對於MyISAM表，加的鎖是表級鎖；寫操作會阻塞讀操作，讀操作會阻塞寫操作，寫操作會阻塞寫操作；讀操作不會阻塞讀操作。

測試一，會話①的讀操作阻塞會話②的寫操作
會話①
mysql> create table t2(id tinyint(3) unsigned not null auto_increment,
-> name varchar(10) not null,
-> primary key(id))
-> engine=myisam auto_increment=8 default charset=gbk;
Query OK, 0 rows affected (0.03 sec)
mysql> insert into t2(name) values('Neo');
Query OK, 1 row affected (0.03 sec)
mysql> insert into t2(name) values('Trinity');
Query OK, 1 row affected (0.00 sec)

mysql> select * from t2;
+----+---------+
| id | name |
+----+---------+
| 8 | Neo |
| 9 | Trinity |
+----+---------+
2 rows in set (0.00 sec)

mysql> desc t2;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| id | tinyint(3) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(10) | NO | | NULL | |
+-------+---------------------+------+-----+---------+----------------+
2 rows in set (0.09 sec)

為表增加讀鎖
mysql> lock table t2 read;
Query OK, 0 rows affected (0.00 sec)

會話②
mysql> select * from t2;
+----+---------+
| id | name |
+----+---------+
| 8 | Neo |
| 9 | Trinity |
+----+---------+
2 rows in set (0.00 sec)

會話會處於等待狀態
mysql> update t2 set name='James' where id=5;

會話①
mysql> show processlist;
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+---------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+---------------------------------------+
| 1 | system user | | NULL | Connect | 748155 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL |
| 2 | system user | | NULL | Connect | 748156 | Connecting to master | NULL |
| 13 | event_scheduler | localhost | NULL | Daemon | 600105 | Waiting on empty queue | NULL |
| 74 | neo | localhost | fire | Query | 0 | NULL | show processlist |
| 75 | neo | localhost | fire | Query | 83 | Waiting for table level lock | update t2 set name='James' where id=5 |
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+---------------------------------------+
5 rows in set (0.00 sec)

mysql> unlock tables;
Query OK, 0 rows affected (0.00 sec)

會話②
mysql> update t2 set name='James' where id=5;
Query OK, 0 rows affected (1 min 48.96 sec)
Rows matched: 0 Changed: 0 Warnings: 0

測試二，會話①的寫操作阻塞會話②的讀操作

會話①
mysql> lock table t2 write;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t2(name) values('Jack');
Query OK, 1 row affected (0.01 sec)

mysql> commit;
Query OK, 0 rows affected (0.00 sec)

mysql> show processlist;
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+
| 1 | system user | | NULL | Connect | 748422 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL |
| 2 | system user | | NULL | Connect | 748423 | Connecting to master | NULL |
| 13 | event_scheduler | localhost | NULL | Daemon | 600372 | Waiting on empty queue | NULL |
| 74 | neo | localhost | fire | Query | 0 | NULL | show processlist |
| 75 | neo | localhost | fire | Query | 2 | Waiting for table metadata lock | select * from t2 |
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+
5 rows in set (0.00 sec)

會話②
查詢會阻塞
mysql> select * from t2;

會話①
mysql> unlock tables;
Query OK, 0 rows affected (0.00 sec)

會話②
mysql> select * from t2;
+----+---------+
| id | name |
+----+---------+
| 8 | Neo |
| 9 | Trinity |
| 10 | Jack |
+----+---------+
3 rows in set (47.89 sec)

當同一個會話中，鎖定一張表後，查詢另外一張表會報錯
mysql> lock table test read;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from test where id=80;
+------+------+
| id   | name |
+------+------+
|   80 | Lily |
+------+------+
1 row in set (0.00 sec)

mysql> select * from t70;
ERROR 1100 (HY000): Table 't70' was not locked with LOCK TABLES

使用表的別名會報錯
mysql> lock table test read;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from test t where id=80;
ERROR 1100 (HY000): Table 't' was not locked with LOCK TABLES

需要對別名進行鎖定

mysql> lock table test t read;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from test t where id=80;
+------+------+
| id   | name |
+------+------+
|   80 | Kame |
+------+------+
1 row in set (0.00 sec)

MySQL 5.5 MyISAM表鎖測試

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