Mysql加锁过程详解（2）-关于mysql 幻读理解出现了幻读，那么不是说mysql的重复读解决了幻读的么？
那么，InnoDB指出的可以避免幻读是怎么回事呢？
http://dev.mysql.com/doc/refman/5.0/en/innodb-record-level-locks.html
By default, InnoDB operates in REPEATABLE READ transaction isolation level and with the innodb_locks_unsafe_for_binlog system variable disabled. In this case, InnoDB uses next-key locks for searches and index scans, which prevents phantom rows (see Section 13.6.8.5, “Avoiding the Phantom Problem Using Next-Key Locking”).
准备的理解是，当隔离级别是可重复读，且禁用innodb_locks_unsafe_for_binlog的情况下，在搜索和扫描index的时候使用的next-key locks可以避免幻读。
关键点在于，是InnoDB默认对一个普通的查询也会加next-key locks，还是说需要应用自己来加锁呢？如果单看这一句，可能会以为InnoDB对普通的查询也加了锁，如果是，那和序列化（SERIALIZABLE）的区别又在哪里呢？
MySQL manual里还有一段：
13.2.8.5. Avoiding the Phantom Problem Using Next-Key Locking (http://dev.mysql.com/doc/refman/5.0/en/innodb-next-key-locking.html)
To prevent phantoms,
InnoDBuses an algorithm called next-key locking that combines index-row locking with gap locking.You can use next-key locking to implement a uniqueness check in your application: If you read your data in share mode and do not see a duplicate for a row you are going to insert, then you can safely insert your row and know that the next-key lock set on the successor of your row during the read prevents anyone meanwhile inserting a duplicate for your row. Thus, the next-key locking enables you to “lock” the nonexistence of something in your table.
我的理解是说，InnoDB提供了next-key locks，但需要应用程序自己去加锁。manual里提供一个例子：
SELECT * FROM child WHERE id > 100 FOR UPDATE;
这样，InnoDB会给id大于100的行（假如child表里有一行id为102），以及100-102，102+的gap都加上锁。
可以使用show innodb status来查看是否给表加上了锁。
下面看列子
例子1
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a |
b |
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SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
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SELECT * FROM test WHERE a='1' FOR UPDATE; |
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SELECT * FROM test |
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INSERT test VALUES(1,1); |
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锁住了 |
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INSERT test VALUES(1,1); |
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成功 |
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COMMIT |
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COMMIT |
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避免幻读可以select锁住，再insert |
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例子2
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a |
b |
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SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
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SELECT * FROM test WHERE a='1' FOR UPDATE; |
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SELECT * FROM test |
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INSERT test VALUES(2,2); |
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连2也被锁住了？ |
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INSERT test VALUES(1,1); |
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成功 |
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COMMIT |
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这次提交成功 |
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COMMIT |
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其他尝试，这种情况无论插入2还是5都被锁住等等 |
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例子3
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a |
b |
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SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
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SELECT * FROM test |
SELECT * FROM test |
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SELECT * FROM test WHERE a='1' FOR UPDATE; |
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SELECT * FROM test |
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INSERT test VALUES(2,2); |
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COMMIT |
COMMIT |
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成功 |
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COMMIT |
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COMMIT |
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例子 4
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a |
b |
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SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
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SELECT * FROM test |
SELECT * FROM test |
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SELECT * FROM test WHERE a='2' FOR UPDATE; |
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SELECT * FROM test |
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INSERT test VALUES(2,2); |
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INSERT test VALUES(5,5); |
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COMMIT |
COMMIT |
例子 5
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a |
b |
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SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
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SELECT * FROM test |
SELECT * FROM test |
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SELECT * FROM test WHERE a='1' FOR UPDATE; |
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INSERT test VALUES(5,5); |
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插入5成功了 |
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UPDATE test SET b=33 WHERE a='3' |
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INSERT test VALUES(2,2); |
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2也可以 |
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UPDATE test SET b=11 WHERE a='1' |
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1锁住了 |
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COMMIT |
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COMMIT |
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SELECT * FROM test |
SELECT * FROM test |
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以上例子说明，forupdate时候，id为主键，RR策略时候，锁住了的条件符合的行，但是如果条件找不到任何列，锁住的是整个表，（主键，唯一索引，非唯一索引，（insert，update对于gab锁不通），参考第一章，第七章,第九章）
------------------------------------------------------------------
再来看大神的解释 ：链接: http://blog.bitfly.cn/post/mysql-innodb-phantom-read/
再看一个实验，要注意，表t_bitfly里的id为主键字段。实验三：
t Session A Session B
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| START
TRANSACTION;
START TRANSACTION;|
| SELECT * FROM t_bitfly
| WHERE id<=1
| FOR UPDATE;
| +------+-------+
| | id | value |
| +------+-------+
| | 1 |
a
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INSERT INTO t_bitfly|
VALUES (2, 'b');|
Query OK, 1 row affected|
| SELECT * FROM t_bitfly;
| +------+-------+
| | id | value |
| +------+-------+
| | 1 |
a
|| +------+-------+
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INSERT INTO t_bitfly|
VALUES (0, '0');|
(waiting for lock ...|
then timeout)|
ERROR 1205 (HY000):|
Lock wait timeout exceeded;|
try restarting transaction|
| SELECT * FROM t_bitfly;
| +------+-------+
| | id | value |
| +------+-------+
| | 1 |
a
|| +------+-------+
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COMMIT;|
| SELECT * FROM t_bitfly;
| +------+-------+
| | id | value |
| +------+-------+
| | 1 |
a
|| +------+-------+
v
可以看到，用id<=1加的锁，只锁住了id<=1的范围，可以成功添加id为2的记录，添加id为0的记录时就会等待锁的释放。
MySQL manual里对可重复读里的锁的详细解释：
http://dev.mysql.com/doc/refman/5.0/en/set-transaction.html#isolevel_repeatable-read
For locking reads (
SELECTwithFORor
UPDATELOCK),
IN SHARE MODEUPDATE,
andDELETEstatements,
locking depends on whether the statement uses a unique index with a
unique search condition, or a range-type search condition. For a
unique index with a unique search
condition,InnoDBlocks
only the index record found, not the gap before it. For other
search conditions,InnoDBlocks
the index range scanned, using gap locks or next-key (gap plus
index-record) locks to block insertions by other sessions into the
gaps covered by the range.
------
一致性读和提交读，先看实验，实验四：
t Session
A
Session B|
| START
TRANSACTION;
START TRANSACTION;|
| SELECT * FROM t_bitfly;
| +----+-------+
| | id | value |
| +----+-------+
| | 1 |
a
|| +----+-------+
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INSERT INTO t_bitfly|
VALUES (2, 'b');|
COMMIT;|
| SELECT * FROM t_bitfly;
| +----+-------+
| | id | value |
| +----+-------+
| | 1 |
a
|| +----+-------+
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| SELECT * FROM t_bitfly LOCK IN SHARE MODE;
| +----+-------+
| | id | value |
| +----+-------+
| | 1 |
a
|| | 2 |
b
|| +----+-------+
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| SELECT * FROM t_bitfly FOR UPDATE;
| +----+-------+
| | id | value |
| +----+-------+
| | 1 |
a
|| | 2 |
b
|| +----+-------+
|
| SELECT * FROM t_bitfly;
| +----+-------+
| | id | value |
| +----+-------+
| | 1 |
a
|| +----+-------+
v
如果使用普通的读，会得到一致性的结果，如果使用了加锁的读，就会读到“最新的”“提交”读的结果。
本身，可重复读和提交读是矛盾的。在同一个事务里，如果保证了可重复读，就会看不到其他事务的提交，违背了提交读；如果保证了提交读，就会导致前后两次读到的结果不一致，违背了可重复读。
可以这么讲，InnoDB提供了这样的机制，在默认的可重复读的隔离级别里，可以使用加锁读去查询最新的数据。
http://dev.mysql.com/doc/refman/5.0/en/innodb-consistent-read.html
If you want to see the “freshest” state of the database, you should
use either the READ COMMITTED isolation level or a locking
read:SELECT * FROM t_bitfly LOCK IN SHARE MODE;
结论：MySQL InnoDB的可重复读并不保证避免幻读，需要应用使用加锁读来保证。而这个加锁度使用到的机制就是next-key
locks。
结论：mysql 的重复读解决了幻读的现象，但是需要 加上 select for update/lock in share mode 变成当前读避免幻读，普通读select存在幻读