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Mysql学习笔记

进击地小白 人气:0

MyISAM和InnoDB

对比 MyISAM InnoDB
主外键 不支持 支持
事务 不支持 支持
行表锁 表锁,操作时即使操作一条记录也会锁住一整张表,不适合高并发的操作 行锁,操作时只锁住某一行,不会影响到其他行,适合高并发
缓存 只缓存索引,不缓存其他数据 缓存索引和真实数据,对内存要求较高,而且内存大小对性能有影响
表空间
关注点 性能 事务
默认安装 Y Y

性能下降SQL慢的原因:

Mysql执行顺序

SQLJoin

a表

 mysql> select * from tbl_dept;
 +----+----------+--------+
 | id | deptName | locAdd |          
 +----+----------+--------+
 |  1 | RD       | 11     |
 |  2 | HR       | 12     |
 |  3 | MK       | 13     |
 |  4 | MIS      | 14     |
 |  5 | FD       | 15     |
 +----+----------+--------+
 5 rows in set (0.00 sec)

b表

 +----+------+--------+
 | id | name | deptId |
 +----+------+--------+
 |  1 | z3   |      1 |
 |  2 | z4   |      1 |
 |  3 | z5   |      1 |
 |  4 | w5   |      2 |
 |  5 | w6   |      2 |
 |  6 | s7   |      3 |
 |  7 | s8   |      4 |
 |  8 | s9   |     51 |
 +----+------+--------+
8 rows in set (0.00 sec)

mysql不支持全连接

使用以下方式可以实现全连接

 mysql> select * from tbl_dept a right join tbl_emp b on a.id=b.deptId
     -> union
     -> select * from tbl_dept a left join tbl_emp b on  a.id=b.deptId;
 +------+----------+--------+------+------+--------+
 | id   | deptName | locAdd | id   | name | deptId |
 +------+----------+--------+------+------+--------+
 |    1 | RD       | 11     |    1 | z3   |      1 |
 |    1 | RD       | 11     |    2 | z4   |      1 |
 |    1 | RD       | 11     |    3 | z5   |      1 |
 |    2 | HR       | 12     |    4 | w5   |      2 |
 |    2 | HR       | 12     |    5 | w6   |      2 |
 |    3 | MK       | 13     |    6 | s7   |      3 |
 |    4 | MIS      | 14     |    7 | s8   |      4 |
 | NULL | NULL     | NULL   |    8 | s9   |     51 |
 |    5 | FD       | 15     | NULL | NULL |   NULL |
 +------+----------+--------+------+------+--------+
 9 rows in set (0.00 sec)

a的独有和b的独有

 mysql> select * from tbl_dept a left join tbl_emp b on  a.id=b.deptId where b.id is null
     -> union
     -> select * from tbl_dept a right join tbl_emp b on  a.id=b.deptId where a.id is null;
 +------+----------+--------+------+------+--------+
 | id   | deptName | locAdd | id   | name | deptId |
 +------+----------+--------+------+------+--------+
 |    5 | FD       | 15     | NULL | NULL |   NULL |
 | NULL | NULL     | NULL   |    8 | s9   |     51 |
 +------+----------+--------+------+------+--------+
 2 rows in set (0.01 sec)

索引

索引的定义:

索引是帮助SQL高效获取数据的数据结构,索引的本质:数据结构

可以简单的理解为:排好序的快速查找数据结构

在数据之外,数据库系统还维护着满足特定查找算法的数据结构,这些数据结构以某种方式(引用)指向数据,这样就可以在这些数据结构上实现高级查找算法。这种数据结构,就是索引,下图就是一种示例:

一般来说索引也很大,因此索引往往以索引文件的方式存储在磁盘上

我们平常所说的索引,如果没有特别指明,一般都是指B树(多路搜索树,不一定是二叉的)结构组织的索引,

其中聚集索引,次要索引,复合索引,前缀索引,唯一索引默认都是使用B+树索引,统称索引,当然除了B+树这种类型的索引之外,还有哈希索引。

索引的优劣

1.优势

类似大学图书馆图书编号建索引,提高了数据检索的效率,降低数据库的IO成本

通过索引对数据进行排序,降低数据排序的成本,降低了CPU的消耗

2.劣势

实际上索引也是一张表,该表保存了主键与存在索引的字段,并指向实体表的记录,所以索引列也是占用空间的

虽然索引大大提高了查询速度,但是会降低更新表的速度,比如 update,insert,delete操作,因为更新表时,MySQL不仅要数据也要保存索引文件每次更新添加了索引的字段,都会调整因为更新所带来的键值变化后的索引信息

索引只是提高效率的一个因素,在一个大数据量的表上,需要建立最为优秀的索引或者写优秀的查询语句,而不是加了索引就能提高效率

索引分类

创建

create [unique] index indexName on mytable(cloumnname(length));
alter mytable add [unique] index [indexName] on (columnname(length));

删除

drop index [indexName] on mytable

查看

show index from table_name\G

有四种方式来添加数据表的索引

mysql索引结构

  1. BTree索引
  2. Hash索引
  3. full-text全文索引
  4. R-Tree

那些情况建索引

  1. 主键自动建立唯一索引
  2. 频繁作为查询条件的字段应该创建索引
  3. 查询中与其他表相关联的字段,外键关系建立索引
  4. 频繁更新的字段不适合创建索引,因为每次更新不单单更新了记录还更新了索引
  5. where条件里用不到的字段不要创建索引
  6. 单键/组合索引的选择问题 who?(高并发下建议组合索引)
  7. 查询中排序的字段,排序字段若通过索引去访问将大大提高排序速度
  8. 查询中统计或分组字段

哪些情况不要建索引

  1. 表记录少
  2. 经常操作dml语句的表
  3. 数据重复且平均分布的表字段,因此只为最经常查询和最经常排序的数据列建立索引,注意,如果某个数据列包含许多重复的内容,为它建立索引就没有太大的实际效果

性能分析

explian重点

能干什么

  1. 表的读取顺序
  2. 数据读取操作的操作类型
  3. 哪些索引可以被使用
  4. 哪些索引被实际使用
  5. 表之间的引用
  6. 每张表有多少行被优化器查询

id三种情况

  1. id 相同,执行顺序由上至下
  2. id不同,如果是子查询,id序号递增,id越大优先级越高
  3. id相同不同 ,同时存在

select_type

  1. SIMPLE 简单查询
  2. PRIMARY 主查询 (最外层的查询)
  3. SUBQUERY 子查询
  4. DERIUED 某个查询的子查询的临时表
  5. UNION 联合查询
  6. UNION RESULT 联合查询结果

type::

type显示的是访问类型排列,是较为重要的一个指标

从最好到最差依次是:

system > const > eq_ref> ref > range > index > ALL;

一般来说,得保证查询至少达到range级别,最好ref

----------------------------------------------type类型-------------------------------------------------------

  1. system:表只有一行记录(等于系统表) 这是const类型的特列 一般不会出现,可忽略不计
  2. const:表示通过索引一次就查询到了,const用来比较primary key或者unique索引。因为只匹配一行数据,所以很快,如将主键置于where列表中,Mysql就能将该查询转换为一个常量
  3. eq_ref:唯一性索引扫描,表中只有一条记录与之匹配,常用于主键或唯一索引扫描(两个表是多对一或者一对一的关系,被连接的表是一的情况下,他的查询是eq_ref)
  4. ref:非唯一性索引扫描,返回匹配某个单独值的所有行,本质上也是一种索引访问,它返回匹配某个单独值的所有行,然而他可能会找到多个复合条件的行,属于查找和扫描的结合体
  5. range:只检索给定范围的行,使用一个索引来选择行,key列显示使用了哪个索引,一般where语句里出现了betweent,<,>,in等的查询,这种范围扫描索引比全表扫描好
  6. index:index与ALL的区别,index只遍历索引树,索引文件通常比数据文件小
  7. ALL:全表扫描

----------------------------------------------type类型-------------------------------------------------------

key_len长度:13是因为char(4)*utf8(3)+允许为null(1)=13

没建立索引时查询t1 t2表 t1表对应t2表的id t2表 col1的值要为'ac'

对于Id这个字段t1表对t2表相当于 一对多

t1表的type为 eq_ref代表唯一性索引扫描,表中只有一条记录与之匹配,t2表对应t1的这个id对应的col值只有一个,根据t2表的主键id索引查询,t1表读取了一行,t2表读取了640行

建立索引后

t1读取一行,t2读取142行,ref非唯一性索引扫描,返回匹配某个单独值的所有行,返回t2对应id的col所有行,而t1对应id的col只有一行,所以type为eq_ref

Extra

包含不适合在其他列展现但十分重要的信息

\G :竖直显示排序

案例

索引优化

单表优化

 CREATE TABLE IF NOT EXISTS `article`(
 ​
 `id` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
 `author_id` INT (10) UNSIGNED NOT NULL,
 `category_id` INT(10) UNSIGNED NOT NULL , 
 `views` INT(10) UNSIGNED NOT NULL , 
 `comments` INT(10) UNSIGNED NOT NULL,
 `title` VARBINARY(255) NOT NULL,
 `content` TEXT NOT NULL
 );
 INSERT INTO `article`(`author_id`,`category_id` ,`views` ,`comments` ,`title` ,`content` )VALUES
 (1,1,1,1,'1','1'),
 (2,2,2,2,'2','2'),
 (1,1,3,3,'3','3');
 ​
 SELECT * FROM ARTICLE;
 mysql>  select id,author_id from article where category_id = 1 and comments > 1 order by views desc limit 1;
 +----+-----------+
 | id | author_id |
 +----+-----------+
 |  3 |         1 |
 +----+-----------+
 1 row in set (0.00 sec)
 ​
 mysql> explain select author_id from article where category_id = 1 and comments > 1 order by views desc li
 imit 1;
 +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+
 | id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                       |
 +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+
 |  1 | SIMPLE      | article | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where; Using filesort |
 +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+
 1 row in set, 1 warning (0.00 sec)

可以看出虽然查询出来了 但是 type是all,Extra里面出现了using filesort证明查询效率很低

需要优化

建立索引

create index idx_article_ccv on article(category_id,comments,views);

查询

 mysql> explain select author_id from article where category_id = 1 and comments > 1 order by views desc limit 1;
 +----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+---------------------------------------+
 | id | select_type | table   | partitions | type  | possible_keys   | key             | key_len | ref  | rows | filtered | Extra                                 |
 +----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+---------------------------------------+
 |  1 | SIMPLE      | article | NULL       | range | inx_article_ccv | inx_article_ccv | 8       | NULL |    1 |   100.00 | Using index condition; Using filesort |
 +----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+---------------------------------------+
 1 row in set, 1 warning (0.00 sec)

这里发现type 变为了 range 查询全表变为了 范围查询 优化了一点

但是 extra 仍然 有 using filesort 证明 索引优化并不成功

所以我们删除索引

drop index idx_article_ccv on article;

建立新的索引,排除掉range

create index idx_article_cv on article(category_id,views);
 mysql> explain select author_id from article where category_id = 1 and comments > 1 order by views desc limit 1;
 +----+-------------+---------+------------+------+----------------+----------------+---------+-------+------+----------+-------------+
 | id | select_type | table   | partitions | type | possible_keys  | key            | key_len | ref   | rows | filtered | Extra       |
 +----+-------------+---------+------------+------+----------------+----------------+---------+-------+------+----------+-------------+
 |  1 | SIMPLE      | article | NULL       | ref  | idx_article_cv | idx_article_cv | 4       | const |    2 |    33.33 | Using where |
 +----+-------------+---------+------------+------+----------------+----------------+---------+-------+------+----------+-------------+
 1 row in set, 1 warning (0.00 sec)
这时候会发现 优化成功 type 变为了ref extra变为了 using where

在这次实验中我又加入了一次试验 发现当建立索引时comments放在最后也是可行的

 mysql> create index idx_article_cvc on article(category_id,views,comments);
 Query OK, 0 rows affected (0.02 sec)
 Records: 0  Duplicates: 0  Warnings: 0
 ​
 mysql> explain select author_id from article where category_id = 1 and comments > 1 order by views desc limit 1;
 +----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------------+
 | id | select_type | table   | partitions | type | possible_keys   | key             | key_len | ref   | rows | filtered | Extra       |
 +----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------------+
 |  1 | SIMPLE      | article | NULL       | ref  | idx_article_cvc | idx_article_cvc | 4       | const |    2 |    33.33 | Using where |
 +----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------------+
 1 row in set, 1 warning (0.00 sec)

这时候会发现 优化成功 type 变为了ref extra变为了 using where

在这次实验中我又加入了一次试验 发现当建立索引时comments放在最后也是可行的

这里发现了 type仍然是ref,extra也是usingwhere,而只是把索引建立的位置换了一换,把范围查询的字段挪到了最后!!!!

双表优化

 CREATE TABLE IF NOT EXISTS `class`(
 `id` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
 `card` INT (10) UNSIGNED NOT NULL
 );
 CREATE TABLE IF NOT EXISTS `book`(
 `bookid` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
 `card` INT (10) UNSIGNED NOT NULL
 );
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO class(card)VALUES(FLOOR(1+(RAND()*20)));
  
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO book(card)VALUES(FLOOR(1+(RAND()*20)));
 ​
 mysql> create index Y on book(card);
  explain select * from book left join class on book.card=class.card;
 +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+----------------------------------------------------+
 | id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                              |
 +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+----------------------------------------------------+
 |  1 | SIMPLE      | book  | NULL       | index | NULL          | Y    | 4       | NULL |   20 |   100.00 | Using index                                        |
 |  1 | SIMPLE      | class | NULL       | ALL   | NULL          | NULL | NULL    | NULL |   20 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
 +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+----------------------------------------------------+
 2 rows in set, 1 warning (0.00 sec)

会发现并无多大区别 还是全表查询 这是因为俩表查询左连接把左表必须全查询 这时候只有对右表建立索引才有用

相反的右链接必须对左表建立索引才有用

对右表建立索引

 create index Y on class;
 explain select * from book left join class on book.card=class.card;
 +----+-------------+-------+------------+-------+---------------+------+---------+----------------+------+----------+-------------+
 | id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref            | rows | filtered | Extra       |
 +----+-------------+-------+------------+-------+---------------+------+---------+----------------+------+----------+-------------+
 |  1 | SIMPLE      | book  | NULL       | index | NULL          | Y    | 4       | NULL           |   20 |   100.00 | Using index |
 |  1 | SIMPLE      | class | NULL       | ref   | Y             | Y    | 4       | db01.book.card |    1 |   100.00 | Using index |
 +----+-------------+-------+------------+-------+---------------+------+---------+----------------+------+----------+-------------+
 2 rows in set, 1 warning (0.00 sec)

会发现 右表只查询了一次。。type为ref

三表优化

 CREATE TABLE IF NOT EXISTS `phone`(
 `phoneid` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
 `card` INT (10) UNSIGNED NOT NULL
 )ENGINE = INNODB;
 ​
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
 INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));

先删除所有索引

 drop index Y on book;
 drop index Y on class;
 explain select * from class left join book on class.card=book.card left join phone on book.card=phone.card;
 +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
 | id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                              |
 +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
 |  1 | SIMPLE      | class | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   20 |   100.00 | NULL                                               |
 |  1 | SIMPLE      | book  | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   20 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
 |  1 | SIMPLE      | phone | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   20 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
 +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
 3 rows in set, 1 warning (0.00 sec)

建立索引

 create index y on book(card);
 ​
 create index z on phone(card);
 explain select * from class left join book on class.card=book.card left join phone on book.card=phone.card;
 +----+-------------+-------+------------+------+---------------+------+---------+-----------------+------+----------+-------------+
 | id | select_type | table | partitions | type | possible_keys | key  | key_len | ref             | rows | filtered | Extra       |
 +----+-------------+-------+------------+------+---------------+------+---------+-----------------+------+----------+-------------+
 |  1 | SIMPLE      | class | NULL       | ALL  | NULL          | NULL | NULL    | NULL            |   20 |   100.00 | NULL        |
 |  1 | SIMPLE      | book  | NULL       | ref  | y             | y    | 4       | db01.class.card |    1 |   100.00 | Using index |
 |  1 | SIMPLE      | phone | NULL       | ref  | z             | z    | 4       | db01.book.card  |    1 |   100.00 | Using index |
 +----+-------------+-------+------------+------+---------------+------+---------+-----------------+------+----------+-------------+
 3 rows in set, 1 warning (0.00 sec)

会发现索引建立的非常成功。。 但是left join 最左表必须全部查询 建立索引

 create index x on class(card);
 explain select * from class left join book on class.card=book.card left join phone on book.card=phone.card;
 +----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
 | id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref             | rows | filtered | Extra       |
 +----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
 |  1 | SIMPLE      | class | NULL       | index | NULL          | x    | 4       | NULL            |   20 |   100.00 | Using index |
 |  1 | SIMPLE      | book  | NULL       | ref   | y             | y    | 4       | db01.class.card |    1 |   100.00 | Using index |
 |  1 | SIMPLE      | phone | NULL       | ref   | z             | z    | 4       | db01.book.card  |    1 |   100.00 | Using index |
 +----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
 3 rows in set, 1 warning (0.00 sec)

结果仍然一样

建立表

 CREATE TABLE staffs(
 id INT PRIMARY KEY AUTO_INCREMENT,
 `name` VARCHAR(24)NOT NULL DEFAULT'' COMMENT'姓名',
 `age` INT NOT NULL DEFAULT 0 COMMENT'年龄',
 `pos` VARCHAR(20) NOT NULL DEFAULT'' COMMENT'职位',
 `add_time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT'入职时间'
 )CHARSET utf8 COMMENT'员工记录表';
 INSERT INTO staffs(`name`,`age`,`pos`,`add_time`) VALUES('z3',22,'manager',NOW());
 INSERT INTO staffs(`name`,`age`,`pos`,`add_time`) VALUES('July',23,'dev',NOW());
 INSERT INTO staffs(`name`,`age`,`pos`,`add_time`) VALUES('2000',23,'dev',NOW());
建立索引

 ALTER TABLE staffs ADD INDEX index_staffs_nameAgePos(`name`,`age`,`pos`);

索引口诀

可以从上图看出 跳过name的都用不了索引

 mysql> explain select * from staffs where name='july';
 +----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-------+
 | id | select_type | table  | partitions | type | possible_keys           | key                     | key_len | ref   | rows | filtered | Extra |
 +----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-------+
 |  1 | SIMPLE      | staffs | NULL       | ref  | index_staffs_nameAgePos | index_staffs_nameAgePos | 74      | const |    1 |   100.00 | NULL  |
 +----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-------+
 1 row in set, 1 warning (0.00 sec)
 ​
 mysql> explain select * from staffs where name='july' and pos='dev';
 +----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-----------------------+
 | id | select_type | table  | partitions | type | possible_keys           | key                     | key_len | ref   | rows | filtered | Extra                 |
 +----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-----------------------+
 |  1 | SIMPLE      | staffs | NULL       | ref  | index_staffs_nameAgePos | index_staffs_nameAgePos | 74      | const |    1 |    33.33 | Using index condition |
 +----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-----------------------+
 1 row in set, 1 warning (0.00 sec)

可以从语句中看出跳过中间的索引后 key_len 不变 证明第索引pos没有被用到

索引案例

假设index(a,b,c)

Y代表索引全部使用了 N全没使用

where语句 索引是否被使用
where a=3 and c=5 (中间b断掉了)使用了a 没使用c
where a=3 and b=4 andc=5 Y
where a=3 and c=5 and b=4 Y这里mysql自动做了优化对语句排序
where a=3 and b>4 and c=5 a,b被使用
where a=3 and b like 'k%' and c=5 Y like后面常量开头索引全用
where b=3 and c=4 N
where a=3 and c>5 and b=4 Y:mysql自动做了优化对语句排序 范围c之后索引才会失效
where b=3 and c=4 and a=2 Y :mysql自动做了优化对语句排序
where c=5 and b=4 and a=3 Y :mysql自动做了优化对语句排序

假设index(a,b,c, d)

 create table test03(
 id int primary key not null auto_increment,
 a int(10),
 b int(10),
 c int(10),
 d int(10),
 ​
 insert into test03(a,b,c,d) values (3,4,5,6);
 insert into test03(a,b,c,d)  values (3,4,5,6);
 insert into test03(a,b,c,d)  values (3,4,5,6);
 insert into test03(a,b,c,d)  values (3,4,5,6);
 ​
 create index idx_test03_abcd on test03(a,b,c,d);

###

where a=3 and b>4 and c=5 使用了a和b ,b后面的索引全失效
where a=3 and b=4 and d=6 order by c 使用了a和b,c其实也用了但是是用在排序,没有统计到mysql中
where a=3 and b=4 order by c 使用了a和b,c其实也用了但是是用在排序,没有统计到mysql中
where a=3 and b=4 order by d 使用了a和b, 这里跳过c 会导致using filesort
where a=3 and d=6 order by b ,c 使用了a, 排序用到了b,c索引
where a=3 and d=6 order by c ,b 使用了 a,会产生using filesort,因为跳过了b对c进行排序
where a=3 and b=4 order by b ,c Y 全使用
where a=3 and b=4 and d&##61;6 order by c , b 使用了a,b,不会产生using filesort 因为在对c,b排序前对b进行了查询,查询时b已经确定了(常量),这样就没有跳过b对c进行排序了,而是相当于直接对c排序 相当于第三格的查询语句

group by 更严重group by先分组再排序 把order by换为 group by 甚至会产生using temporary,与order by差不多,但是更严重 而且与group by产生的效果差不多就不做演示了

Order By 索引优化

orderBy 条件 Extra
where a>4 order by a using where using index
where a>4 order by a,b using where using index
where a>4 order by b using where, using index ,using filesort(order by 后面带头大哥不在)
where a>4 order by b,a using where, using index ,using filesort(order by 后面顺序)
where a=const order by b,c 如果where使用索引的最左前缀定义为常量,则order by能使用索引
where a=const and b=const order by c where使用索引的最左前缀定义为常量,则order by能使用索引
where a=const and b>3 order by b c using where using index
order by a asc, b desc ,c desc 排序不一致 升降机

exsites

 select a.* from A a where exists(select 1 from B b where a.id=b.id)
 以上查询使用了exists语句,exists()会执行A.length次,它并不缓存exists()结果集,因为exists()结果集的内容并不重要,重要的是结果集中是否有记录,如果有则返回true,没有则返回false. 它的查询过程类似于以下过程
 ​
 List resultSet=[]; Array A=(select * from A)
 for(int  i=0;i<A.length;i++) {   if(exists(A[i].id) {  //执行select 1 from B b where b.id=a.id是否有记录返回     resultSet.add(A[i]);   } } return  resultSet;
 ​
 当B表比A表数据大时适合使用exists(),因为它没有那么遍历操作,只需要再执行一次查询就行.  如:A表有10000条记录,B表有1000000条记录,那么exists()会执行10000次去判断A表中的id是否与B表中的id相等.  如:A表有10000条记录,B表有100000000条记录,那么exists()还是执行10000次,因为它只执行A.length次,可见B表数据越多,越适合exists()发挥效果.  再如:A表有10000条记录,B表有100条记录,那么exists()还是执行10000次,还不如使用in()遍历10000*100次,因为in()是在内存里遍历比较,而exists()需要查询数据库,我们都知道查询数据库所消耗的性能更高,而内存比较很快.

Mysql慢查询日志命令

show VARIABLES like '%slow_query_log%';

显示是否开启mysql慢查询日志

set global slow_query_log=0;

关闭mysql慢查询日志

set global slow_query_log=1;

开启mysql慢查询日志

show VARIABLES like '%long_query_time%';

显示超过多长时间即为 慢查询

set global long_quert_time=10;

修改慢查询时间为10秒,当查询语句时间超过10秒即为慢查询

show global status like '%Slow_queries%';

显示一共有几条慢查询语句

[root@iZ0jlh1zn42cgftmrf6p6sZ data]# cat mysql-slow.log

linux查询慢sql

函数操作批量插入数据

 CREATE TABLE dept(
    id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    deptno MEDIUMINT UNSIGNED NOT NULL DEFAULT 0,
    dname VARCHAR(20) NOT NULL DEFAULT '',
    loc VARCHAR(13) NOT NULL DEFAULT ''
 )ENGINE=INNODB DEFAULT CHARSET=GBK;
 CREATE TABLE emp(
    id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    empno MEDIUMINT UNSIGNED NOT NULL DEFAULT 0, #编号
    enname VARCHAR(20) NOT NULL DEFAULT '', #名字
    job VARCHAR(9) NOT NULL DEFAULT '', #工作
    mgr MEDIUMINT UNSIGNED NOT NULL DEFAULT 0, #上级编号
    hiredate DATE NOT NULL, #入职时间
    sal DECIMAL(7,2) NOT NULL, #薪水
    comm DECIMAL(7,2) NOT NULL, #红利
    deptno MEDIUMINT UNSIGNED NOT NULL DEFAULT 0 #部门编号
 )ENGINE=INNODB DEFAULT CHARSET=GBK;

 show variables like 'log_bin_trust_function_creators';
 set global log_bin_trust_function_creators=1;

创建函数:随机产生部门编号 随机产生字符串

DELIMITER $$是因为sql都是;进行结尾但是创建函数过程要多次使用;所以改变sql执行结束的条件为输入$$,相当于代替了分号' ;'

//定义函数1

 DELIMITER $$
 CREATE FUNCTION rand_string(n INT) RETURNS VARCHAR(255)
 BEGIN
    DECLARE chars_set VARCHAR(100) DEFAULT 'abcdefghigklmnopqrstuvwxyzABCDEFGHIGKLMNOPQRSTUVWXYZ';
    DECLARE return_str VARCHAR(255) DEFAULT '';
    DECLARE i INT DEFAULT 0;
    WHILE i < n DO
       SET return_str = CONCAT(return_str,SUBSTRING(chars_set,FLOOR(1 + RAND()*52),1));
       SET i = i + 1;
    END WHILE;
    RETURN return_str;
 END   $$
//定义函数2

 DELIMITER $$
 CREATE FUNCTION rand_num() RETURNS INT(5)
 BEGIN
    DECLARE i INT DEFAULT 0;
    SET i = FLOOR(100 + RAND()*10);
    RETURN i;
 END $$
//定义存储过程1

 DELIMITER $$
 CREATE PROCEDURE insert_emp(IN start INT(10), IN max_num INT(10))
 BEGIN
    DECLARE i INT DEFAULT 0;
    SET autocommit = 0;
    REPEAT
    SET i = i + 1;
    INSERT INTO emp(empno, enname, job, mgr, hiredate, sal, comm, deptno) VALUES((start + i),rand_string(6),'SALESMAN',0001,CURDATE(),2000,400,rand_num());
    UNTIL i = max_num  
    END REPEAT;
    COMMIT;
 END $$
//定义存储过程2

 DELIMITER $$
 CREATE PROCEDURE insert_dept(IN start INT(10), IN max_num INT(10))
 BEGIN
    DECLARE i INT DEFAULT 0;
    SET autocommit = 0;
    REPEAT
    SET i = i + 1;
    INSERT INTO dept(deptno,dname,loc) VALUES((start + i),rand_string(10),rand_string(8));
    UNTIL i = max_num  
    END REPEAT;
    COMMIT;
 END $$
//开始插入数据

 DELIMITER ;
 call insert_dept(100,10);
 call insert_emp(100001,500000);

show Profile分析sql

 mysql> show variables like 'profiling';
 +---------------+-------+
 | Variable_name | Value |
 +---------------+-------+
 | profiling     | OFF   |
 +---------------+-------+
 1 row in set (0.00 sec)
 ​
 mysql> set profiling=on;
 Query OK, 0 rows affected, 1 warning (0.00 sec)
 ​
 mysql> show variables like 'profiling';
 +---------------+-------+
 | Variable_name | Value |
 +---------------+-------+
 | profiling     | ON    |
 +---------------+-------+
 1 row in set (0.01 sec)

随便写几条插入语句‘

显示查询操作语句的速度

 mysql> show profiles;
 +----------+------------+----------------------------------------------------------------+
 | Query_ID | Duration   | Query                                                          |
 +----------+------------+----------------------------------------------------------------+
 |        1 | 0.00125325 | show variables like 'profiling'                                |
 |        2 | 0.00018850 | select * from dept                                             |
 |        3 | 0.00016825 | select * from tb1_emp e inner join tbl_dept d on e.deptId=d.id |
 |        4 | 0.00023900 | show tables                                                    |
 |        5 | 0.00031125 | select * from tbl_emp e inner join tbl_dept d on e.deptId=d.id |
 |        6 | 0.00024775 | select * from tbl_emp e inner join tbl_dept d on e.deptId=d.id |
 |        7 | 0.00023725 | select * from tbl_emp e inner join tbl_dept d on e.deptId=d.id |
 |        8 | 0.00023825 | select * from tbl_emp e left join tbl_dept d on e.deptId=d.id  |
 |        9 | 0.35058075 | select * from emp group by id%10 limit 15000                   |
 |       10 | 0.35542250 | select * from emp group by id%10 limit 15000                   |
 |       11 | 0.00024550 | select * from tbl_emp e left join tbl_dept d on e.deptId=d.id  |
 |       12 | 0.36441850 | select * from emp group by id%20 order by 5                    |
 +----------+------------+----------------------------------------------------------------+
 12 rows in set, 1 warning (0.00 sec)

显示查询过程 sql生命周期

 mysql> show profile cpu,block io for query 3;
 +----------------------+----------+----------+------------+--------------+---------------+
 | Status               | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
 +----------------------+----------+----------+------------+--------------+---------------+
 | starting             | 0.000062 | 0.000040 |   0.000021 |            0 |             0 |
 | checking permissions | 0.000004 | 0.000003 |   0.000001 |            0 |             0 |
 | checking permissions | 0.000015 | 0.000006 |   0.000003 |            0 |             0 |
 | Opening tables       | 0.000059 | 0.000039 |   0.000020 |            0 |             0 |
 | query end            | 0.000004 | 0.000002 |   0.000001 |            0 |             0 |
 | closing tables       | 0.000002 | 0.000001 |   0.000000 |            0 |             0 |
 | freeing items        | 0.000014 | 0.000010 |   0.000005 |            0 |             0 |
 | cleaning up          | 0.000009 | 0.000006 |   0.000003 |            0 |             0 |
 +----------------------+----------+----------+------------+--------------+---------------+
 8 rows in set, 1 warning (0.00 sec)
 ​
 mysql> show profile cpu,block io for query 12;
 +----------------------+----------+----------+------------+--------------+---------------+
 | Status               | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
 +----------------------+----------+----------+------------+--------------+---------------+
 | starting             | 0.000063 | 0.000042 |   0.000021 |            0 |             0 |
 | checking permissions | 0.000006 | 0.000003 |   0.000002 |            0 |             0 |
 | Opening tables       | 0.000013 | 0.000009 |   0.000004 |            0 |             0 |
 | init                 | 0.000028 | 0.000017 |   0.000008 |            0 |             0 |
 | System lock          | 0.000007 | 0.000004 |   0.000002 |            0 |             0 |
 | optimizing           | 0.000004 | 0.000002 |   0.000002 |            0 |             0 |
 | statistics           | 0.000014 | 0.000010 |   0.000004 |            0 |             0 |
 | preparing            | 0.000008 | 0.000005 |   0.000003 |            0 |             0 |
 | Creating tmp table   | 0.000028 | 0.000018 |   0.000009 |            0 |             0 |
 | Sorting result       | 0.000003 | 0.000002 |   0.000001 |            0 |             0 |
 | executing            | 0.000002 | 0.000002 |   0.000001 |            0 |             0 |
 | Sending data         | 0.364132 | 0.360529 |   0.002426 |            0 |             0 |
 | Creating sort index  | 0.000053 | 0.000034 |   0.000017 |            0 |             0 |
 | end                  | 0.000004 | 0.000002 |   0.000002 |            0 |             0 |
 | query end            | 0.000007 | 0.000005 |   0.000002 |            0 |             0 |
 | removing tmp table   | 0.000005 | 0.000003 |   0.000002 |            0 |             0 |
 | query end            | 0.000003 | 0.000002 |   0.000001 |            0 |             0 |
 | closing tables       | 0.000006 | 0.000004 |   0.000002 |            0 |             0 |
 | freeing items        | 0.000023 | 0.000016 |   0.000007 |            0 |             0 |
 | cleaning up          | 0.000012 | 0.000007 |   0.000004 |            0 |             0 |
 +----------------------+----------+----------+------------+--------------+---------------+
 20 rows in set, 1 warning (0.00 sec)

如果出现以上这四个 中的任何一个就需要 优化查询语句

全局查询日志

  set global general_log=1;
 set global log_output='TABLE';

此后你编写的sql语句将会记录到mysql库里的general_log表,可以用下面的命令查看

 select * from mysql.general_log;
 mysql> select * from mysql.general_log;
 +----------------------------+---------------------------+-----------+-----------+--------------+---------------------------------+
 | event_time                 | user_host                 | thread_id | server_id | command_type | argument                        |
 +----------------------------+---------------------------+-----------+-----------+--------------+---------------------------------+
 | 2021-12-06 11:53:53.457242 | root[root] @ localhost [] |        68 |         1 | Query        | select * from mysql.general_log |
 +----------------------------+---------------------------+-----------+-----------+--------------+---------------------------------+
 1 row in set (0.00 sec)

Mysql锁

在下面进行表锁的测试

 use big_data;
 ​
 create table mylock (
 id int not null primary key auto_increment,
 name varchar(20) default ''
 ) engine myisam;
 ​
 insert into mylock(name) values('a');
 insert into mylock(name) values('b');
 insert into mylock(name) values('c');
 insert into mylock(name) values('d');
 insert into mylock(name) values('e');
 ​
 select * from mylock;

锁命令

 lock table mylock read,book write;## 读锁锁mylock  写锁锁book
 show open tables;  ##显示哪些表被加锁了
 unlock tables;##取消锁

表锁:读锁

 ##添加读锁后不可修改
 mysql> lock table mylock  read;##1
 Query OK, 0 rows affected (0.00 sec)
 ​
 mysql> select * from mylock;##1
 +----+------+
 | id | name |
 +----+------+
 |  1 | a    |
 |  2 | b    |
 |  3 | c    |
 |  4 | d    |
 |  5 | e    |
 +----+------+
 5 rows in set (0.00 sec)
 ​
 mysql> update mylock set name='a2' where id=1; ##1
 ERROR 1099 (HY000): Table 'mylock' was locked with a READ lock and can't be updated
 ##改不了当前读锁锁住的表
 ##读不了其他表
 mysql> select * from book;##1
 ERROR 1100 (HY000): Table 'book' was not locked with LOCK TABLES

为了区分两个命令 把1当作原有的mysql命令终端上的操作,2当作新建的mysql终端

新建一个mysql终端命令操作

 ##新建一个mysql终端命令操作
 mysql> update mylock set name='a3' where id=1;  ##2

发现会出现阻塞操作

在原有的mysql命令终端上取消锁

 unlock tables;##1
 Query OK, 1 row affected (2 min 1.46 sec)  ##2
 Rows matched: 1  Changed: 1  Warnings: 0   ##2

会发现阻塞了两分钟多

总结 :当读锁锁表mylock之后:1.查询操作:当前client(终端命令操作1)可以进行查询表mylock

其他client(终端命令操作2)也可以查询表mylock 2.DML操作(增删改)当前client会失效报错 ERROR 1099 (HY000): Table 'mylock' was locked with a READ lock and can't be updated 其他client进行DML操作会让mysql陷入阻塞状态直到当前session释放锁

表锁:写锁

 mysql> lock table mylock write;
 Query OK, 0 rows affected (0.00 sec)
给当前session mylock表加上写锁

 mysql> update mylock set name='a4'where id=1 ;
 Query OK, 1 row affected (0.00 sec)
 Rows matched: 1  Changed: 1  Warnings: 0
 ​
 mysql> select * from mylock;
 +----+------+
 | id | name |
 +----+------+
 |  1 | a4   |
 |  2 | b    |
 |  3 | c    |
 |  4 | d    |
 |  5 | e    |
 +----+------+
 mysql> select * from book;
 ERROR 1100 (HY000): Table 'book' was not locked with LOCK TABLES

会发现无法操其他表但是可以操作加上锁的表

再开启一个新的客户端测试被锁住的表

 mysql> select * from mylock;
 ​
 5 rows in set (2 min 30.92 sec)

发现新的客户端上操作(增删改查)被写锁锁住的表会陷入阻塞状态

分析表锁定

 mysql> show status like 'table%';
 +----------------------------+-------+
 | Variable_name              | Value |
 +----------------------------+-------+
 | Table_locks_immediate      | 194   |
 | Table_locks_waited         | 0     |
 | Table_open_cache_hits      | 18    |
 | Table_open_cache_misses    | 2     |
 | Table_open_cache_overflows | 0     |
 +----------------------------+-------+
 5 rows in set (0.00 sec)

行锁

InnoDB 的行锁模式

InnoDB 实现了以下两种类型的行锁。

对于UPDATE、DELETE和INSERT语句,InnoDB会自动给涉及数据集加排他锁(X);

对于普通SELECT语句,InnoDB不会加任何锁;

可以通过以下语句显示给记录集加共享锁或排他锁 。

 共享锁(S):SELECT * FROM table_name WHERE ... LOCK IN SHARE MODE
 ​
 排他锁(X) :SELECT * FROM table_name WHERE ... FOR UPDATE

由于行锁支持事务,在此复习一下

事务

事务是一组由SQL语句组成的逻辑处理单元,事务具有四个属性:ACID

并发事务带来的问题:

更新丢失,脏读,不可重复读,幻读

ACID属性 含义
原子性(Atomicity) 事务是一个原子操作单元,其对数据的修改,要么全部成功,要么全部失败。
一致性(Consistent) 在事务开始和完成时,数据都必须保持一致状态。
隔离性(Isolation) 数据库系统提供一定的隔离机制,保证事务在不受外部并发操作影响的 “独立” 环境下运行。
持久性(Durable) 事务完成之后,对于数据的修改是永久的。

并发事务处理带来的问题

问题 含义
丢失更新(Lost Update) 当两个或多个事务选择同一行,最初的事务修改的值,会被后面的事务修改的值覆盖。
脏读(Dirty Reads) 当一个事务正在访问数据,并且对数据进行了修改,而这种修改还没有提交到数据库中,这时,另外一个事务也访问这个数据,然后使用了这个数据。
不可重复读(Non-Repeatable Reads) 一个事务在读取某些数据后的某个时间,再次读取以前读过的数据,却发现和以前读出的数据不一致。
幻读(Phantom Reads) 一个事务按照相同的查询条件重新读取以前查询过的数据,却发现其他事务插入了满足其查询条件的新数据。

事务隔离级别

为了解决上述提到的事务并发问题,数据库提供一定的事务隔离机制来解决这个问题。数据库的事务隔离越严格,并发副作用越小,但付出的代价也就越大,因为事务隔离实质上就是使用事务在一定程度上“串行化” 进行,这显然与“并发” 是矛盾的。

数据库的隔离级别有4个,由低到高依次为Read uncommitted、Read committed、Repeatable read、Serializable,这四个级别可以逐个解决脏写、脏读、不可重复读、幻读这几类问题。

隔离级别 丢失更新 脏读 不可重复读 幻读
Read uncommitted ×
Read committed × ×
Repeatable read(默认) × × ×
Serializable × × × ×

备注 : √ 代表可能出现 , × 代表不会出现

Mysql 的数据库的默认隔离级别为 Repeatable read , 查看方式:

 show variables like 'tx_isolation';

行锁测试建表, 案例准备工作

 create table test_innodb_lock(
    id int(11),
    name varchar(16),
    sex varchar(1)
 )engine = innodb default charset=utf8;
 ​
 insert into test_innodb_lock values(1,'100','1');
 insert into test_innodb_lock values(3,'3','1');
 insert into test_innodb_lock values(4,'400','0');
 insert into test_innodb_lock values(5,'500','1');
 insert into test_innodb_lock values(6,'600','0');
 insert into test_innodb_lock values(7,'700','0');
 insert into test_innodb_lock values(8,'800','1');
 insert into test_innodb_lock values(9,'900','1');
 insert into test_innodb_lock values(1,'200','0');
 ​
 create index idx_test_innodb_lock_id on test_innodb_lock(id);
 create index idx_test_innodb_lock_name on test_innodb_lock(name);

行锁测试

还是开俩个终端测试,关闭事自动事务提交,因为自动事务提交会自动加锁释放锁;

 mysql> set autocommit=0;

 mysql> set autocommit=0;

会发现查询无影响

对左边进行更新操作

 mysql> update test_innodb_lock set name='100' where id=3;
 Query OK, 0 rows affected (0.00 sec)
 Rows matched: 1  Changed: 0  Warnings: 0

对左边进行更新操作

对右边进行更新操作后停止操作

 mysql> update test_innodb_lock set name='340' where id=3;
 ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

会发现进行阻塞了 直到锁释放或者提交事务(commit)为止

对于innodb引擎来说,对某一行数据进行DML(增删改)操作会对操作的那行添加排它锁

别的事务就不能执行这行语句了,但是可以操作其他行的数据

无索引行锁会升级成表锁:如果不通过索引条件检索数据,那么innodb会对表中所有记录加锁,实际效果和表锁一样

记住进行操作时使用索引:innodb引擎索引失效时时行锁会升级为表锁

 mysql> update test_innodb_lock set sex='2' where name=400;
 Query OK, 0 rows affected (0.00 sec)
 Rows matched: 2  Changed: 0  Warnings: 0

注意这里name没有加单引号 索引失效

 mysql> update test_innodb_lock set sex='3' where id=3;
 Query OK, 1 row affected (23.20 sec)
 Rows matched: 1  Changed: 1  Warnings: 0

发现了对其他行操作也陷入了阻塞状态,这是没加索引导致的行锁升级为表锁

本来只对一行数据加锁 但是由于忘记给name字段加单引号导致索引失效给全表都加上了锁;

间隙锁

当我们使用范围条件而不是想等条件进行检索数据,并请求共享或排它锁,在那个范围条件中有不存在的记录,叫做间隙,innodb也会对这个间隙进行加锁,这种锁机制就叫做间隙锁

 mysql> select * from test_innodb_lock;
 +------+------+------+
 | id   | name | sex  |
 +------+------+------+
 |    1 | 100  | 2    |
 |    3 | 100  | 3    |
 |    4 | 400  | 0    |
 |    5 | 500  | 1    |
 |    6 | 600  | 0    |
 |    7 | 700  | 3    |
 |    8 | 800  | 1    |
 |    9 | 900  | 2    |
 |    1 | 200  | 0    |
 +------+------+------+
 没有id为2的数据

行锁征用情况查看

 mysql> show status like 'innodb_row_lock%';
 +-------------------------------+--------+
 | Variable_name                 | Value  |
 +-------------------------------+--------+
 | Innodb_row_lock_current_waits | 0      |
 | Innodb_row_lock_time          | 284387 |
 | Innodb_row_lock_time_avg      | 21875  |
 | Innodb_row_lock_time_max      | 51003  |
 | Innodb_row_lock_waits         | 13     |
 +-------------------------------+--------+
 5 rows in set (0.00 sec)
 Innodb_row_lock_current_waits: 当前正在等待锁定的数量
 ​
 Innodb_row_lock_time: 从系统启动到现在锁定总时间长度
 ​
 Innodb_row_lock_time_avg:每次等待所花平均时长
 ​
 Innodb_row_lock_time_max:从系统启动到现在等待最长的一次所花的时间
 ​
 Innodb_row_lock_waits: 系统启动后到现在总共等待的次数

行锁总结

InnoDB存储引擎由于实现了行级锁定,虽然在锁定机制的实现方面带来了性能损耗可能比表锁会更高一些,但是在整体并发处理能力方面要远远由于MyISAM的表锁的。当系统并发量较高的时候,InnoDB的整体性能和MyISAM相比就会有比较明显的优势。

但是,InnoDB的行级锁同样也有其脆弱的一面,当我们使用不当的时候,可能会让InnoDB的整体性能表现不仅不能比MyISAM高,甚至可能会更差。

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