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mysql 索引相关知识
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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 inset (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
3)
[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=0 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 inset (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，
我们可以通过vi编辑器来查看文件的format格式。步骤如下：
上传到Linux下执行时经常会报这种错误：
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。
[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=dos3.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 [ "${dbenv}" = "dev" ]; then mysql_user=root mysql_pass=root mysql_host= fiif [ "${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" ${mysql_cmd}<<EOF use test; drop table if exists t_user; create table t_user ( f_uin bigint(20) NOT NULL auto_increment, f_name varchar(256) NOT NULL DEFAULT ‘‘, f_createtime datetime not null default ‘0000-00-00 00:00:00‘, f_status int(11) NOT NULL DEFAULT ‘0‘, f_rangenum int(11) NOT NULL DEFAULT ‘0‘, PRIMARY KEY (f_uin) )ENGINE=InnoDB DEFAULT CHARSET=utf8; EOF echo"create table t_user ok"
2）通过python脚本来创建sql脚本
#!/usr/bin/python #-*-coding:UTF-8-*-import sys def Insert(num): if(num<=10000000): str = "insert into t_user (f_name,f_createtime,f_status,f_rangenum) values "for i in range(num): name = "zhantao_%d"%i status = 1 rangenum = i if(i==0): strsql = "(‘%s‘,now(),%d,%d)"%(name,status,rangenum) else: strsql = ",(‘%s‘,now(),%d,%d)"%(name,status,rangenum) str += strsql f = open("insert.sql","w") f.write(str) f.close if__name__ == ‘__main__‘: #num = int(sys.argv[1]) Insert(1000000)
可以看到创建出来的sql脚本 一个就33兆 如果前面没有修改sql语句最大长度，是肯定执行不了的
[ztao@localhost 0418]$ ll -alhtotal 33Mdrwxrwxr-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 oftableandcolumn 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>selectcount(*) from t_user; +----------+|count(*) |+----------+|5000000|+----------+1 row inset (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 |2015-04-1820:11:36|1|123568||1123569| zhantao_123568 |2015-04-1820:11:54|1|123568||2123569| zhantao_123568 |2015-04-1820:12:05|1|123568||3123569| zhantao_123568 |2015-04-1820:12:15|1|123568||4123569| zhantao_123568 |2015-04-1820:12:25|1|123568|+---------+----------------+---------------------+----------+------------+5 rows inset (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 inset (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>altertable t_user addindex f_rangenum_index(f_rangenum); Query OK, 5000000 rows affected (1min45.16 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 |2015-04-1820:11:36|1|123199||1123200| zhantao_123199 |2015-04-1820:11:54|1|123199||2123200| zhantao_123199 |2015-04-1820:12:05|1|123199||3123200| zhantao_123199 |2015-04-1820:12:15|1|123199||4123200| zhantao_123199 |2015-04-1820:12:25|1|123199|+---------+----------------+---------------------+----------+------------+5 rows inset (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 inset (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 |2015-04-1820:11:36|1|123199||1123200| zhantao_123599002 |2015-04-1820:11:54|1|123199||2123200| zhantao_123199 |2015-04-1820:12:05|1|123199||3123200| zhantao_123199 |2015-04-1820:12:15|1|123199||4123200| zhantao_123199 |2015-04-1820:12:25|1|123199|+---------+-------------------------+---------------------+----------+------------+5 rows inset (0.00 sec) mysql>select*from t_user where f_rangenum =123199and f_name = "zhantao_123599001123200"; +--------+-------------------------+---------------------+----------+------------+| f_uin | f_name | f_createtime | f_status | f_rangenum |+--------+-------------------------+---------------------+----------+------------+|123200| zhantao_123599001123200 |2015-04-1820:11:36|1|123199|+--------+-------------------------+---------------------+----------+------------+1 row inset (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 |2015-04-1820:11:36|1|123199|+--------+-------------------------+---------------------+----------+------------+1 row inset (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 inset (0.00 sec) mysql> explain select*from t_user where f_rangenum =123199and 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 inset (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 where1=1and 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 inset (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 inset (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 |2015-04-1820:11:36|1|123199||1123200| zhantao_123599002 |2015-04-1820:11:54|1|123199||2123200| zhantao_123199 |2015-04-1820:12:05|1|123199||3123200| zhantao_123199 |2015-04-1820:12:15|1|123199||4123200| zhantao_123199 |2015-04-1820:12:25|1|123199|+---------+-------------------------+---------------------+----------+------------+5 rows inset (0.00 sec) mysql>select*from t_user where1=1and f_rangenum =123199; +---------+-------------------------+---------------------+----------+------------+| f_uin | f_name | f_createtime | f_status | f_rangenum |+---------+-------------------------+---------------------+----------+------------+|123200| zhantao_123599001123200 |2015-04-1820:11:36|1|123199||1123200| zhantao_123599002 |2015-04-1820:11:54|1|123199||2123200| zhantao_123199 |2015-04-1820:12:05|1|123199||3123200| zhantao_123199 |2015-04-1820:12:15|1|123199||4123200| zhantao_123199 |2015-04-1820:12:25|1|123199|+---------+-------------------------+---------------------+----------+------------+5 rows inset (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>altertable t_user_bak addindex f_namerangenum_index(f_name,f_rangenum); Query OK, 5000000 rows affected, 2 warnings (2min54.18 sec) Records: 5000000 Duplicates: 0 Warnings: 0 mysql> show createtable t_user_bak\G; ***************************1. row ***************************Table: t_user_bak CreateTable: CREATETABLE `t_user_bak` ( `f_uin` bigint(20) NOTNULL AUTO_INCREMENT, `f_name` varchar(256) NOTNULLDEFAULT‘‘, `f_createtime` datetimeNOTNULLDEFAULT‘0000-00-00 00:00:00‘, `f_status` int(11) NOTNULLDEFAULT‘0‘, `f_rangenum` int(11) NOTNULLDEFAULT‘0‘, PRIMARYKEY (`f_uin`), KEY `f_namerangenum_index` (`f_name`(255),`f_rangenum`) ) ENGINE=InnoDB AUTO_INCREMENT=5000001DEFAULT CHARSET=utf8 1 row inset (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=155465and 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 inset (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 inset (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 inset (0.00 sec) mysql> explain select*from t_user_bak where1=1and 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 inset (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 inset (0.00 sec)
3） 各个查询条件下的速度
mysql> show createtable t_user_bak\G; ***************************1. row ***************************Table: t_user_bak CreateTable: CREATETABLE `t_user_bak` ( `f_uin` bigint(20) NOTNULL AUTO_INCREMENT, `f_name` varchar(256) NOTNULLDEFAULT‘‘, `f_createtime` datetimeNOTNULLDEFAULT‘0000-00-00 00:00:00‘, `f_status` int(11) NOTNULLDEFAULT‘0‘, `f_rangenum` int(11) NOTNULLDEFAULT‘0‘, PRIMARYKEY (`f_uin`), KEY `f_namecreatetimerangenum_index` (`f_name`(255),`f_createtime`,`f_rangenum`) ) ENGINE=InnoDB AUTO_INCREMENT=5000001DEFAULT CHARSET=utf8 1 row inset (0.00 sec) ERROR: No query specified mysql> show createtable t_user\G; ***************************1. row ***************************Table: t_user CreateTable: CREATETABLE `t_user` ( `f_uin` bigint(20) NOTNULL AUTO_INCREMENT, `f_name` varchar(256) NOTNULLDEFAULT‘‘, `f_createtime` datetimeNOTNULLDEFAULT‘0000-00-00 00:00:00‘, `f_status` int(11) NOTNULLDEFAULT‘0‘, `f_rangenum` int(11) NOTNULLDEFAULT‘0‘, PRIMARYKEY (`f_uin`), KEY `f_name_index` (`f_name`(255)), KEY `f_rangenum_index` (`f_rangenum`) ) ENGINE=InnoDB AUTO_INCREMENT=6000001DEFAULT CHARSET=utf8 1 row inset (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 ||0000-00-0000:00:00||| f_status |int(11) | NO ||0||| f_rangenum |int(11) | NO ||0||+--------------+--------------+------+-----+---------------------+----------------+5 rows inset (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 ||0000-00-0000:00:00||| f_status |int(11) | NO ||0||| f_rangenum |int(11) | NO | MUL |0||+--------------+--------------+------+-----+---------------------+----------------+5 rows inset (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 where1=1and f_name = "zhantao_155465" and f_status !=2and f_rangenum=155465 ; +---------+----------------+---------------------+----------+------------+| f_uin | f_name | f_createtime | f_status | f_rangenum |+---------+----------------+---------------------+----------+------------+|155466| zhantao_155465 |2015-04-1821:21:11|1|155465||1155466| zhantao_155465 |2015-04-1821:21:20|1|155465||2155466| zhantao_155465 |2015-04-1821:21:29|1|155465||3155466| zhantao_155465 |2015-04-1821:21:38|1|155465||4155466| zhantao_155465 |2015-04-1821:21:48|1|155465|+---------+----------------+---------------------+----------+------------+5 rows inset (0.03 sec) mysql>select*from t_user_bak where1=1and f_name = "zhantao_155465" and f_status !=2and f_rangenum=155465 ; +---------+----------------+---------------------+----------+------------+| f_uin | f_name | f_createtime | f_status | f_rangenum |+---------+----------------+---------------------+----------+------------+|155466| zhantao_155465 |2015-04-1821:19:35|1|155465||1155466| zhantao_155465 |2015-04-1821:19:47|1|155465||2155466| zhantao_155465 |2015-04-1821:20:10|1|155465||3155466| zhantao_155465 |2015-04-1821:20:20|1|155465||4155466| zhantao_155465 |2015-04-1821:20:37|1|155465|+---------+----------------+---------------------+----------+------------+5 rows inset (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。
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。
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 inset (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 inset (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 inset (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 inset (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~~~~
原文：http://www.cnblogs.com/ztteng/p/4438466.html
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