MySQL表锁和行锁

　　本次主要是测试一下MySQL的select ... for update这种悲观锁的表锁和行锁，版本：MySQL5.7.20，存储引擎 InnoDB 操作系统：Windows10 64位，工具：navicat for mysql 11

表名称：test_order，主键是id；

　　要使用悲观锁，我们必须关闭mysql数据库的自动提交属性，因为MySQL默认使用autocommit模式，也就是说，当你执行一个更新操作后，MySQL会立刻将结果进行提交，所以我们需要手动设置autocommit=0;

1. 对主键进行加锁：

navicat起一个命令行，设置autocommit=0，并且对id=1的记录进行加锁；

mysql> select * from test_order;
+----+--------+--------+----------+
| id | status | name   | goods_id |
+----+--------+--------+----------+
|  1 |      1 | 茅台   | 123      |
|  2 |      2 | 五粮液 | 345      |
|  3 |      3 | 剑南春 | 567      |
+----+--------+--------+----------+
3 rows in set

mysql> set autocommit = 0;
Query OK, 0 rows affected

mysql> select * from test_order where id = 1 for update;
+----+--------+------+----------+
| id | status | name | goods_id |
+----+--------+------+----------+
|  1 |      1 | 茅台 | 123      |
+----+--------+------+----------+
1 row in set

navicat再起一个命令行

mysql> select * from test_order where id = 1;
+----+--------+------+----------+
| id | status | name | goods_id |
+----+--------+------+----------+
|  1 |      1 | 茅台 | 123      |
+----+--------+------+----------+
1 row in set

mysql> select * from test_order where id = 1 for update;
1205 - Lock wait timeout exceeded; try restarting transaction
mysql> select * from test_order where id = 2 for update;
+----+--------+--------+----------+
| id | status | name   | goods_id |
+----+--------+--------+----------+
|  2 |      2 | 五粮液 | 345      |
+----+--------+--------+----------+
1 row in set

mysql> 

结论：对主键进行锁定时，对常规的select操作无影响，再次锁id=1的记录时，发现已经被锁，而查询其他的数据则没问题；

2. 对非主键非索引加锁

mysql> select * from test_order where status = 1 for update;
+----+--------+------+----------+
| id | status | name | goods_id |
+----+--------+------+----------+
|  1 |      1 | 茅台 | 123      |
+----+--------+------+----------+
1 row in set

另起一个命令行

mysql> select * from test_order where id = 1 for update;
1205 - Lock wait timeout exceeded; try restarting transaction
mysql> select * from test_order where id = 2 for update;
1205 - Lock wait timeout exceeded; try restarting transaction

结论：对非主键非索引加锁时，锁的是整张表；

3. 对非主键索引加锁

我们对status这个字段加上索引，然后再测试：

mysql> set autocommit = 0;
Query OK, 0 rows affected

mysql> select * from test_order where status = 1 for update;
+----+--------+------+----------+
| id | status | name | goods_id |
+----+--------+------+----------+
|  1 |      1 | 茅台 | 123      |
+----+--------+------+----------+
1 row in set

再起一个命令行

mysql> select * from test_order where id = 1 for update;
1205 - Lock wait timeout exceeded; try restarting transaction
mysql> select * from test_order where id = 2 for update;
+----+--------+--------+----------+
| id | status | name   | goods_id |
+----+--------+--------+----------+
|  2 |      2 | 五粮液 | 345      |
+----+--------+--------+----------+
1 row in set

结论：对于索引字段来说，使用的是行锁；

总结：MySQL的select ... for update这种悲观锁，有两种锁定级别：行锁(Row Lock )，表锁(Table Lock)。在对表进行进行加锁的时候，由于主键本身就是索引，所以，对包含索引的字段加的是行锁，而对于非索引字段，加的是表锁；Mysql InnoDB存储引擎默认的是Row Lock，所以查询的时候尽量使用索引。