14.3.4 Phantom Rows 幻影行
所谓的幻读为发生在一个事务 当相同的查询产生不同的结果集在不同的时间。
比如,如果一个SELECT被执行2次, 但是第2次返回的记录不是第一次返回的记录,
行是幻行
假设在child 表的id列上有一个索引 ,你需要读取和锁定所有的行 值大于100的,
以便更新选择行的一些列:
SELECT * FROM child WHERE id > 100 FOR UPDATE;
查询扫描索引从第一个记录id大于100开始, 让表包含记录90和102.
如果锁设置在index records 在扫描的范围 不堵塞插入 在区间上(在这个例子里,区间是90和102）
另外一个会话可以插入新值到表 id值为101.
如果你执行相同的查询 在相同的session，你会看到新的值id=101（一个幻读）
在查询返回的结果。 如果我们将一组行作为数据项, 新的幻读的child 会违背事务隔离原理,
一个事务应该在事务期间所读取的数据不会改变。
mysql> show variables like '%tx_isolation%';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.01 sec)
CREATE TABLE `child` (
`sn` int(11) NOT NULL AUTO_INCREMENT,
`id` int(11) DEFAULT NULL,
`info` varchar(40) DEFAULT NULL,
PRIMARY KEY (`sn`)
);
mysql> show variables like '%tx_isolation%';
+---------------+----------------+
| Variable_name | Value |
+---------------+----------------+
| tx_isolation | READ-COMMITTED |
+---------------+----------------+
1 row in set (0.00 sec)
Session 1:
mysql> SELECT * FROM child WHERE id > 100 FOR UPDATE;
Empty set (0.00 sec)
Session 2:
mysql> select * from child;
+----+------+------+
| sn | id | info |
+----+------+------+
| 1 | 1 | a1 |
| 2 | 99 | a99 |
+----+------+------+
2 rows in set (0.00 sec)
mysql> insert into child(id,info)
values(101,'a101');
Query OK, 1 row affected (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.01 sec)
继续测试:
Session 1:
mysql> show variables like '%tx_isolation%';
+---------------+----------------+
| Variable_name | Value |
+---------------+----------------+
| tx_isolation | READ-COMMITTED |
+---------------+----------------+
1 row in set (0.00 sec)
mysql> SELECT * FROM child;
+----+------+------+
| sn | id | info |
+----+------+------+
| 1 | 1 | a1 |
| 2 | 99 | a99 |
| 3 | 101 | a101 |
+----+------+------+
3 rows in set (0.00 sec)
mysql> insert into child(id,info) values(110,'a110');
Query OK, 1 row affected (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.01 sec)
mysql> select * from child;
+----+------+------+
| sn | id | info |
+----+------+------+
| 1 | 1 | a1 |
| 2 | 99 | a99 |
| 3 | 101 | a101 |
| 4 | 110 | a110 |
+----+------+------+
4 rows in set (0.00 sec)
mysql> SELECT * FROM child WHERE id > 100 FOR UPDATE;
+----+------+------+
| sn | id | info |
+----+------+------+
| 3 | 101 | a101 |
| 4 | 110 | a110 |
+----+------+------+
2 rows in set (0.00 sec)
Session 2:
mysql> select * from
child;
+----+------+------+
| sn | id | info |
+----+------+------+
| 1 | 1 | a1 |
| 2 | 99 | a99 |
| 3 | 101 | a101 |
| 4 | 110 | a110 |
+----+------+------+
4 rows in set (0.00 sec)
mysql> insert into child
(id,info) values(105,'a105');
Query OK, 1 row affected
(0.00 sec)
mysql> commit;
Query OK, 0 rows affected
(0.01 sec)
mysql> select * from child;
+----+------+------+
| sn | id | info |
+----+------+------+
| 1 | 1 | a1 |
| 2 | 99 | a99 |
| 3 | 101 | a101 |
| 4 | 110 | a110 |
| 5 | 105 | a105 |
+----+------+------+
5 rows in set (0.00 sec)
为了防止幻读,InnoDB使用一个算法叫做 next-key locking 由 index-row locking和gap locking 组成。
InnoDB 执行 row-level locking 以这种方式 当它搜索或者扫描表的索引,
它设置共享锁或者排它锁 在它遇到的index records上.
因此, row-level locks 实际上是 index-record locks.
此外, a next-key lock 在一个Index record 也影响了 “gap” before that index record.
也就是说,一个next-key lock 是一个index record lock 加上一个gap lock
在index record 之前的区间。
如果一个会话有一个共享锁或者排它锁在记录R上 在一个index里,
另外的会话不能立即插入到一个新的index record 在一个gap (在记录R之前)
当InnoDB 扫描一个Index, 它也可以lock gap 在最后一条记录后面 在index里,
比如之前的例子,防止任何插入到表 id值大于100的数据,
InnoDB设置锁包含一个lock区间 在id=102以后的区间
测试:
Session 1:
mysql> show variables like '%tx_isolation%';
+---------------+-----------------+
| Variable_name | Value |
S+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.00 sec)
Database changed
mysql> select * from child;
+----+------+------+
| sn | id | info |
+----+------+------+
| 1 | 90 | a90 |
| 2 | 102 | a102 |
+----+------+------+
2 rows in set (0.00 sec)
mysql> select * from child where id>100 for update;
+----+------+------+
| sn | id | info |
+----+------+------+
| 2 | 102 | a102 |
+----+------+------+
1 row in set (0.00 sec)
mysql> show index from child;
+-------+------------+------------+--------------+-------------+-----------+-------------
+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality |
Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+------------+--------------+-------------+-----------+-------------
+----------+--------+------+------------+---------+---------------+
| child | 0 | PRIMARY | 1 | sn | A | 4 |
NULL | NULL | | BTREE | | |
| child | 0 | child_idx1 | 1 | id | A | 4 |
NULL | NULL | YES | BTREE | | |
+-------+------------+------------+--------------+-------------+-----------+-------------
+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
Session 2:
mysql> insert into child(id,info) values(91,'a91');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(90,'a90');
ERROR 1062 (23000): Duplicate entry '90' for key 'child_idx1'
mysql> insert into child(id,info) values(89,'a89');
Query OK, 1 row affected (0.00 sec)
mysql> rollback;
Query OK, 0 rows affected (0.01 sec)
mysql> insert into child(id,info) values(88,'a88');
Query OK, 1 row affected (0.00 sec)
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into child(id,info) values(92,'a92');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(93,'a93');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(94,'a94');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(95,'a95');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(96,'a96');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(97,'a97');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(98,'a98');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(99,'a99');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(100,'a100');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(101,'a101');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(102,'a102');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(103,'a103');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into child(id,info) values(104,'a104');
x∈[3,4]表示3≤x≤4 因为两端有等号,所以叫闭区间
x∈(3,4)表示3＜x＜4 因为两端没等号,所以叫开区间
锁了（90，无穷）
你可以使用 next-key locking 来实现一个唯一性检查 在你的应用里:
如果你读取你的数据 在共享模式,没有看到重复的 对于一条记录准备被插入,
然后你可以安全的插入你的记录,知道 next-key lock 设置在你的行的继承者 在读取阻止任何
同时插入一个重复的记录。因此,the next-key locking 让你可以lock 你表中不存在的东西