Oracle PL/SQL之SET TRANSACTION READ ONLY（事务隔离性）

SET TRANSACTION READ ONLY实际上是实现数据库四大事务（ACID）中隔离性（Isolation）的一种手段，用来将数据的读一致性定在某一时间点，即不管其他事务如何更改数据（不能在当前session中再使用自治事务），在当前事务中进行查询的结果始终不变。由于Oracle的读一致性是通过undo段来实现的，所以如果在此期间DML修改的数据量很大而undo空间设置过小可能会导致ORA-01555（快照过旧）错误。

Test Code:

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

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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):

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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):

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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):

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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):

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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