mysql 的这些坑你踩过吗？快来看看怎么优化 mysql？

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发布于: 2020 年 12 月 04 日
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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)
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mysql的逻辑分层
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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）
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# 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
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mysql> show engines
    -> ;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                        | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| 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         |
| FEDERATED          | NO      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
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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
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存储层
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mysql的索引类型Mysql支持的索引类型：我们最常用的是B-TREE索引，但是mysql还有另外一种索引，就是HASH索引https://blog.csdn.net/oChangWen/article/details/54024063
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# hash
create table testhash(
fname varchar(50) not null,
lname varchar(50) not null,
key using hash(fname)) engine=memory;
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# 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
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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
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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;
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# 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
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mysql> select * from t_user where id='2424786gafafdfdsa';
+---------+------+-------+------+-------------+--------+------------+------------+
| id      | name | email | age  | telephone   | status | created_at | updated_at |
+---------+------+-------+------+-------------+--------+------------+------------+
| 2424786 | jimi | ffdsa |   11 | 15001262936 |      0 | NULL       | NULL       |
+---------+------+-------+------+-------------+--------+------------+------------+
1 row in set, 1 warning (0.00 sec)
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mysql> explain  select * from t_user where id='2426gafafdfdsa';
+----+-------------+--------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | t_user | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
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
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mysql> select * from t_user where  email=0 limit 10;
+----+------+-------+------+-------------+--------+---------------------+---------------------+
| id | name | email | age  | telephone   | status | created_at          | updated_at          |
+----+------+-------+------+-------------+--------+---------------------+---------------------+
|  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）
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mysql> select * from t_user where email='ffdsaADFG';
+---------+------+-----------+------+-------------+--------+------------+------------+
| id      | name | email     | age  | telephone   | status | created_at | updated_at |
+---------+------+-----------+------+-------------+--------+------------+------------+
| 2424786 | jimi | ffdsaADFG |   11 | 15001262936 |      0 | NULL       | NULL       |
+---------+------+-----------+------+-------------+--------+------------+------------+
1 row in set (0.00 sec)
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mysql> select * from t_user where email='ffdsaadfg';
+---------+------+-----------+------+-------------+--------+------------+------------+
| id      | name | email     | age  | telephone   | status | created_at | updated_at |
+---------+------+-----------+------+-------------+--------+------------+------------+
| 2424786 | jimi | ffdsaADFG |   11 | 15001262936 |      0 | NULL       | NULL       |
+---------+------+-----------+------+-------------+--------+------------+------------+
1 row in set (0.00 sec)
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# 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
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mysql> select * from t_user where email=123;
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;
+---------+------+--------+------+-------------+--------+------------+------------+
| id      | name | email  | age  | telephone   | status | created_at | updated_at |
+---------+------+--------+------+-------------+--------+------------+------------+
| 2424789 | jimi | 123abc |   11 | 15001262936 |      0 | NULL       | NULL       |
+---------+------+--------+------+-------------+--------+------------+------------+
1 row in set, 65535 warnings (2.57 sec)
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mysql> explain  select * from t_user where email=123;
+----+-------------+--------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+---------+----------+-------------+
|  1 | SIMPLE      | t_user | NULL       | ALL  | idx_email     | NULL | NULL    | NULL | 2090340 |    10.00 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+---------+----------+-------------+
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
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    /**
     * 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
    }
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    /**
     * 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
    }
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/**
     * 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
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        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
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        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;
+----+-------------+--------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+---------+----------+-------------+
|  1 | SIMPLE      | t_user | NULL       | ALL  | idx_email     | NULL | NULL    | NULL | 2090340 |    10.00 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+---------+----------+-------------+
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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
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#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)
);
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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);
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insert into teacher values(1,'tz',1);
insert into teacher values(2,'tw',2);
insert into teacher values(3,'tl',3);
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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)
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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 ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                              |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
|  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
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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
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mysql> explain select * from   teacherCard limit 1;
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table       | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | teacherCard | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL  |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
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mysql> explain select  cr.cname from ( select * from course where tid = 1  union select * from course where tid = 2 ) cr ;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | <derived2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |   100.00 | NULL            |
|  2 | DERIVED      | course     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |    25.00 | Using where     |
|  3 | UNION        | course     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |    25.00 | Using where     |
| NULL | UNION RESULT | <union2,3> | NULL       | ALL  | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
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type
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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）
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mysql> create table test01
    -> (
    -> tid int(3),
    -> tname varchar(20)
    -> );
Query OK, 0 rows affected (0.04 sec)
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mysql>
mysql> insert into test01 values(1,'a') ;
Query OK, 1 row affected (0.01 sec)
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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
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mysql> explain select * from (select * from test01 )t where tid =1 ;
+----+-------------+--------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | test01 | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
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）
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mysql>
mysql> alter table test01 drop primary key ;
Query OK, 1 row affected (0.08 sec)
Records: 1  Duplicates: 0  Warnings: 0
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mysql> create index test01_index on test01(tid) ;
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0
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mysql> explain select * from (select * from test01 )t where tid =1 ;
+----+-------------+--------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
| id | select_type | table  | partitions | type | possible_keys | key          | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
|  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）
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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>
mysql> explain select t.tcid from teacher t,teacherCard tc where t.tcid = tc.tcid ;
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------+------+----------+--------------------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref         | rows | filtered | Extra                    |
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------+------+----------+--------------------------+
|  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)
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mysql> alter table teacher add index tid_index (tid) ;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0
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mysql> explain select t.* from teacher t where t.tid in (1,2) ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t     | NULL       | ALL  | tid_index     | NULL | NULL    | NULL |    3 |    66.67 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
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mysql> explain select t.* from teacher t where t.tid <3 ;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
|  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
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mysql> explain select tid from teacher ;
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | teacher | NULL       | index | NULL          | tid_index | 5       | NULL |    3 |   100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
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
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mysql> explain select *  from teacher
    -> ;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | teacher | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
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。
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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' ;
+----+-------------+-------+------------+--------+-------------------+-----------+---------+-------------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys     | key       | key_len | ref         | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+-------------------+-----------+---------+-------------+------+----------+-------------+
|  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）。
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mysql> create table test_kl
    -> (
    -> name char(20) not null default ''
    -> );
Query OK, 0 rows affected (0.03 sec)
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mysql> alter table test_kl add index index_name(name) ;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0
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mysql> explain select * from test_kl where name =''
    -> ;
+----+-------------+---------+------------+------+---------------+------------+---------+-------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key        | key_len | ref   | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------------+---------+-------+------+----------+-------------+
|  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)
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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)
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# 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
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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
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mysql> explain select * from test_kl where name1 = '' ;
+----+-------------+---------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table   | partitions | type  | possible_keys | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+---------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test_kl | NULL       | index | NULL          | name_name1_index | 161     | NULL |    1 |   100.00 | Using where; Using index |
+----+-------------+---------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
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mysql> explain select * from test_kl where name = ''
    ->
    -> ;
+----+-------------+---------+------------+------+-----------------------------+------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys               | key        | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+-----------------------------+------------+---------+-------+------+----------+-------+
|  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)
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mysql> explain select * from myTest  where b=3 and c=4;
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | myTest | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 32893 |     1.00 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
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mysql> explain select * from myTest  where a=3 and c=4;
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
|  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。
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mysql> alter table course  add index tid_index (tid) ;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0
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mysql> explain select * from course c,teacher t where c.tid = t.tid  and t.tname ='tw' ;
+----+-------------+-------+------------+------+---------------+-----------+---------+------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref        | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+-----------+---------+------------+------+----------+-------------+
|  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)
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mysql> explain select * from course c,teacher t  where c.tid = t.tid
    -> and t.tname = 'tz' ;
+----+-------------+-------+------------+------+---------------+-----------+---------+------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref        | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+-----------+---------+------------+------+----------+-------------+
|  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)
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Extra：
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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。
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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)
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mysql>
mysql> explain select * from test02 where a1 ='' order by a1 ;
+----+-------------+--------+------------+------+---------------+--------+---------+-------+------+----------+-------+
| id | select_type | table  | partitions | type | possible_keys | key    | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+--------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | test02 | NULL       | ref  | idx_a1        | idx_a1 | 13      | const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+------+---------------+--------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)
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mysql> explain select * from test02 where a1 ='' order by a2 ;
+----+-------------+--------+------------+------+---------------+--------+---------+-------+------+----------+---------------------------------------+
| id | select_type | table  | partitions | type | possible_keys | key    | key_len | ref   | rows | filtered | Extra                                 |
+----+-------------+--------+------------+------+---------------+--------+---------+-------+------+----------+---------------------------------------+
|  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)
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using temporary:性能损耗大 ，用到了临时表。一般出现在group by 语句中。
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避免：查询那些列，就根据那些列 group by .
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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'
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关于数据表格式规范谈谈mysql中utf8和utf8mb4区别简介
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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。
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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 )，和很多不常用的汉字，以及任何新增的 Unicode 字符等等。
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- 我认为 合理表应该这样设计
CREATE TABLE `demo`  (
                         `id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '主键Id',
                         `uuid` varchar(12) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL COMMENT '业务id',
                         `create_time` timestamp(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6) COMMENT '创建时间',
                         `update_time` timestamp(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6) COMMENT '更新时间',
                         `status` tinyint(2) NOT NULL DEFAULT 0 COMMENT '状态 0:正常 1:下线',
                         `logical_del` tinyint(2) NOT NULL DEFAULT 0 COMMENT '逻辑删除标识',
                         PRIMARY KEY `id`(`Id`) USING BTREE,
                         INDEX `uuid`(`uuid`) USING BTREE
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) ENGINE = InnoDB  CHARSET=utf8mb4  COMMENT = 'demo';
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