How To Set a Transaction To Be READ ONLY? - Oracle DBA FAQ - Understanding SQL Transaction Management

If you want a transaction to be set as READ ONLY, you need to the transaction with the SET TRANSACTION READ ONLY statement. Note that a DML statement will start the transaction automatically. So you have to issue the SET TRANSACTION statement before any DML statements. The tutorial exercise below shows you a good example of READ ONLY transaction:

SQL> connect HR/fyicenter

SQL> SET TRANSACTION READ ONLY;
Transaction set.

SQL> SELECT * FROM fyi_links;
     ID URL              NOTES          COUNTS CREATED
------- ---------------- ---------- ---------- ---------
    101 FYICENTER.COM                          07-MAY-06
    110 CENTERFYI.COM                          07-MAY-06
    112 oracle.com                             07-MAY-06
    113 sql.com                                07-MAY-06

Keep the "HR" SQL*Plus window as is, and open another window to run another instance of SQL*Plus.

>cd (OracleXE home directory)
>.\bin\sqlplus /nolog

SQL> connect SYSTEM/password
Connected.

SQL>  DELETE FROM hr.fyi_links where id = 112;
1 row deleted.

SQL>  DELETE FROM hr.fyi_links where id = 113;
1 row deleted.

SQL> COMMIT;
Commit complete.

Go back to the "HR" SQL*Plus window.

SQL> SELECT * FROM fyi_links;
     ID URL              NOTES          COUNTS CREATED
------- ---------------- ---------- ---------- ---------
    101 FYICENTER.COM                          07-MAY-06
    110 CENTERFYI.COM                          07-MAY-06
    112 oracle.com                             07-MAY-06
    113 sql.com                                07-MAY-06

SQL> COMMIT;
Commit complete.

SQL> SELECT * FROM fyi_links;
     ID URL              NOTES          COUNTS CREATED
------- ---------------- ---------- ---------- ---------
    101 FYICENTER.COM                          07-MAY-06
    110 CENTERFYI.COM                          07-MAY-06

As you can see that two records were deleted from another session after the HR session started the READ ONLY transaction. The deleted records was not impacting any query statements until the transaction was ended with the COMMIT statement.