根据网络资料学习并整理
一、
MyISAM并发插入
＃语法：lock tables [tablename] read／write Local
＃说明：
１、local的作用：在满足MyISAM表并发插入条件的情况下，允许其他用户在表尾并发插入记录
２、在用lock tables显示加锁时
- 必须同时取得所有涉及到的表的锁
- 不支持锁升级，也就是说只能访问显示加锁的这些表，不能访问未加锁的表
- 如果加的是［读锁］，那么当前事务和其他事务只能执行查询操作，不能执行更新｜插入｜删除操作
- 如果加的是［写锁］，那么其他事务不能查询｜更新｜插入｜删除操作。
- 若同一个表在ＳＱＬ语句中出现多次，那么也需要将ＳＱＬ语句中相同表的表别名进行锁定，否则通过［别名］访问也会出错
mysql> lock table user as a read,user as b read;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from user a,user b where a.id=b.id;
+----+---------+-----------+----+---------+-----------+
| id | user_no | user_name | id | user_no | user_name |
+----+---------+-----------+----+---------+-----------+
| 1 | 2 | guan | 1 | 2 | guan |
| 3 | 3 | NULL | 3 | 3 | NULL |
| 4 | 2 | NULL | 4 | 2 | NULL |
| 5 | 7 | NULL | 5 | 7 | NULL |
| 9 | 7 | NULL | 9 | 7 | NULL |
| 10 | 7 | NULL | 10 | 7 | NULL |
+----+---------+-----------+----+---------+-----------+
6 rows in set (0.00 sec)
＃若未给别名也加锁，会报错，如下：
mysql> lock table user read;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from user a,user b where a.id=b.id;
ERROR 1100 (HY000): Table 'a' was not locked with LOCK TABLES
mysql>
３、除了显示加锁，还能［隐式加锁］，下面几种情况也会加上表锁
session1中给user表
插入／删除／更新
记录时，session２中用lock tables user read命令显示锁表会阻塞，直到session1提交事务
４
、查询表级锁争用情况，
Table_locks_waited比较高，那么说明存在严重的表级锁争用情况
mysql>
show status like'table%';
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Table_locks_immediate | 133 |
| Table_locks_waited | 0 |
| Table_
open_cache_hits | 15 |
| Table_open_cache_misses | 6 |
| Table_open_cache_overflows | 0 |
+----------------------------+-------+
5 rows in set (0.00 sec)
［补充］行级锁争用情况查询：
show status like 'innodb_row_lock%'
mysql>
show status like 'innodb_row_lock%';
+-------------------------------+-------+
| Variable_name | Value |
+-------------------------------+-------+
|
Innodb_row_lock_current_waits | 1 |
| Innodb_row_lock_time | 98982 |
| Innodb_row_lock_time_avg | 32994 |
| Innodb_row_lock_time_max | 51036 |
| Innodb_row_lock_waits | 3 |
+-------------------------------+-------+
5 rows in set (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
mysql> show status like 'innodb_row_lock%';
+-------------------------------+--------+
| Variable_name | Value |
+-------------------------------+--------+
| Innodb_row_lock_current_waits | 0 |
| Innodb_row_lock_time | 107017 |
| Innodb_row_lock_time_avg | 35672 |
| Innodb_row_lock_time_max | 51036 |
| Innodb_row_lock_waits | 3 |
+-------------------------------+--------+
5 rows in set (0.00 sec)
５、ＭyISAM并发插入
＃系统变量
- concurrent_insert:0 >>不允许并发插入
- concurrent_insert:1 >>若表中间没有空洞（没有被删除的行），则允许另外一个线程在表尾并发插入？对空洞不是很理解
- concurrent_insert:2 >>无论有没有窑洞，都允许另外一个线程在表尾并发插入
＃如何实现？加local选项
语法：lock tables user read | write
local
| session1 | session2 | |
| lock tables user read | write local | ||
|
当前session1不能对锁定的表进行
更新｜插入｜删除
操作
mysql> insert into user values(25,'6',null);
ERROR 1099 (HY000): Table 'user' was locked with a READ lock and can't be updated
mysql> update user set user_no=25 where id=25;
ERROR 1099 (HY000): Table 'user' was locked with a READ lock and can't be updated
mysql> delete from user where id=25;
ERROR 1099 (HY000): Table 'user' was locked with a READ lock and can't be updated
|
session2可以查询该表的记录
select * from user;
+----+---------+-----------+
| id | user_no | user_name |
+----+---------+-----------+
| 1 | 2 | guan |
| 3 | 3 | NULL |
| 22 | 6 | NULL |
| 25 | 6 | NULL |
| 24 | 6 | NULL |
+----+---------+-----------+
13 rows in set (0.00 sec)
|
|
|
当前session1不能查询没有锁定的表
mysql> select * from student;
ERROR 1100 (HY000): Table 'student' was not locked with LOCK TABLES
|
session2可以进行［
插入
］操作，但是［
更新｜删除
］会阻塞
mysql> insert into user values(27,7,'hui');
Query OK, 1 row affected (0.01 sec)
|
|
|
当前session1不能访问session2插入的记录
mysql> select * from user;
+----+---------+-----------+
| id | user_no | user_name |
+----+---------+-----------+
| 3 | 3 | NULL |
| 4 | 2 | NULL |
| 5 | 7 | NULL |
| 9 | 17 | NULL |
| 10 | 7 | NULL |
| 15 | 15 | NULL |
| 19 | 6 | NULL |
| 20 | 6 | NULL |
| 21 | 6 | NULL |
| 22 | 6 | NULL |
| 25 | 6 | NULL |
| 24 | 6 | NULL |
+----+---------+-----------+
12 rows in set (0.00 sec)
|
二、间隙锁问题分析（行锁是给索引加锁）
概念：什么是间隙锁？
与oracle不同的是，mysql加锁主要是对索引加锁（二级索引上间隙锁）
在进行删除或者修改操作时，如果过滤条件列是【非唯一索引】，为了保证当前读的数据一致性，mysql通过间隙锁对数据之间区域进行锁定。（实际上是通过锁定索引达到效果）
这种锁叫间隙锁，这种锁定会造成许多误杀，很多并不冲突的数据会因为间隙锁而无法插入
Ｓ锁：共享锁>>lock in share mode
Ｘ锁：排他锁>>for update
，除了for update显示加锁，在更新｜删除｜插入默认会加上排他锁。
##表结构：主键>>id;二级索引>>number
mysql> select * from student;
+----+--------+
| id | number |
+----+--------+
| 10 | 1 |
| 1 | 2 |
| 2 | 2 |
| 3 | 3 |
| 5 | 5 |
| 7 | 5 |
| 8 | 88 |
+----+--------+
| session1 | session2 |
|
通过
select * from student where number=5 for update;
加上排他锁
注：number＝５的记录是存在的，所以是记录锁？好像也不对呀
１、number=3,number=4时可以更新｜删除，但是不能［插入］
２、更新｜删除 number=2的记录
（
set id=5）
，会阻塞
综合案例，select ...for update给number=5的间隙也加上了锁
|
select * from user where
number
=5 for update 阻塞
》》》
阻塞
insert into student values(6,5);
insert into student values(4,5);
insert into student values(4,4);
insert into student values(12,9);
》》》
阻塞
delete from student where
number
=5;
》》》
阻塞，删除的记录包含在（３，５），（５，５），（５，８８）的间隙之间
delete from student where
number
=3;
delete from student where
number
=４;
注：number=4也可以删除
》》》
执行成功
，
临界值
３，可以删除
update student set
id=88
where
number
=5;
update student set
id=2
where number=5;
》》》
阻塞
update student set id=2 where number=4;
》》》
执行成功
insert into student values(4,2);
》》》
执行成功
更新｜删除 number=2的记录
（
set id=5）
，会阻塞
|
|
delete from student where id=8;
更新｜删除｜插入会默认加上行锁，即使记录不存在
|
select * from student where id=8 for update;
insert into student values(8,8);
update student set number=8 where id=8;
delete from student where id=8;
》》》以上
查询｜插入｜更新｜删除
的四种操作中，不论
id=８的记录存不存在，都
阻塞
|
|
间隙锁问题：锁定不存在的记录，那么会将不存在记录的前面和后面的可疑数据都加锁。
如下ＳＱＬ中，id=9的记录不存在，那么会给id=8,id=10的记录上锁
mysql> select * from student;
+----+--------+
| id | number |
+----+--------+
| 1 | 2 |
| 2 | 2 |
| 3 | 3 |
| 5 | 5 |
| 7 | 5 |
| 8 | 8 |
| 10 | 1 |
+----+--------+
7 rows in set (0.00 sec)
mysql> select * from student where id=9 for update;
|
update student set number=88 where id=8;
更新｜删除｜插入
id=8的记录都是阻塞状态
|