oracle multiple database link read only access error - database

Question

The query is like this
select 1 from dual#link1
union
select 1 from dual#link2
then i got the read only access error.
I have users in all those 3 databases and they are all read only users, so when i do the query "select 1 from dual#link1" then i got read only error, then i tried to change the query as
set transaction read only;
select 1 from dual#link1;
then it's solved.
then i tried the query as below:
select 1 from dual#link1
union
select 1 from dual#link2
it error again, i am not sure why 1 link works but 2 links won't work.
any one knows?

Answer 1

It is quite a surprising limitation of having the database in read-only mode, and multiple dblinks involved.
When you have read from the first dblink, you should close the current transaction (!) and only then read from the second dblink. This obviously prevents you from having a SELECT that joins tables from both dblinks.
It is documented, but rather hard to find. In Starting Up and Shutting Down you can see:
When executing on a read-only database, you must commit or roll back any in-progress transaction that involves one database link before you use another database link. This is true even if you execute a generic SELECT statement on the first database link and the transaction is currently read-only.
To reiterate the concept, if you have access to metalink, there is an oracle note in response to a customer which shows an example (Document 1296288.1) clarifying the limitation
SQL> select * from emp#link_emp_chicago;
select * from emp#link_emp_chicago
*
ERROR at line 1:
ORA-16000: database open for read-only access
Solution
SQL> select open_mode,database_role from v$database;
OPEN_MODE DATABASE_ROLE
---------- ----------------
READ ONLY PHYSICAL STANDBY
SQL> select owner,db_link from all_db_links;
OWNER
------------------------------
DB_LINK
PUBLIC
LINK_EMP_CHICAGO
SQL> select * from emp#link_emp_chicago;
select * from emp#link_emp_chicago
*
ERROR at line 1:
ORA-16000: database open for read-only access
SQL> set transaction read only;
Transaction set.
SQL> select * from emp#link_emp_chicago;
EMPNO ENAME JOB MGR HIREDATE SAL COMM
---------- ---------- --------- ---------- --------- ---------- ----------
DEPTNO
----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
Just to speculate, the reason may be related to the fact that, in the words of Tom Kyte
distributed stuff starts a transaction "just in case".