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><H1
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><A
NAME="TUTORIAL-TRANSACTIONS"
>3.4. Transactions</A
></H1
><P
> <I
CLASS="FIRSTTERM"
>Transactions</I
> are a fundamental concept of all database
systems. The essential point of a transaction is that it bundles
multiple steps into a single, all-or-nothing operation. The intermediate
states between the steps are not visible to other concurrent transactions,
and if some failure occurs that prevents the transaction from completing,
then none of the steps affect the database at all.
</P
><P
> For example, consider a bank database that contains balances for various
customer accounts, as well as total deposit balances for branches.
Suppose that we want to record a payment of $100.00 from Alice's account
to Bob's account. Simplifying outrageously, the SQL commands for this
might look like:
</P><PRE
CLASS="PROGRAMLISTING"
>UPDATE accounts SET balance = balance - 100.00
WHERE name = 'Alice';
UPDATE branches SET balance = balance - 100.00
WHERE name = (SELECT branch_name FROM accounts WHERE name = 'Alice');
UPDATE accounts SET balance = balance + 100.00
WHERE name = 'Bob';
UPDATE branches SET balance = balance + 100.00
WHERE name = (SELECT branch_name FROM accounts WHERE name = 'Bob');</PRE
><P>
</P
><P
> The details of these commands are not important here; the important
point is that there are several separate updates involved to accomplish
this rather simple operation. Our bank's officers will want to be
assured that either all these updates happen, or none of them happen.
It would certainly not do for a system failure to result in Bob
receiving $100.00 that was not debited from Alice. Nor would Alice long
remain a happy customer if she was debited without Bob being credited.
We need a guarantee that if something goes wrong partway through the
operation, none of the steps executed so far will take effect. Grouping
the updates into a <I
CLASS="FIRSTTERM"
>transaction</I
> gives us this guarantee.
A transaction is said to be <I
CLASS="FIRSTTERM"
>atomic</I
>: from the point of
view of other transactions, it either happens completely or not at all.
</P
><P
> We also want a
guarantee that once a transaction is completed and acknowledged by
the database system, it has indeed been permanently recorded
and won't be lost even if a crash ensues shortly thereafter.
For example, if we are recording a cash withdrawal by Bob,
we do not want any chance that the debit to his account will
disappear in a crash just after he walks out the bank door.
A transactional database guarantees that all the updates made by
a transaction are logged in permanent storage (i.e., on disk) before
the transaction is reported complete.
</P
><P
> Another important property of transactional databases is closely
related to the notion of atomic updates: when multiple transactions
are running concurrently, each one should not be able to see the
incomplete changes made by others. For example, if one transaction
is busy totalling all the branch balances, it would not do for it
to include the debit from Alice's branch but not the credit to
Bob's branch, nor vice versa. So transactions must be all-or-nothing
not only in terms of their permanent effect on the database, but
also in terms of their visibility as they happen. The updates made
so far by an open transaction are invisible to other transactions
until the transaction completes, whereupon all the updates become
visible simultaneously.
</P
><P
> In <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, a transaction is set up by surrounding
the SQL commands of the transaction with
<TT
CLASS="COMMAND"
>BEGIN</TT
> and <TT
CLASS="COMMAND"
>COMMIT</TT
> commands. So our banking
transaction would actually look like:
</P><PRE
CLASS="PROGRAMLISTING"
>BEGIN;
UPDATE accounts SET balance = balance - 100.00
WHERE name = 'Alice';
-- etc etc
COMMIT;</PRE
><P>
</P
><P
> If, partway through the transaction, we decide we do not want to
commit (perhaps we just noticed that Alice's balance went negative),
we can issue the command <TT
CLASS="COMMAND"
>ROLLBACK</TT
> instead of
<TT
CLASS="COMMAND"
>COMMIT</TT
>, and all our updates so far will be canceled.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> actually treats every SQL statement as being
executed within a transaction. If you do not issue a <TT
CLASS="COMMAND"
>BEGIN</TT
>
command,
then each individual statement has an implicit <TT
CLASS="COMMAND"
>BEGIN</TT
> and
(if successful) <TT
CLASS="COMMAND"
>COMMIT</TT
> wrapped around it. A group of
statements surrounded by <TT
CLASS="COMMAND"
>BEGIN</TT
> and <TT
CLASS="COMMAND"
>COMMIT</TT
>
is sometimes called a <I
CLASS="FIRSTTERM"
>transaction block</I
>.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Some client libraries issue <TT
CLASS="COMMAND"
>BEGIN</TT
> and <TT
CLASS="COMMAND"
>COMMIT</TT
>
commands automatically, so that you might get the effect of transaction
blocks without asking. Check the documentation for the interface
you are using.
</P
></BLOCKQUOTE
></DIV
><P
> It's possible to control the statements in a transaction in a more
granular fashion through the use of <I
CLASS="FIRSTTERM"
>savepoints</I
>. Savepoints
allow you to selectively discard parts of the transaction, while
committing the rest. After defining a savepoint with
<TT
CLASS="COMMAND"
>SAVEPOINT</TT
>, you can if needed roll back to the savepoint
with <TT
CLASS="COMMAND"
>ROLLBACK TO</TT
>. All the transaction's database changes
between defining the savepoint and rolling back to it are discarded, but
changes earlier than the savepoint are kept.
</P
><P
> After rolling back to a savepoint, it continues to be defined, so you can
roll back to it several times. Conversely, if you are sure you won't need
to roll back to a particular savepoint again, it can be released, so the
system can free some resources. Keep in mind that either releasing or
rolling back to a savepoint
will automatically release all savepoints that were defined after it.
</P
><P
> All this is happening within the transaction block, so none of it
is visible to other database sessions. When and if you commit the
transaction block, the committed actions become visible as a unit
to other sessions, while the rolled-back actions never become visible
at all.
</P
><P
> Remembering the bank database, suppose we debit $100.00 from Alice's
account, and credit Bob's account, only to find later that we should
have credited Wally's account. We could do it using savepoints like
this:
</P><PRE
CLASS="PROGRAMLISTING"
>BEGIN;
UPDATE accounts SET balance = balance - 100.00
WHERE name = 'Alice';
SAVEPOINT my_savepoint;
UPDATE accounts SET balance = balance + 100.00
WHERE name = 'Bob';
-- oops ... forget that and use Wally's account
ROLLBACK TO my_savepoint;
UPDATE accounts SET balance = balance + 100.00
WHERE name = 'Wally';
COMMIT;</PRE
><P>
</P
><P
> This example is, of course, oversimplified, but there's a lot of control
possible in a transaction block through the use of savepoints.
Moreover, <TT
CLASS="COMMAND"
>ROLLBACK TO</TT
> is the only way to regain control of a
transaction block that was put in aborted state by the
system due to an error, short of rolling it back completely and starting
again.
</P
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