一、Oracle的锁
锁是Oracle管理共享数据库资源并发访问并防止并发数据库事务之间“相互干涉”的核心机制。
1、锁的类型
(1)DML锁:允许并发执行数据修改,特定数据行上的锁或表中的所有行的锁。
(2)DDL锁:保护对象结构定义。
(3)Latch锁:这是Oracle内部锁,用来协调对期共享数据结构的访问。
2、LOCK锁的模式
(1)空锁(NULL)
(2)排它锁(X)
(3)共享锁(S)
二、DML锁
1、TX锁:修改数据的事务在执行期间会获得这种锁。
TX锁的模式:排它锁(X)
TX锁和行级锁:TX锁不是行锁,一个事务不管修改了多少行,都只会有一个TX锁。TX锁算是行锁的代表,行锁上发生了等待,会表 现为TX锁的等待。行锁是属于事务的,事务开始,行锁产生,事务结束,行锁也被释放。
两个会话时DML操作同一行数据,会产生阻塞,操作如下:
会话125号,更新id=1这行数据。
gyj@OCM> select distinct sid from v$mystat;
SID
----------
125
gyj@OCM> update t5 set name='aaaaaa' where id=1;
1 row updated.
事务未提交!!
会话145,同时也更新id=1这行数据。
gyj@OCM> select distinct sid from v$mystat;
SID
----------
145
gyj@OCM> update t5 set name='bbbbb' where id=1;
这里被阻塞了!
查v$lock视图
sys@OCM>select TYPE,ID1,ID2,LMODE,REQUEST,CTIME,BLOCK from v$lock where sidin(125,145);
TY ID1 ID2LMODE REQUEST CTIMEBLOCK
-- ---------- ---------- ---------- ---------- ---------- ----------
AE 100 0 4 0 1647 0
AE 100 0 4 0 682 0
TX 65539 2978 0 6 770
TO 65908 1 3 0 676 0
TM 74709 0 3 0 77 0
TM 74709 0 3 0 206 0
TX 65539 2978 6 0206 1
TX锁的标识:TX-ID1-ID2 --ID1表示事务使用的回滚段编号以及在事务表中对应的记录编号,ID2表示该记录编号被重用的次数(wrap)
将ID1拆解:ID1是由4个字节组成的,高位2个字节和低位2个字节分别表示v$trasaction中的xidusn,xidslot
selecttrunc(id1/power(2,16)) as undo_blk#,bitand(id1,to_number('ffff','xxxx')) + 0 asslot# from dual;
另一计算方法:selectto_char(id1,'xxxxxxx') from dual;
查询持有锁和请求锁的信息:v$lock
gyj@OCM> desc v$lock
名称是否为空?类型
----------------- -------- ------------
ADDR RAW(4):锁状态对象的地址V$transaction.addr V$session.taddr这个其实是事务地址
KADDR RAW(4):锁的地址。此列是真正的锁地址。V$session.lockwait
SID NUMBER:持有或获得锁的会话
TYPE VARCHAR2(2):用户或系统锁的类型
ID1 NUMBER:锁的标识
ID2 NUMBER:锁的标识
LMODE NUMBER:会话持有的锁的模式,0 - none、1 - null(NULL)、2 - row-S (SS)、3 - row-X (SX)、4 - share (S)、5 - S/Row-X (SSX)、6 - exclusive (X)
REQUEST NUMBER:进程正在请求锁的模式,模式类型同上。
CTIME NUMBER:当前模式持续的时间
BLOCK NUMBER:当前锁是否正在阻塞其他的锁(1:是,0:不是)
系统进程也可能持有锁,系统锁被持有很短的时间,系统锁有如下类型:
System Type |
Description |
System Type |
Description |
AT |
Lock held for the ALTER TABLE statement |
NA..NZ |
Library cache pin instance (A..Z = namespace) |
BL |
Buffer hash table instance |
PF |
Password File |
CF |
Control file schema global enqueue |
PI, PS |
Parallel operation |
CI |
Cross-instance function invocation instance |
PR |
Process startup |
CU |
Cursor bind |
QA..QZ |
Row cache instance (A..Z = cache) |
DF |
datafile instance |
RT |
Redo thread global enqueue |
DL |
Direct loader parallel index create |
SC |
System change number instance |
DM |
Mount/startup db primary/secondary instance |
SM |
SMON |
DR |
Distributed recovery process |
SN |
Sequence number instance |
DX |
Distributed transaction entry |
SQ |
Sequence number enqueue |
FS |
File set |
SS |
Sort segment |
HW |
Space management operations on a specific segment |
ST |
Space transaction enqueue |
IN |
Instance number |
SV |
Sequence number value |
IR |
Instance recovery serialization global enqueue |
TA |
Generic enqueue |
IS |
Instance state |
TS |
Temporary segment enqueue (ID2=0) |
IV |
Library cache invalidation instance |
TS |
New block allocation enqueue (ID2=1) |
JQ |
Job queue |
TT |
Temporary table enqueue |
KK |
Thread kick |
UN |
User name |
LA .. LP |
Library cache lock instance lock (A..P = namespace) |
US |
Undo segment DDL |
MM |
Mount definition global enqueue |
WL |
Being-written redo log instance |
MR |
Media recovery |
2、TM锁:表级锁
TM锁的主要作用,在DML期间,防止对表的DDL操作,可以确保对象的结构不被修改,例如,如果你已经更新了一个表,会得到这个表的一个TM锁。这会防止另一个用户在该表上执行DROP或ALTER命令。如果你有表的一个TM锁,而另一个用户试图在这个表上执行DDL,操作如下,他不会得到以下错误消息:
会话一
gyj@OCM> select distinct sid from v$mystat;
SID
----------
125
gyj@OCM> delete from t5 where id=1;
1 row deleted.
事务没提交!
会话二:
gyj@OCM> drop table t5;
drop table t5
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified ortimeout expired
查v$lock视图
gyj@OCM> select TYPE,ID1,ID2,LMODE,REQUEST,CTIME,BLOCK fromv$lock where sid=125;
TY ID1 ID2LMODE REQUEST CTIMEBLOCK
-- ---------- ---------- ---------- ---------- ---------- ----------
AE 100 0 4 0 1171 0
TM 74709 0 3 0 241 0
TX 196641 3996 6 0 241 0
锁的标识:TM-ID1-ID2 --ID1表示被锁定的对象的对象ID,ID2始终为0
TM锁的模式:
代码 |
锁模式 |
表 |
行 |
0-None |
没有锁 |
- |
- |
1-Null |
空锁 |
- |
- |
2-SS(RS) |
行级共享锁。Row Share |
- |
共享 |
3-SX(RX) |
行级排它锁。Row Exclusive |
- |
排他 |
4-S |
表共享锁Share |
共享 |
- |
5-SSX(SRX) |
表共享行排它锁。Share Row Exclusive |
共享 |
排他 |
6-X |
表排它锁。Exclusive |
排他 |
- |
通过lock tablet10 in row share mode;命令添加RS锁
通过lock tablet10 in row exclusive mode;命令对表添加RX锁定;
通过select … from forupdate命令添加RX锁
通过lock tablet10 in share mode;命令添加该S锁(wait for ITL release)
通过lock tablet10 in share row exclusive mode;命令添加SRX锁
通过lock tablet10 in exclusive mode命令添加X锁
语法:lock table in [row share][rowexclusive][share][share row exclusive][exclusive] mode;
各种TM锁的兼容性:
持有/得到 |
1-Null |
2-SS(RS) |
3-SX(RX) |
4-S |
5-SSX(SRX) |
6-X |
1-Null |
√ |
√ |
√ |
√ |
√ |
√ |
2-SS(RS) |
√ |
√ |
√ |
√ |
√ |
|
3-SX(RX) |
√ |
√ |
√ |
|
|
|
4-S |
√ |
√ |
|
√ |
|
|
5-SSX(SRX) |
√ |
√ |
|
|
|
|
6-X |
√ |
|
|
|
|
|
二、DDL锁
(省,单独讲)
三、Latch锁
(省,单独讲)
四、相关查询锁的语句
1、查询持有锁和请求锁的信息:v$lock
sys@OCM> selectsid,type,id1,id2,decode(lmode,0,'None',1,'Null',2,'Row share',3,'RowExclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive')lock_mode,decode(request,0,'None',1,'Null',2,'Row share',3,'RowExclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive') request_mode,blockfrom v$lock where sid in(125,145);
SID TY ID1 ID2 LOCK_MODE REQUEST_MODE BLOCK
---------- -- ---------- ----------------------------- ------------------- ----------
125 AE 100 0 Share None 0
145 AE 100 0 Share None 0
145 TX 65539 2978 None Exclusive 0
145 TO 659081 Row Exclusive None 0
145 TM 74709 0 Row Exclusive None 0
125 TM 74709 0 Row Exclusive None 0
125 TX 65539 2978 Exclusive None
2、查询请求锁的信息:v$enqueue_lock
sys@OCM>select sid,type,decode(request,0,'None',1,'Null',2,'Row share',3,'RowExclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive') request_mode fromv$enqueue_lock where sid in(125,145);
SID TY REQUEST_MODE
---------- -- -------------------
125 AE None
145 AE None
145 TX Exclusive
145 TO None
3、查询请求锁时间过长的信息
sys@OCM> select a.sidblocker_sid,a.serial#,a.username asblocker_username,b.type,decode(b.lmode,0,'None',1,'Null',2,'Row share',3,'RowExclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive') lock_mode,b.ctimeas time_held,c.sid as waiter_sid,decode(c.request,0,'None',1,'Null',2,'Rowshare',3,'Row Exclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive')request_mode,c.ctime time_waited fromv$lock b, v$enqueue_lock c, v$session a
2 where a.sid = b.sid and b.id1= c.id1(+) and b.id2 = c.id2(+) andc.type(+) = 'TX' and b.type = 'TX'and b.block = 1
3 order by time_held,time_waited;
BLOCKER_SID SERIAL# BLOCKER_USERNAME TY LOCK_MODE TIME_HELD WAITER_SID REQUEST_MODE TIME_WAITED
----------- ---------------------------------------- -- ------------------- ---------- ----------------------------- -----------
125 9 GYJ TX Exclusive 605 145 Exclusive 476
把125号会话给KILL掉,操作命令如下:
alter system kill session '125,9';
五、死锁
1、两个session(以A和B来表示),如果A持有B正在申请的锁定,同时B也持有A正在申请的锁定时,这时发生死锁现象。
操作如下:
session A
update t1 set name='A1' where id=1;
session B
update t1 set name='B1' where id=2;
session A
update t1 set name='B2' where id=2;
session B
update t1 set name='A2' where id=1;
alter_
2、实际举个产生死锁的例子
步骤一:通过主外键创建班级代码表和学生基本信息表。
gyj@OCM> create table bjdmb(bjdm number(8) primary key,bjmc varchar2(20));
Table created.
gyj@OCM> create table xsjbxxb(xh number(10),xm varchar2(10),bjdm number(8),
2 foreign key(bjdm) references bjdmb(bjdm) on delete cascade,primary key(xh));
Table created.
步骤二:加载数据
gyj@OCM> insert into bjdmb values(01,'bj01');
1 row created.
gyj@OCM>insert into bjdmb values(02,'bj02');
1 row created.
gyj@OCM> insert into bjdmb values(03,'bj03');
1 row created.
gyj@OCM>insert into bjdmb values(04,'bj04');
1 row created.
gyj@OCM>commit;
Commit complete.
gyj@OCM> insert into xsjbxxb values(001,'tom',01);
1 row created.
gyj@OCM> insert into xsjbxxb values(002,'joe',02);
1 row created.
gyj@OCM> commit;
Commit complete.
步骤三:产生死锁
会话A,插入学生一条信息记录:
gyj@OCM>insert into xsjbxxb values(00,'joe',03);
1 row created.
会话B:删除4班信息
gyj@OCM> delete from bjdmb where bjdm=04;
会话B这时被阻塞了!!!!!
步骤四:回滚会话A和会话B
会话A回滚:
gyj@OCM> insert into xsjbxxbvalues(00,'joe',03);
1 row created.
gyj@OCM> rollback;
Rollback complete.
会话B回滚:
gyj@OCM> delete from bjdmb wherebjdm=04;
1 row deleted.
gyj@OCM> rollback;
Rollback complete.
步骤五:在外键上创建索引
gyj@OCM> create index idx_xsjbxxb_bjdm onxsjbxxb(bjdm);
Index created.
步骤六:然后重复执行步骤三的操作:
会话A上操作:
gyj@OCM> insert into xsjbxxbvalues(00,'joe',03);
1 row created.
会话B上操作:
gyj@OCM> delete from bjdmb wherebjdm=04;
1 row deleted.
没有被阻塞!!!!
死锁99%是外键上没加索引原因!!!!!!!!!!!!!
最后总结:外键要不需要加索引的情况
A)没有从交表删除行
B)没有更新父表的惟一键/主键值
C)没有从父表联结子表
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