mysql var目录很快_mysql的这些坑你踩过吗？快来看看怎么优化mysql

什么是mysql?

如果你的回答是关系型数据库，那就会显得有些浅薄。我们平时工作中肯定会用到mysql,但是谈到mysql,就不能只说增删改查。

接下来我们从另一个角度认识一下mysql(其实不仅仅是mysql,对于任何一个产品、服务，我们都应该有一个抽象化的架构，而不能局限于这个产品的某一个区域)

mysql的逻辑分层

image.png

连接层：提供客户端的连接功能和权限认证,

服务层：

提供用户使用的接口(curd,主从配置,数据备份等)

sql优化器(mysql query optimizer)

# 联合索引 a b c

select * from table1 where a=xxx and c=xxx and b=xxx；#经过优化器优化后可以使用索引，

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引擎层 :提供存储数据的方式(innodb myisam archive ,memory,csv,federated )，Mysql在V5.1之前默认存储引擎是MyISAM；在此之后默认存储引擎是InnoDB，myisam 和innodb的区别：https://segmentfault.com/a/1190000021995700

image.png

mysql> show engines

-> ;

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

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

| InnoDB | DEFAULT | Supports transactions, row-level locking, and foreign keys | YES | YES | YES |

| MRG_MYISAM | YES | Collection of identical MyISAM tables | NO | NO | NO |

| MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO |

| BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO |

| MyISAM | YES | MyISAM storage engine | NO | NO | NO |

| CSV | YES | CSV storage engine | NO | NO | NO |

| ARCHIVE | YES | Archive storage engine | NO | NO | NO |

| PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO |

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

mysql> show variables like '%storage_engine%';

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

| Variable_name | Value |

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

| default_storage_engine | InnoDB |

| default_tmp_storage_engine | InnoDB |

| disabled_storage_engines | |

| internal_tmp_disk_storage_engine | InnoDB |

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

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TODO 具体存储引擎和相关使用场景待补充

存储层

mysql的索引类型

# hash

create table testhash(

fname varchar(50) not null,

lname varchar(50) not null,

key using hash(fname)) engine=memory;

# b-tree

CREATE TABLE t(

aid int unsigned not null auto_increment,

userid int unsigned not null default 0,

username varchar(20) not null default ‘’,

detail varchar(255) not null default ‘’,

primary key(aid),

unique key(uid) USING BTREE,

key (username(12)) USING BTREE — 此处 uname 列只创建了最左12个字符长度的部分索引

)engine=InnoDB;

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使用场景

image.png

b-tree 索引原理示意图(二叉树为例) 中序

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mysql的这些坑你踩过吗？

创建数据表，插入数据

CREATE TABLE `t_user` (

`id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '主键Id',

`name` varchar(30) DEFAULT NULL COMMENT '姓名',

`email` varchar(30) DEFAULT NULL COMMENT '邮箱',

`age` int(11) DEFAULT NULL COMMENT '年龄',

`telephone` varchar(30) DEFAULT NULL COMMENT '电话',

`status` tinyint(4) DEFAULT NULL COMMENT '0:正常 1:下线 ',

`created_at` datetime DEFAULT CURRENT_TIMESTAMP comment '创建时间',

`updated_at` datetime DEFAULT CURRENT_TIMESTAMP comment '更新时间',

PRIMARY KEY (`id`),

KEY `idx_email` (`email`),

KEY `idx_name` (`name`),

KEY `idx_telephone` (`telephone`)

) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4;

# 插入一条数据

INSERT INTO test.t_user (name, email, age, telephone, status, created_at, updated_at) VALUES ('jimi', 'ffdsa', 11, '15001262936', 0, DEFAULT, DEFAULT);

# 批量插入数据

INSERT INTO test.t_user select null, name, email, age, telephone, 0, null, null from t_user;

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字符串转数字,通过以下可以看到，主键id的类型是int,但是查询的关键字是string,这个时候就会转换

mysql> select * from t_user where id='2424786gafafdfdsa';

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

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

| 2424786 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

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

1 row in set, 1 warning (0.00 sec)

mysql> explain select * from t_user where id='2426gafafdfdsa';

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

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

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

1 row in set, 2 warnings (0.00 sec)

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字符串类型的字段0会全匹配

mysql> select * from t_user where email=0 limit 10;

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

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

| 2 | jimi | ffdsa | 11 | 15001262936 | 0 | 2020-11-27 14:33:57 | 2020-11-27 14:33:57 |

| 3 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

| 4 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

| 5 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

| 7 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

| 8 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

| 9 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

| 10 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

| 14 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

| 15 | jimi | ffdsa | 11 | 15001262936 | 0 | NULL | NULL |

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

10 rows in set, 10 warnings (0.00 sec)

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大小写敏感问题(造成线上缓存击穿，如语音模块,视频模块已控制)

mysql> select * from t_user where email='ffdsaADFG';

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

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

| 2424786 | jimi | ffdsaADFG | 11 | 15001262936 | 0 | NULL | NULL |

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

1 row in set (0.00 sec)

mysql> select * from t_user where email='ffdsaadfg';

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

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

| 2424786 | jimi | ffdsaADFG | 11 | 15001262936 | 0 | NULL | NULL |

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

1 row in set (0.00 sec)

# 解决大小写问题

#utf8_general_ci，表示不区分大小写；utf8_general_cs表示区分大小写；utf8_bin表示二进制比较，也可以比较大小写

ALTER TABLE t_user MODIFY COLUMN email VARCHAR(30) BINARY CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL;

mysql> select * from t_user where email='ffdsaadfg';

Empty set (0.00 sec)

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数字转字符串,但是这种转化是用不上索引的

mysql> select * from t_user where email=123;

;

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

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

| 2424789 | jimi | 123abc | 11 | 15001262936 | 0 | NULL | NULL |

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

1 row in set, 65535 warnings (2.57 sec)

mysql> explain select * from t_user where email=123;

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

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

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

1 row in set, 3 warnings (0.00 sec)

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作为一个phper,此处也吐槽一下php的弱类型

/**

* Notes:布尔类型转换

* User: zhangguofu

* Date: 2020/12/1

* Time: 4:35 下午

public function test1()

{

$a = 2;

$b = 3;

if ($a = 3 || $b = 6) {

$a++;

$b++;

}

echo $a . " " . $b;//1 4

}

/**

* Notes:字符串数字类型转换

* User: zhangguofu

* Date: 2020/11/26

* Time: 8:01 下午

public function test2()

{

$a = 'a';

$b = 'b';

$a++;

var_dump($a == $b);//true

}

/**

* Notes:字符串数字弱类型对比和转换

* User: zhangguofu

* Date: 2020/12/4

* Time: 3:12 下午

function test3()

{

var_dump(md5('240610708') == md5('QNKCDZO')); //true

var_dump("admin" == 0); //true

var_dump("1admin" == 1); //true

var_dump("admin1" == 1);//false

var_dump("admin1" == 0);//true

var_dump("0e123456" == "0e4456789"); //true

var_dump(0 == "a"); // 0 == 0 -> true

var_dump("1" == "01"); // 1 == 1 -> true

var_dump("10" == "1e1"); // 10 == 10 -> true

var_dump(100 == "1e2"); // 100 == 100 -> true

}

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怎么优化mysql?Explain 分析查看mysql性能

mysql> explain select * from t_user where email=123;

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

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

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

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id : 编号 select_type ：查询类型 table ：表 type ：类型 possible_keys ：预测用到的索引 key ：实际使用的索引 key_len ：实际使用索引的长度

ref :表之间的引用 rows ：通过索引查询到的数据量 Extra :额外的信息

解释：

插入数据

#课程表

create table course

(

cid int(3),

cname varchar(20),

tid int(3)

);

#教师表

create table teacher

(

tid int(3),

tname varchar(20),

tcid int(3)

);

# 教师证

create table teacherCard

(

tcid int(3),

tcdesc varchar(200)

);

insert into course values(1,'java',1);

insert into course values(2,'html',1);

insert into course values(3,'sql',2);

insert into course values(4,'web',3);

insert into teacher values(1,'tz',1);

insert into teacher values(2,'tw',2);

insert into teacher values(3,'tl',3);

insert into teacherCard values(1,'tzdesc') ;

insert into teacherCard values(2,'twdesc') ;

insert into teacherCard values(3,'tldesc') ;

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id: id值相同，从上往下顺序执行。id值不同：id值越大越优先查询 (本质：在嵌套子查询时，先查内层再查外层)

mysql> explain select tc.tcdesc from teacherCard tc,course c,teacher t where c.tid = t.tid

-> and t.tcid = tc.tcid and c.cid = 2 or tc.tcid=3 ;

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

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

| 1 | SIMPLE | tc | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | NULL |

| 1 | SIMPLE | t | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | Using where; Using join buffer (Block Nested Loop) |

| 1 | SIMPLE | c | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | Using where; Using join buffer (Block Nested Loop) |

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

3 rows in set, 1 warning (0.00 sec)

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select_type:查询类型

simple 简单子查询，不包含子查询和union primary 包含union或者子查询，最外层的部分标记为primary subquery 一般子查询中的子查询被标记为subquery，也就是位于select列表中的查询 derived 派生表——该临时表是从子查询派生出来的，位于form中的子查询 union 位于union中第二个及其以后的子查询被标记为union，第一个就被标记为primary如果是union位于from中则标记为derived union result 用来从匿名临时表里检索结果的select被标记为union result dependent union 顾名思义，首先需要满足UNION的条件，及UNION中第二个以及后面的SELECT语句，同时该语句依赖外部的查询 subquery 子查询中第一个SELECT语句 dependent subquery 和DEPENDENT UNION相对UNION一样

mysql> explain select * from teacherCard limit 1;

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

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

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

mysql> explain select cr.cname from ( select * from course where tid = 1 union select * from course where tid = 2 ) cr ;

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

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

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

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type

system: 只有一条数据的系统表；或衍生表只有一条数据的主查询 const:仅仅能查到一条数据的SQL ,用于Primary key 或unique索引 (类型与索引类型有关)

mysql> create table test01

-> (

-> tid int(3),

-> tname varchar(20)

-> );

Query OK, 0 rows affected (0.04 sec)

mysql>

mysql> insert into test01 values(1,'a') ;

Query OK, 1 row affected (0.01 sec)

mysql> alter table test01 add constraint tid_pk primary key(tid) ;

Query OK, 0 rows affected (0.15 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> explain select * from (select * from test01 )t where tid =1 ;

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

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

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

1 row in set, 1 warning (0.00 sec)

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ref：非唯一性索引，对于每个索引键的查询，返回匹配的所有行(0，多)

mysql>

mysql> alter table test01 drop primary key ;

Query OK, 1 row affected (0.08 sec)

Records: 1 Duplicates: 0 Warnings: 0

mysql> create index test01_index on test01(tid) ;

Query OK, 0 rows affected (0.04 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> explain select * from (select * from test01 )t where tid =1 ;

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

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

| 1 | SIMPLE | test01 | NULL | ref | test01_index | test01_index | 4 | const | 1 | 100.00 | NULL |

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

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eq_ref:唯一性索引：对于每个索引键的查询，返回匹配唯一行数据(有且只有1个，不能多、不能0)

mysql> alter table teacherCard add constraint pk_tcid primary key(tcid);

ERROR 1068 (42000): Multiple primary key defined

mysql> alter table teacher add constraint uk_tcid unique index(tcid) ;

ERROR 1061 (42000): Duplicate key name 'uk_tcid'

mysql>

mysql> explain select t.tcid from teacher t,teacherCard tc where t.tcid = tc.tcid ;

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

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

| 1 | SIMPLE | t | NULL | index | uk_tcid | uk_tcid | 5 | NULL | 3 | 100.00 | Using where; Using index |

| 1 | SIMPLE | tc | NULL | eq_ref | PRIMARY | PRIMARY | 4 | test.t.tcid | 1 | 100.00 | Using index |

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

2 rows in set, 1 warning (0.00 sec)

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range：检索指定范围的行 ,where后面是一个范围查询(between ,> < >=, 特殊:in有时候会失效，从而转为无索引all--5.7以前的版本)

mysql> alter table teacher add index tid_index (tid) ;

Query OK, 0 rows affected (0.02 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> explain select t.* from teacher t where t.tid in (1,2) ;

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

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

| 1 | SIMPLE | t | NULL | ALL | tid_index | NULL | NULL | NULL | 3 | 66.67 | Using where |

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

1 row in set, 1 warning (0.00 sec)

mysql> explain select t.* from teacher t where t.tid <3 ;

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

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

| 1 | SIMPLE | t | NULL | range | tid_index | tid_index | 5 | NULL | 2 | 100.00 | Using index condition |

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

1 row in set, 1 warning (0.00 sec)

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index：查询全部索引中数据,不需要回表查找，黄金索引

mysql> explain select tid from teacher ;

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

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

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

1 row in set, 1 warning (0.00 sec)

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all：查询全部表中的数据，全表扫描

mysql> explain select * from teacher

-> ;

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

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

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

1 row in set, 1 warning (0.00 sec)

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possible_keys ：可能用到的索引，是一种预测，不准。

mysql> explain select tc.tcdesc from teacherCard tc,course c,teacher t where c.tid = t.tid

-> and t.tcid = tc.tcid and c.cname = 'sql' ;

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

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

| 1 | SIMPLE | c | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where |

| 1 | SIMPLE | t | NULL | ref | uk_tcid,tid_index | tid_index | 5 | test.c.tid | 1 | 100.00 | Using where |

| 1 | SIMPLE | tc | NULL | eq_ref | PRIMARY | PRIMARY | 4 | test.t.tcid | 1 | 100.00 | NULL |

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

3 rows in set, 1 warning (0.00 sec)

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key ：实际使用到的索引

key_len ：索引的长度 ; 作用：用于判断复合索引是否被完全使用 (a,b,c)。

mysql> create table test_kl

-> (

-> name char(20) not null default ''

-> );

Query OK, 0 rows affected (0.03 sec)

mysql> alter table test_kl add index index_name(name) ;

Query OK, 0 rows affected (0.03 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> explain select * from test_kl where name =''

-> ;

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

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

| 1 | SIMPLE | test_kl | NULL | ref | index_name | index_name | 80 | const | 1 | 100.00 | Using index |

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

1 row in set, 1 warning (0.00 sec)

mysql> show variables like '%char%';

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

| Variable_name | Value |

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

| character_set_client | latin1 |

| character_set_connection | latin1 |

| character_set_database | utf8mb4 |

| character_set_filesystem | binary |

| character_set_results | latin1 |

| character_set_server | utf8mb4 |

| character_set_system | utf8 |

| character_sets_dir | /usr/local/mysql/share/charsets/ |

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

8 rows in set (0.06 sec)

# 字符集utf8mb4 char 20 就是 80，如果有null 则null 占一个字节，如果是varchar 则需要1-2个字节存储值的长度

mysql> alter table test_kl add column name1 char(20) ;

Query OK, 0 rows affected (0.06 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> alter table test_kl add index name_name1_index (name,name1) ;

Query OK, 0 rows affected (0.02 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> explain select * from test_kl where name1 = '' ;

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

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

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

1 row in set, 1 warning (0.00 sec)

mysql> explain select * from test_kl where name = ''

-> ;

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

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

| 1 | SIMPLE | test_kl | NULL | ref | index_name,name_name1_index | index_name | 80 | const | 1 | 100.00 | NULL |

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

1 row in set, 1 warning (0.00 sec)

mysql> explain select * from myTest where b=3 and c=4;

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

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

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

1 row in set, 1 warning (0.00 sec)

mysql> explain select * from myTest where a=3 and c=4;

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

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

| 1 | SIMPLE | myTest | NULL | ref | a | a | 5 | const | 1 | 10.00 | Using index condition |

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

1 row in set, 1 warning (0.01 sec)

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ref 指明当前表所参照的字段。

mysql> alter table course add index tid_index (tid) ;

Query OK, 0 rows affected (0.03 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> explain select * from course c,teacher t where c.tid = t.tid and t.tname ='tw' ;

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

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

| 1 | SIMPLE | t | NULL | ALL | tid_index | NULL | NULL | NULL | 3 | 33.33 | Using where |

| 1 | SIMPLE | c | NULL | ref | tid_index | tid_index | 5 | test.t.tid | 1 | 100.00 | NULL |

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

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rows: 被索引优化查询的数据个数 (实际通过索引而查询到的数据个数)

mysql> explain select * from course c,teacher t where c.tid = t.tid

-> and t.tname = 'tz' ;

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

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

| 1 | SIMPLE | t | NULL | ALL | tid_index | NULL | NULL | NULL | 3 | 33.33 | Using where |

| 1 | SIMPLE | c | NULL | ref | tid_index | tid_index | 5 | test.t.tid | 1 | 100.00 | NULL |

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

2 rows in set, 1 warning (0.00 sec)

复制代码

Extra：

using filesort ：性能消耗大；需要“额外”的一次排序(查询) 。常见于 order by 语句中。对于单索引，如果排序和查找是同一个字段，则不会出现using filesort；如果排序和查找不是同一个字段，则会出现using filesort；怎么避免： where哪些字段，就order by那些字段 where和order by 按照复合索引的顺序使用，不要跨列或无序使用。

mysql> create table test02

-> (

-> a1 char(3),

-> a2 char(3),

-> a3 char(3),

-> index idx_a1(a1),

-> index idx_a2(a2),

-> index idx_a3(a3)

-> );

Query OK, 0 rows affected (0.03 sec)

mysql>

mysql> explain select * from test02 where a1 ='' order by a1 ;

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

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

| 1 | SIMPLE | test02 | NULL | ref | idx_a1 | idx_a1 | 13 | const | 1 | 100.00 | NULL |

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

1 row in set, 1 warning (0.01 sec)

mysql> explain select * from test02 where a1 ='' order by a2 ;

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

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

| 1 | SIMPLE | test02 | NULL | ref | idx_a1 | idx_a1 | 13 | const | 1 | 100.00 | Using index condition; Using filesort |

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

1 row in set, 1 warning (0.00 sec)

复制代码

using temporary:性能损耗大，用到了临时表。一般出现在group by 语句中。

避免：查询那些列，就根据那些列 group by .

using index :性能提升; 索引覆盖(覆盖索引)。原因：不读取原文件，只从索引文件中获取数据 (不需要回表查询),只要使用到的列全部都在索引中，就是索引覆盖using index

using where (需要回表查询)

impossible where ： where子句永远为false select * from test02 where a1='x' and a1='y'

关于数据表格式规范

谈谈mysql中utf8和utf8mb4区别

简介

MySQL在5.5.3之后增加了这个utf8mb4的编码，mb4就是most bytes 4的意思，专门用来兼容四字节的unicode。好在utf8mb4是utf8的超集，除了将编码改为utf8mb4外不需要做其他转换。当然，为了节省空间，一般情况下使用utf8也就够了。

那上面说了既然utf8能够存下大部分中文汉字,那为什么还要使用utf8mb4呢? 原来mysql支持的 utf8 编码最大字符长度为 3 字节，如果遇到 4 字节的宽字符就会插入异常了。三个字节的 UTF-8 最大能编码的 Unicode 字符是 0xffff，也就是 Unicode 中的基本多文种平面(BMP)。也就是说，任何不在基本多文本平面的 Unicode字符，都无法使用 Mysql 的 utf8 字符集存储。包括 Emoji 表情(Emoji 是一种特殊的 Unicode 编码，常见于 ios 和 android 手机上

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