锁分类

锁的分类(oracle)

按操作划分，可分为DML锁、DDL锁
按锁的粒度划分，可分为表级锁、行级锁、页级锁（mysql）
按锁级别划分，可分为共享锁、排他锁
按加锁方式划分，可分为自动锁、显示锁
按使用方式划分，可分为乐观锁、悲观锁

DML锁（data locks，数据锁），用于保护数据的完整性，其中包括行级锁(Row Locks (TX锁))、表级锁(table lock(TM锁))。 DDL锁（dictionary locks，数据字典锁），用于保护数据库对象的结构，如表、索引等的结构定义。其中包排他DDL锁（Exclusive DDL lock）、共享DDL锁（Share DDL lock）、可中断解析锁（Breakable parse locks）

MyISAM和MEMORY采用表级锁(table-level locking)
BDB采用页面锁(page-level locking)或表级锁，默认为页面锁
InnoDB支持行级锁(row-level locking)和表级锁,默认为行级锁

行级锁

行级锁是MySQL中锁定粒度最细的一种锁，表示只针对当前操作的行进行加锁。行级锁能大大减少数据库操作的冲突。其加锁粒度最小，但加锁的开销也最大。行级锁分为共享锁和排他锁。

另外，为了允许行锁和表锁共存，实现多粒度锁机制，InnoDB还有两种内部使用的意向锁（Intention Locks），这两种意向锁都是表锁。

意向共享锁（IS）：事务打算给数据行加行共享锁，事务在给一个数据行加共享锁前必须先取得该表的IS锁。
意向排他锁（IX）：事务打算给数据行加行排他锁，事务在给一个数据行加排他锁前必须先取得该表的IX锁。

InnoDB行锁是通过给索引上的索引项加锁来实现的，这一点MySQL与Oracle不同，后者是通过在数据块中对相应数据行加锁来实现的。InnoDB这种行锁实现特点意味着：只有通过索引条件检索数据，InnoDB才使用行级锁，否则，InnoDB将使用表锁！

在MySQL中，行级锁并不是直接锁记录，而是锁索引。索引分为主键索引和非主键索引两种，如果一条sql语句操作了主键索引，MySQL就会锁定这条主键索引；如果一条语句操作了非主键索引，MySQL会先锁定该非主键索引，再锁定相关的主键索引。在UPDATE、DELETE操作时，MySQL不仅锁定WHERE条件扫描过的所有索引记录，而且会锁定相邻的键值，即所谓的next-key locking。

当两个事务同时执行，一个锁住了主键索引，在等待其他相关索引。另一个锁定了非主键索引，在等待主键索引。这样就会发生死锁。

发生死锁后，InnoDB一般都可以检测到，并使一个事务释放锁回退，另一个获取锁完成事务。

-- 明确指定主键，并且有此笔资料，row lock
SELECT * FROM products WHERE id='3' FOR UPDATE;
SELECT * FROM products WHERE id='3' and type=1 FOR UPDATE;

-- 明确指定主键，若查无此笔资料，无lock
SELECT * FROM products WHERE id='-1' FOR UPDATE;

-- 无主键，table lock
SELECT * FROM products WHERE name='Mouse' FOR UPDATE;

-- 主键不明确，table lock
SELECT * FROM products WHERE id<>'3' FOR UPDATE;
SELECT * FROM products WHERE id LIKE '3' FOR UPDATE;

表级锁

表级锁是MySQL中锁定粒度最大的一种锁，表示对当前操作的整张表加锁，它实现简单，资源消耗较少，被大部分MySQL引擎支持。最常使用的MYISAM与INNODB都支持表级锁定。表级锁定分为表共享读锁（共享锁）与表独占写锁（排他锁）。

页级锁

页级锁是MySQL中锁定粒度介于行级锁和表级锁中间的一种锁。表级锁速度快，但冲突多，行级冲突少，但速度慢。所以取了折衷的页级，一次锁定相邻的一组记录。BDB支持页级锁

InnoDB锁类型

Shared and Exclusive Locks

共享锁（S): 允许一个事务去读一行，阻止其他事务获得相同数据集的排他锁。
排他锁（X): 允许获得排他锁的事务更新数据，阻止其他事务取得相同数据集的共享读锁和排他写锁。

If transaction T1 holds a shared (S) lock on row r, then requests from some distinct transaction T2 for a lock on row r are handled as follows:

A request by T2 for an S lock can be granted immediately. As a result, both T1 and T2 hold an S lock on r.
A request by T2 for an X lock cannot be granted immediately.

If a transaction T1 holds an exclusive (X) lock on row r, a request from some distinct transaction T2 for a lock of either type on r cannot be granted immediately. Instead, transaction T2 has to wait for transaction T1 to release its lock on row r.

Intention Locks

Intention locks are table-level locks that indicate which type of lock (shared or exclusive) a transaction requires later for a row in a table.

意向共享锁（IS）: An intention shared lock (IS) indicates that a transaction intends to set a shared lock on individual rows in a table. Before a transaction can acquire a shared lock on a row in a table, it must first acquire an IS lock or stronger on the table. eg. SELECT ... LOCK IN SHARE MODE
意向排他锁（IX）: An intention exclusive lock (IX) indicates that a transaction intends to set an exclusive lock on individual rows in a table. Before a transaction can acquire an exclusive lock on a row in a table, it must first acquire an IX lock on the table. eg. SELECT ... FOR UPDATE

Intention locks do not block anything except full table requests (for example, LOCK TABLES ... WRITE). The main purpose of intention locks is to show that someone is locking a row, or going to lock a row in the table.

InnoDB行锁是通过给索引上的索引项加锁来实现的，这一点MySQL与Oracle不同，后者是通过在数据块中对相应数据行加锁来实现的。InnoDB这种行锁实现特点意味着：只有通过索引条件检索数据，InnoDB才使用行级锁，否则，InnoDB将使用表锁！

Record Locks

A record lock is a lock on an index record. For example, SELECT c1 FROM t WHERE c1 = 10 FOR UPDATE; prevents any other transaction from inserting, updating, or deleting rows where the value of t.c1 is 10.

Gap Locks

A gap lock is a lock on a gap between index records, or a lock on the gap before the first or after the last index record. For example, SELECT c1 FROM t WHERE c1 BETWEEN 10 and 20 FOR UPDATE; prevents other transactions from inserting a value of 15 into column t.c1, whether or not there was already any such value in the column, because the gaps between all existing values in the range are locked.

Next-Key Locks

A next-key lock is a combination of a record lock on the index record and a gap lock on the gap before the index record.

Suppose that an index contains the values 10, 11, 13, and 20. The possible next-key locks for this index cover the following intervals, where a round bracket denotes exclusion of the interval endpoint and a square bracket denotes inclusion of the endpoint:

(negative infinity, 10]
(10, 11]
(11, 13]
(13, 20]
(20, positive infinity)


By default, InnoDB operates in <span class="article-label">REPEATABLE READ</span> transaction isolation level. In this case, InnoDB uses next-key locks for searches and index scans, which prevents <span class="article-label">phantom rows</span>

Insert Intention Locks

AUTO-INC Locks

Predicate Locks for Spatial Indexes