DML會產(chǎn)生行鎖，只有該行的鎖解開后才可繼續(xù)對改行進行其他的DML，若新的DML和舊的DML操作的行相同，且舊DML以把行改變，則新DML不起作用
準備工作：
SQL>create table lck (a number, b number);
SQL>insert into lck values (1,1);
SQL>insert into lck values (2,2);
SQL>insert into lck values (3,3);
SQL>insert into lck values (4,4);
SQL>insert into lck values (5,5);
SQL>insert into lck values (6,6);
SQL>insert into lck values (7,7);
SQL>commit;
實驗過程：
| 步 驟 | Session 14 | Session 10 | 說明 |
| 1 | SQL> select sid from v$session
where audsid=userenv('SESSIONID');
SID
----------
14 | SQL> select sid from v$session
where audsid=userenv('SESSIONID');
SID
----------
10 | 獲得當前session的SID |
| 2 | SQL> insert into lck values (1000,1001);
1 row created. | | 未提交 |
| 3 | SQL> select sid,type,id1,lmode,request from v$lock
where sid in (10,14);
SID TY ID1 LMODE REQUEST
---------- -- ---------- ---------- ----------
14 TX 262153 6 0
14 TM 4145 3 0 | | 因為Session14的DML未提交，因此在v$lock里產(chǎn)生2個鎖，一個為：transaction lock(TX)，另一個為:DML/table lock(TM).LMODE=3代表：行排它.LMODE=6代表：對象排它 |
| 4 | SQL> insert into lck values (1001,1000);
1 row created. | | 未提交 |
| 5 | SQL> select sid,type,id1,lmode,request from v$lock
where sid in (10,14);
SID TY ID1 LMODE REQUEST
---------- -- ---------- ---------- ----------
14 TX 262153 6 0
14 TM 4145 3 0 | SQL> select * from lck;
A B
---------- ----------
1 2
2 4
3 6
4 8
5 3
6 5
7 7
7 rows selected. | v$lock中并不會因為對該表的再一次DML且未提交而新生成鎖 |
| 6 | | SQL>update lck set a=2000,b=2001
where a=1;
1 row updated. | 未提交 |
| 7 | SQL> select * from lck;
A B
---------- ----------
1 2
2 4
3 6
4 8
5 3
6 5
7 7
1000 1001
1001 1000
9 rows selected. | SQL> select sid,type,id1,lmode,request from v$lock
where sid in (10,14);
SID TY ID1 LMODE REQUEST
---------- -- ---------- ---------- ----------
10 TX 327698 6 0
10 TM 4145 3 0
14 TX 262153 6 0
14 TM 4145 3 0 | v$lock中對另外session10的DML產(chǎn)生了鎖 |
| 8 | SQL>update lck set a=4000,b=2001
where a=1; | | session10發(fā)出更新a=1行的SQL后，session14也發(fā)出更新a=1行的SQL，則后者的SQL懸在那里，無法結束 |
| 9 | SQL> select event, seconds_in_wait, sid from v$session_wait where sid in (10,14);
EVENT SECONDS_IN_WAIT SID
---------------------------------------------------------------- --------------- ----------
enq: TX - row lock contention 1593 14
SQL*Net message from client 2862 10 | v$session_wait中可以看到哪個session在等待，等待原因和已經(jīng)等待的時間 |
| 10 | 0 rows updated.
| SQL>commit;
Commit complete. | 只有在session10提交該DML后,session14才執(zhí)行完，但更新為0行 |
| 11 | SQL> select * from lck;
A B
---------- ----------
1 2
2 4
3 6
4 8
5 3
6 5
7 7
1000 1001
1001 1000
9 rows selected. | SQL> select * from lck;
A B
---------- ----------
1 2
2 4
3 6
4 8
5 3
6 5
7 7
1000 1001
1001 1000
9 rows selected. | |
筆記來源：
http://www.adp-gmbh.ch/ora/concepts/lock.html
posted on 2006-08-24 13:14
七匹狼 閱讀(1871)
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