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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/start_transaction.sgml,v 1.3 2002/08/30 22:45:25 tgl Exp $
PostgreSQL documentation
-->
<refentry id="SQL-START-TRANSACTION">
<docinfo>
<date>2002-07-26</date>
</docinfo>
<refmeta>
<refentrytitle id="SQL-START-TRANSACTION-TITLE">START TRANSACTION</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>START TRANSACTION</refname>
<refpurpose>start a transaction block</refpurpose>
</refnamediv>
<refsynopsisdiv>
<synopsis>
START TRANSACTION [ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ]
</synopsis>
<refsect2 id="R2-SQL-START-TRANSACTION-1">
<refsect2info>
<date>1998-09-27</date>
</refsect2info>
<title>
Inputs
</title>
<para>
None.
</para>
</refsect2>
<refsect2 id="R2-SQL-START-TRANSACTION-2">
<refsect2info>
<date>1998-09-27</date>
</refsect2info>
<title>
Outputs
</title>
<para>
<variablelist>
<varlistentry>
<term><computeroutput>
START TRANSACTION
</computeroutput></term>
<listitem>
<para>
Message returned if successful.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><computeroutput>
WARNING: BEGIN: already a transaction in progress
</computeroutput></term>
<listitem>
<para>
If there is already a transaction in progress when the
command is issued.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
This command begins a new transaction. If the isolation level is
specified, the new transaction has that isolation level. In all other
respects, the behavior of this command is identical to the
<xref linkend="sql-begin" endterm="sql-begin-title"> command.
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
The isolation level of a transaction can also be set with the <xref
linkend="sql-set-transaction" endterm="sql-set-transaction-title">
command. If no isolation level is specified, the default isolation
level is used.
</para>
</refsect1>
<refsect1 id="R1-SQL-START-TRANSACTION-3">
<title>Compatibility</title>
<refsect2 id="R2-SQL-START-TRANSACTION-4">
<title>SQL99</title>
<para>
<option>SERIALIZABLE</option> is the default isolation level in
<acronym>SQL99</acronym>, but it is not the usual default in
<productname>PostgreSQL</productname>: the factory default setting
is READ COMMITTED.
<productname>PostgreSQL</productname>
does not provide the isolation levels <option>READ UNCOMMITTED</option>
and <option>REPEATABLE READ</option>. Because of lack of predicate
locking, the <option>SERIALIZABLE</option> level is
not truly serializable. See the <citetitle>User's Guide</citetitle>
for details.
</para>
<para>
In <acronym>SQL99</acronym> this statement can specify two other
properties of the new transaction: whether the transaction is
read-only and the size of the diagnostics area. Neither of these
concepts are currently supported in
<productname>PostgreSQL</productname>.
</para>
</refsect2>
</refsect1>
</refentry>
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