL    | NULL | 6001509 | Using where              |

|  2 | DEPENDENT SUBQUERY | t_globe_num | unique_subquery | PRIMARY       | PRIMARY | 8       | func |       1 | Using index; Using where |

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

2 rows in set (0.00 sec)

mysql> explain select * from t_user where f_rangenum in (1,2,3,4,5,6);

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

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

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

|  1 | SIMPLE      | t_user | range | f_rangenum_index | f_rangenum_index | 4       | NULL |   36 | Using where |

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

1 row in set (0.04 sec)

可以看到如果in里面是值那么会启用index

但是如果是子句则不走index是全表扫描

t_user表600万个数据

t_globe_num 表1万个数据

rows 那里显示6001509 ~~~

mysql要查600多万条数据卡死你~~~~

秒客网

mysql 索引相关知识

相关文章