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):

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

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

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> 

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