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This creates a named point in the transaction that can be used in a subsequent ROLLBACK command The following table illustrates the number of rows in a table at various stages in a transaction The table is a very simple table called TAB, with one column
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Command Rows Visible to the User Rows Visible to Others
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truncate table tab; insert into tab values ('one'); savepoint first; insert into tab values ('two'); savepoint second; insert into tab values ('three'); rollback to savepoint second; rollback to savepoint first; commit; delete from tab; rollback;
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The example in the table shows two transactions: the first terminated with a COMMIT, the second with a ROLLBACK It can be seen that the use of savepoints is visible only within the transaction: other sessions see nothing that is not committed
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SELECT FOR UPDATE
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One last transaction control statement is SELECT FOR UPDATE Oracle, by default, provides the highest possible level of concurrency: readers do not block writers, and writers do not block readers Or in plain language, there is no problem with one
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session querying data that another session is updating, or one session updating data that another session is querying However, there are times when you may wish to change this behavior and prevent changes to data that is being queried It is not unusual for an application to retrieve a set of rows with a SELECT command, present them to a user for perusal, and prompt them for any changes Because Oracle is a multiuser database, it is not impossible that another session has also retrieved the same rows If both sessions attempt to make changes, there can be some rather odd effects The following table depicts such a situation
First User Second User
select * from regions;
select * from regions; delete from regions where region_id=5; commit;
update regions set region_name='GB'where region_id=5;
This is what the first user will see, from a SQL*Plus prompt:
SQL> select * from regions; REGION_ID REGION_NAME ---------- ------------------------5 UK 1 Europe 2 Americas 3 Asia 4 Middle East and Africa SQL> update regions set region_name='GB' where region_id=5; 0 rows updated
This is a bit disconcerting One way around this problem is to lock the rows in which one is interested:
select * from regions for update;
The FOR UPDATE clause will place a lock on all the rows retrieved No changes can be made to them by any session other than that which issued the command, and therefore the subsequent updates will succeed: it is not possible for the rows to have been changed This means that one session will have a consistent view of the data (it won t change), but the price to be paid is that other sessions will hang if they try to update any of the locked rows (they can, of course, query them) The locks placed by a FOR UPDATE clause will be held until the session issuing the command issues a COMMIT or ROLLBACK This must be done to release the locks, even if no DML commands have been executed
The So-Called Autocommit
To conclude this discussion of commit processing, it is necessary to remove any confusion about what is often called autocommit, or sometimes implicit commit You
8: DML and Concurrency
will often hear it said that in some situations Oracle will autocommit One of these situations is when doing DDL, which is described in the preceding section; another is when you exit from a user process such as SQL*Plus Quite simply, there is no such thing as an automatic commit When you execute a DDL statement, there is a perfectly normal COMMIT included in the source code that implements the DDL command But what about when you exit from your user process If you are using SQL*Plus on a Windows terminal and you issue a DML statement followed by an EXIT, your transaction will be committed This is because built into the SQL*Plus EXIT command there is a COMMIT statement But what if you click in the top-right corner of the SQL*Plus window The window will close, and if you log in again, you will see that the transaction has been rolled back This is because the programmers who wrote SQL*Plus for Microsoft Windows included a ROLLBACK statement in the code that is executed when you close the window The behavior of SQL*Plus on other platforms may well be different; the only way to be sure is to test it So whether you get an autocommit when you exit from a program in various ways is entirely dependent on how your programmers wrote your user process The Oracle server will simply do what it is told to do There is a SQL*Plus command SET AUTOCOMMIT ON This will cause SQL*Plus to modify its behavior: it will append a COMMIT to every DML statement issued So all statements are committed immediately as soon as they are executed and cannot be rolled back But this is happening purely on the user process side; there is still no autocommit in the database, and the changes made by a long-running statement will be isolated from other sessions until the statement completes Of course, a disorderly exit from SQL*Plus in these circumstances, such as killing it with an operating system utility while the statement is running, will be detected by PMON and the active transaction will always be rolled back Exercise 8-4: Manage Data Using DML In this exercise, you will demonstrate transaction isolation and control Use two SQL*Plus sessions (or SQL Developer if you prefer), each connected as user SYSTEM Run the commands in the steps that follow in the two sessions in the correct order
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