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InnoDB支持行鎖，有時(shí)升級(jí)為表鎖。
MyISAM只支持表鎖。
表鎖：開小小，加鎖快，不會(huì)出現(xiàn)死鎖；鎖粒度大，鎖沖突概率高，并發(fā)度低。
行鎖：開銷大，加鎖慢，會(huì)出現(xiàn)死鎖，鎖粒度小，鎖沖突概率低，并發(fā)高。
主要分為：讀鎖（共享鎖），寫鎖（排他鎖），意向鎖，和MDL鎖。
讀鎖，S鎖，一個(gè)事物在讀取一個(gè)數(shù)據(jù)行時(shí)，其他事務(wù)也可以讀，但不能對(duì)該數(shù)據(jù)行增刪改的操作。兩種select方式的應(yīng)用。
l 自動(dòng)提交模式下的select查詢語(yǔ)句，不需加任何鎖返回結(jié)果，是一致性非鎖定讀。
l 通過select....lock in share mode在被讀取的行記錄或行記錄的范圍上加一個(gè)讀鎖，讓其他事務(wù)可讀不可申請(qǐng)加寫鎖。
寫鎖簡(jiǎn)稱X鎖，一個(gè)事務(wù)獲取一行的寫鎖，其他事務(wù)就不能獲取該行其它鎖，優(yōu)先級(jí)最高。
select for update，會(huì)對(duì)讀取的行記錄上加一個(gè)寫鎖，其他任何事務(wù)就不能加任何鎖。
mysql5.5引入meta data lock，簡(jiǎn)稱MDL鎖，用于保護(hù)表中元數(shù)據(jù)的信息。即一個(gè)事務(wù)查詢表將自動(dòng)給表加MDL鎖，其他事務(wù)不能做任何DDL操作。
InnoDB引擎中，意向鎖是表級(jí)鎖，作用和MDL類似，防止事務(wù)進(jìn)行過程中，執(zhí)行DDL語(yǔ)句的操作而導(dǎo)致數(shù)據(jù)不一致。有兩種意向鎖類型：
l 意向共享鎖（IS）：數(shù)據(jù)行加共享鎖前必須先取得該表的IS鎖。
l 意向排他鎖（IX）：數(shù)據(jù)行加排他鎖前必須先取得該表的IX鎖。
InnoDB默認(rèn)事務(wù)隔離級(jí)別為RR，且參數(shù)innodb_locks_unsafe_for_binlog=0的模式下，行鎖有三種。
l 單個(gè)行記錄的鎖（record lock）,主鍵和唯一索引都是。
l 間隙鎖（GAP lock）
l 記錄鎖和間隙鎖的組合叫next-key lock。普通索引默認(rèn)。
InnoDB上的行鎖就是加在索引上。有索引，更新只鎖指定行，無索引，更新鎖所有行。
RR隔離級(jí)別，為了避免幻讀，引入Gap lock，只鎖定行記錄數(shù)據(jù)的范圍，不包含記錄本身，即不允許在此范圍內(nèi)插入任何數(shù)據(jù)。
RC隔離級(jí)別允許出現(xiàn)幻讀現(xiàn)象。
Next-key lock是記錄鎖（Record Lock）與間隔鎖（Gap Lock）的組合，當(dāng)InnoDB掃描索引記錄時(shí)，會(huì)先對(duì)選中的索引記錄加上記錄鎖（Record lock）,再對(duì)索引記錄兩邊的間隙加上間隙鎖（Gap lock）。
鎖等待，是一個(gè)事務(wù)產(chǎn)生鎖，其他事務(wù)等待上個(gè)事務(wù)釋放它的鎖。鎖等待超時(shí)閾值innodb_lok_wait_timeout控制，單位秒。
死鎖，多個(gè)事務(wù)爭(zhēng)奪資源相互等待的現(xiàn)象，即鎖資源請(qǐng)求產(chǎn)生了回路，就是死循環(huán)。
避免死鎖的方法：
l 如不同的程序會(huì)并發(fā)存取多個(gè)表，或涉及多汗記錄，盡量約定以相同的順序訪問表。
l 業(yè)務(wù)中盡量采用小事務(wù)，避免大事務(wù)，及時(shí)提交或回滾。
l 在同一個(gè)事務(wù)中，盡可能做到一次鎖定所需的所有資源。
l 對(duì)容易產(chǎn)生死鎖的業(yè)務(wù)，可以嘗試使用升級(jí)鎖粒度，通過表鎖定減少鎖產(chǎn)生的概率。
通過show engine innodb sttus查看死鎖展示信息 。
[(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
============================
通過show full processlist和show engine Innodb status來判斷事務(wù)中鎖問題情況，另外還有三張表可查：
information_schema.INNODB_TRX
information_schema.INNODB_LOCKS
information_schema.INNODB_LOCK_WAITS
innodb_trx表部分字段
trx_id唯一的事務(wù)id號(hào)；
trx_state:事務(wù)狀態(tài)；
trx_wait_started:事務(wù)開始等待時(shí)間。
trx_mysql_thread_id:線程ID，與show full processlist相互對(duì)應(yīng)。
trx_query:事務(wù)運(yùn)行的SQL;
trx_operation_state:事務(wù)運(yùn)行的狀態(tài)。
[(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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