2 <!DOCTYPE appendix SYSTEM "/usr/share/xml/docbook/schema/dtd/5.0/docbook.dtd"
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7 <!ENTITY % entities SYSTEM "entities">
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11 <!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook V5.0//EN"
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40 <appendix xml:id="transaction" xreflabel="transaction"
41 xmlns="http://docbook.org/ns/docbook"
43 <title>&transaction_appendix;</title>
46 <para>This appendix is primarily excerpted from the &postgresql;
47 9.1 documentation chapter titled <quote><link
48 xlink:href="http://www.postgresql.org/docs/9.1/static/tutorial-transactions.html">Transactions</link></quote>.</para>
51 <para>Transactions are a fundamental concept of all database
52 systems. The essential point of a transaction is that it bundles
53 multiple steps into a single, all-or-nothing operation. The
54 intermediate states between the steps are not visible to other
55 concurrent transactions, and if some failure occurs that prevents
56 the transaction from completing, then none of the steps affect the
57 database at all.</para>
59 <para>Another important property of transactional databases is
60 closely related to the notion of atomic updates: when multiple
61 transactions are running concurrently, each one should not be able
62 to see the incomplete changes made by others.</para>
64 <para>In PostgreSQL, a transaction is set up by surrounding the
65 SQL commands of the transaction with <code>BEGIN</code> and
66 <code>COMMIT</code> commands. So a banking transaction would
67 actually look like:</para>
71 UPDATE accounts SET balance = balance - 100.00
75 </code></literallayout>
77 <para>If, partway through the transaction, we decide we do not
78 want to commit (perhaps we just noticed that Alice's balance went
79 negative), we can issue the command <code>ROLLBACK</code> instead
80 of <code>COMMIT</code>, and all our updates so far will be
83 <para>PostgreSQL actually treats every SQL statement as being
84 executed within a transaction. If you do not issue a
85 <code>BEGIN</code> command, then each individual statement has an
86 implicit <code>BEGIN</code> and (if successful)
87 <code>COMMIT</code> wrapped around it.</para>
89 <para>The entirely of each file uploaded using &ppa; is committed
90 to the database in a transaction.</para>
98 <!-- LocalWords: xreflabel ulink url postgresql checkbox