InnoDB支持行锁，有时升级为表锁。
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MyISAM只支持表锁。
表锁：开小小，加锁快，不会出现死锁；锁粒度大，锁冲突概率高，并发度低。
行锁：开销大，加锁慢，会出现死锁，锁粒度小，锁冲突概率低，并发高。
主要分为：读锁（共享锁），写锁（排他锁），意向锁，和MDL锁。
读锁，S锁，一个事物在读取一个数据行时，其他事务也可以读，但不能对该数据行增删改的操作。两种select方式的应用。
l 自动提交模式下的select查询语句，不需加任何锁返回结果，是一致性非锁定读。
l 通过select....lock in share mode在被读取的行记录或行记录的范围上加一个读锁，让其他事务可读不可申请加写锁。
写锁简称X锁，一个事务获取一行的写锁，其他事务就不能获取该行其它锁，优先级最高。
select for update，会对读取的行记录上加一个写锁，其他任何事务就不能加任何锁。
MySQL5.5引入meta data lock，简称MDL锁，用于保护表中元数据的信息。即一个事务查询表将自动给表加MDL锁，其他事务不能做任何DDL操作。
InnoDB引擎中，意向锁是表级锁，作用和MDL类似，防止事务进行过程中，执行DDL语句的操作而导致数据不一致。有两种意向锁类型：
l 意向共享锁（IS）：数据行加共享锁前必须先取得该表的IS锁。
l 意向排他锁（IX）：数据行加排他锁前必须先取得该表的IX锁。
InnoDB默认事务隔离级别为RR，且参数innodb_locks_unsafe_for_binlog=0的模式下，行锁有三种。
l 单个行记录的锁（record lock）,主键和唯一索引都是。
l 间隙锁（GAP lock）
l 记录锁和间隙锁的组合叫next-key lock。普通索引默认。
InnoDB上的行锁就是加在索引上。有索引，更新只锁指定行，无索引，更新锁所有行。
RR隔离级别，为了避免幻读，引入Gap lock，只锁定行记录数据的范围，不包含记录本身，即不允许在此范围内插入任何数据。
RC隔离级别允许出现幻读现象。
Next-key lock是记录锁（Record Lock）与间隔锁（Gap Lock）的组合，当InnoDB扫描索引记录时，会先对选中的索引记录加上记录锁（Record lock）,再对索引记录两边的间隙加上间隙锁（Gap lock）。
锁等待，是一个事务产生锁，其他事务等待上个事务释放它的锁。锁等待超时阈值innodb_lok_wait_timeout控制，单位秒。
死锁，多个事务争夺资源相互等待的现象，即锁资源请求产生了回路，就是死循环。
避免死锁的方法：
l 如不同的程序会并发存取多个表，或涉及多汗记录，尽量约定以相同的顺序访问表。
l 业务中尽量采用小事务，避免大事务，及时提交或回滚。
l 在同一个事务中，尽可能做到一次锁定所需的所有资源。
l 对容易产生死锁的业务，可以尝试使用升级锁粒度，通过表锁定减少锁产生的概率。
通过show engine innodb sttus查看死锁展示信息 。
[(none)]>show engine innodb status;
| Type | Name | Status
| InnoDB | |
=====================================
2018-11-07 22:49:40 0x7f1320202700 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 11 seconds
-----------------
BACKGROUND THREAD
-----------------
srv_master_thread loops: 98 srv_active, 0 srv_shutdown, 49465 srv_idle
srv_master_thread log flush and writes: 49563
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 20
OS WAIT ARRAY INFO: signal count 20
RW-shared spins 0, rounds 34, OS waits 16
RW-excl spins 0, rounds 200, OS waits 2
RW-sx spins 0, rounds 0, OS waits 0
Spin rounds per wait: 34.00 RW-shared, 200.00 RW-excl, 0.00 RW-sx
------------
TRANSACTIONS
------------
Trx id counter 65440
Purge done for trx's n:o < 65438 undo n:o < 0 state: running but idle
History list length 12
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 421197684710112, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
---TRANSACTION 421197684709200, not started
0 lock struct(s), heap size 1136, 0 row lock(s)
--------
FILE I/O
--------
I/O thread 0 state: waiting for completed aio requests (insert buffer thread)
I/O thread 1 state: waiting for completed aio requests (log thread)
I/O thread 2 state: waiting for completed aio requests (read thread)
I/O thread 3 state: waiting for completed aio requests (read thread)
I/O thread 4 state: waiting for completed aio requests (read thread)
I/O thread 5 state: waiting for completed aio requests (read thread)
I/O thread 6 state: waiting for completed aio requests (read thread)
I/O thread 7 state: waiting for completed aio requests (read thread)
I/O thread 8 state: waiting for completed aio requests (read thread)
I/O thread 9 state: waiting for completed aio requests (read thread)
I/O thread 10 state: waiting for completed aio requests (write thread)
I/O thread 11 state: waiting for completed aio requests (write thread)
I/O thread 12 state: waiting for completed aio requests (write thread)
I/O thread 13 state: waiting for completed aio requests (write thread)
I/O thread 14 state: waiting for completed aio requests (write thread)
I/O thread 15 state: waiting for completed aio requests (write thread)
I/O thread 16 state: waiting for completed aio requests (write thread)
I/O thread 17 state: waiting for completed aio requests (write thread)
Pending normal aio reads: [0, 0, 0, 0, 0, 0, 0, 0] , aio writes: [0, 0, 0, 0, 0, 0, 0, 0] ,
ibuf aio reads:, log i/o's:, sync i/o's:
Pending flushes (fsync) log: 0; buffer pool: 0
271 OS file reads, 61118 OS file writes, 60451 OS fsyncs
0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf: size 1, free list len 0, seg size 2, 0 merges
merged operations:
insert 0, delete mark 0, delete 0
discarded operations:
insert 0, delete mark 0, delete 0
Hash table size 553193, node heap has 0 buffer(s)
Hash table size 553193, node heap has 0 buffer(s)
Hash table size 553193, node heap has 0 buffer(s)
Hash table size 553193, node heap has 0 buffer(s)
Hash table size 553193, node heap has 0 buffer(s)
Hash table size 553193, node heap has 1 buffer(s)
Hash table size 553193, node heap has 0 buffer(s)
Hash table size 553193, node heap has 0 buffer(s)
0.00 hash searches/s, 0.00 non-hash searches/s
---
LOG
---
Log sequence number 20357616
Log flushed up to 20357616
Pages flushed up to 20357616
Last checkpoint at 20357607
0 pending log flushes, 0 pending chkp writes
60048 log i/o's done, 0.00 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total large memory allocated 2198863872
Dictionary memory allocated 156387
Buffer pool size 131056
Free buffers 130465
Database pages 590
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 238, created 352, written 805
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 590, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
----------------------
INDIVIDUAL BUFFER POOL INFO
----------------------
---BUFFER POOL 0
Buffer pool size 16382
Free buffers 16279
Database pages 102
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 38, created 64, written 95
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 102, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 1
Buffer pool size 16382
Free buffers 16312
Database pages 70
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 6, created 64, written 64
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 70, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 2
Buffer pool size 16382
Free buffers 16319
Database pages 63
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 8, created 55, written 56
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 63, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 3
Buffer pool size 16382
Free buffers 16303
Database pages 79
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 73, created 6, written 59
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 79, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 4
Buffer pool size 16382
Free buffers 16265
Database pages 117
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 76, created 41, written 120
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 117, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 5
Buffer pool size 16382
Free buffers 16307
Database pages 75
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 11, created 64, written 91
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 75, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 6
Buffer pool size 16382
Free buffers 16363
Database pages 19
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 13, created 6, written 12
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 19, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
---BUFFER POOL 7
Buffer pool size 16382
Free buffers 16317
Database pages 65
Old database pages 0
Modified db pages 0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 0, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 13, created 52, written 308
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 65, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
--------------
ROW OPERATIONS
--------------
0 queries inside InnoDB, 0 queries in queue
0 read views open inside InnoDB
Process ID=21556, Main thread ID=139720374998784, state: sleeping
Number of rows inserted 60824, updated 0, deleted 0, read 121836
0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
============================
通过show full processlist和show engine Innodb status来判断事务中锁问题情况，另外还有三张表可查：
information_schema.INNODB_TRX
information_schema.INNODB_LOCKS
information_schema.INNODB_LOCK_WAITS
innodb_trx表部分字段
trx_id唯一的事务id号；
trx_state:事务状态；
trx_wait_started:事务开始等待时间。
trx_mysql_thread_id:线程ID，与show full processlist相互对应。
trx_query:事务运行的SQL;
trx_operation_state:事务运行的状态。
[(none)]>show full processlist;
+----+------+-----------+------+---------+------+----------+-----------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+----------+-----------------------+
| 57 | root | localhost | NULL | Query | 0 | starting | show full processlist |
+----+------+-----------+------+---------+------+----------+-----------------------+
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