Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 8 ■ TRANSACTIONS
Transaction Control Statements
There is no “begin transaction” statement needed in Oracle. A transaction implicitly begins
with the first statement that modifies data (the first statement that gets a TX lock). You may
explicitly begin a transaction using SET TRANSACTION or the DBMS_TRANSACTION package, but it is
not a necessary step, unlike in various other databases. Issuing either a COMMIT or ROLLBACK
statement explicitly ends a transaction.
■Note A ROLLBACK TO SAVEPOINT command will not end a transaction! Only a full, proper ROLLBACK
will.
You should always explicitly terminate your transactions with a COMMIT or ROLLBACK; otherwise,
the tool/environment you are using will pick one or the other for you. If you exit your
SQL*Plus session normally, without committing or rolling back, SQL*Plus will assume you
wish you commit your work and will do so for you. If you just exit from a Pro*C program, on
the other hand, an implicit rollback will take place. Never rely on implicit behavior, as it could
change in the future. Always explicitly COMMIT or ROLLBACK your transactions.
Transactions are atomic in Oracle, meaning that either every statement that comprises
the transaction is committed (made permanent) or all of the statements are rolled back. This
protection is extended to individual statements as well. Either a statement entirely succeeds
or the statement is entirely rolled back. Note that I said the “statement” is rolled back. The
failure of one statement does not cause previously executed statements to be automatically
rolled back. Their work is preserved and must be either committed or rolled back by you.
Before we get into the details of exactly what it means for a statement and transaction to be
atomic, let’s take a look at the various transaction control statements available to us:
• COMMIT: To use this statement’s simplest form, you would just issue COMMIT. You could
be more verbose and say COMMIT WORK, but the two are equivalent. A COMMIT ends your
transaction and makes any changes permanent (durable). There are extensions to the
COMMIT statement used in distributed transactions. These extensions allow you to label a
COMMIT (label a transaction) with some meaningful comment and force the commit of
an in-doubt distributed transaction.
• ROLLBACK: To use this statement’s simplest form, you would just issue ROLLBACK. Again,
you could be more verbose and say ROLLBACK WORK, but the two are equivalent. A rollback
ends your transaction and undoes any uncommitted changes you have outstanding. It
does this by reading information stored in the rollback/undo segments (going forward
I’ll refer to these exclusively as undo segments, the favored terminology for Oracle 10g)
and restoring the database blocks to the state they were in prior to your transaction
beginning.
• SAVEPOINT: A SAVEPOINT allows you to create a “marked point” within a transaction.
You may have multiple SAVEPOINTs within a single transaction.