Oracle PL/SQL之SET TRANSACTION READ ONLY（事務隔離性）

SET TRANSACTION READ ONLY實際上是實現數據庫四大事務（ACID）中隔離性（Isolation）的一種手段，用來將數據的讀一致性定在某一時間點，即不管其他事務如何更改數據（不能在當前session中再使用自治事務），在當前事務中進行查詢的結果始終不變。由於Oracle的讀一致性是通過undo段來實現的，所以如果在此期間DML修改的數據量很大而undo空間設置過小可能會導致ORA-01555（快照過舊）錯誤。

 

Test Code:

Step 1, @session 1(SET TRANSACTION READ ONLY):

Connected to Oracle Database 11g Enterprise Edition Release 11.2.0.1.0
Connected as tuser1

SQL> select * from dept;

DEPTNO DNAME          LOC
------ -------------- -------------
    10 ACCOUNTING     NEW YORK01
    20 RESEARCH       DALLAS
    30 SALES          CHICAGO
    40 OPERATIONS     BOSTON

SQL> SET TRANSACTION READ ONLY;

Transaction set

Connected to Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 Connected as tuser1 SQL> select * from dept; DEPTNO DNAME LOC ------ -------------- ------------- 10 ACCOUNTING NEW YORK01 20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTON SQL> SET TRANSACTION READ ONLY; Transaction set

 

Step 2, @session 2(DML, insert a new record):

SQL> select * from dept;

DEPTNO DNAME          LOC
------ -------------- -------------
    10 ACCOUNTING     NEW YORK01
    20 RESEARCH       DALLAS
    30 SALES          CHICAGO
    40 OPERATIONS     BOSTON

SQL> insert into dept(deptno) values(50);

1 row inserted

SQL> commit;

Commit complete

SQL> select * from dept;

DEPTNO DNAME          LOC
------ -------------- -------------
    10 ACCOUNTING     NEW YORK01
    20 RESEARCH       DALLAS
    30 SALES          CHICAGO
    40 OPERATIONS     BOSTON
    50

SQL> select * from dept; DEPTNO DNAME LOC ------ -------------- ------------- 10 ACCOUNTING NEW YORK01 20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTON SQL> insert into dept(deptno) values(50); 1 row inserted SQL> commit; Commit complete SQL> select * from dept; DEPTNO DNAME LOC ------ -------------- ------------- 10 ACCOUNTING NEW YORK01 20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTON 50

 

Step 3, @session 1(query the same object and get the same result):

SQL> select * from dept;

DEPTNO DNAME          LOC
------ -------------- -------------
    10 ACCOUNTING     NEW YORK01
    20 RESEARCH       DALLAS
    30 SALES          CHICAGO
    40 OPERATIONS     BOSTON

SQL> select * from dept; DEPTNO DNAME LOC ------ -------------- ------------- 10 ACCOUNTING NEW YORK01 20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTON

 

Step 4, @session 1(autonomous transaction is not permitted):

SQL> declare
  2  pragma AUTONOMOUS_TRANSACTION;
  3  begin
  4  update dept set loc = loc || '-XXX' where deptno=20;
  5  end;
  6  /

declare
pragma AUTONOMOUS_TRANSACTION;
begin
update dept set loc = loc || '-XXX' where deptno=20;
end;

ORA-06519: active autonomous transaction detected and rolled back
ORA-06512: at line 6

SQL> select * from dept;

DEPTNO DNAME          LOC
------ -------------- -------------
    10 ACCOUNTING     NEW YORK01
    20 RESEARCH       DALLAS
    30 SALES          CHICAGO
    40 OPERATIONS     BOSTON

SQL>

SQL> declare 2 pragma AUTONOMOUS_TRANSACTION; 3 begin 4 update dept set loc = loc || '-XXX' where deptno=20; 5 end; 6 / declare pragma AUTONOMOUS_TRANSACTION; begin update dept set loc = loc || '-XXX' where deptno=20; end; ORA-06519: active autonomous transaction detected and rolled back ORA-06512: at line 6 SQL> select * from dept; DEPTNO DNAME LOC ------ -------------- ------------- 10 ACCOUNTING NEW YORK01 20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTON SQL>

 

Step 5, @session 1(commit current transaction and we get the change):

SQL> commit;

Commit complete

SQL> select * from dept;

DEPTNO DNAME          LOC
------ -------------- -------------
    10 ACCOUNTING     NEW YORK01
    20 RESEARCH       DALLAS
    30 SALES          CHICAGO
    40 OPERATIONS     BOSTON
    50