How To Set a Transaction To Be READ ONLY?

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.