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<!--
doc/src/sgml/ref/begin.sgml
PostgreSQL documentation
-->
<refentry id="sql-begin">
<indexterm zone="sql-begin">
<primary>BEGIN</primary>
</indexterm>
<refmeta>
<refentrytitle>BEGIN</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>BEGIN</refname>
<refpurpose>start a transaction block</refpurpose>
</refnamediv>
<refsynopsisdiv>
<synopsis>
BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</replaceable> [, ...] ]
<phrase>where <replaceable class="parameter">transaction_mode</replaceable> is one of:</phrase>
ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
READ WRITE | READ ONLY
[ NOT ] DEFERRABLE
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>BEGIN</command> initiates a transaction block, that is,
all statements after a <command>BEGIN</command> command will be
executed in a single transaction until an explicit <link
linkend="sql-commit"><command>COMMIT</command></link> or <link
linkend="sql-rollback"><command>ROLLBACK</command></link> is given.
By default (without <command>BEGIN</command>),
<productname>PostgreSQL</productname> executes
transactions in <quote>autocommit</quote> mode, that is, each
statement is executed in its own transaction and a commit is
implicitly performed at the end of the statement (if execution was
successful, otherwise a rollback is done).
</para>
<para>
Statements are executed more quickly in a transaction block, because
transaction start/commit requires significant CPU and disk
activity. Execution of multiple statements inside a transaction is
also useful to ensure consistency when making several related changes:
other sessions will be unable to see the intermediate states
wherein not all the related updates have been done.
</para>
<para>
If the isolation level, read/write mode, or deferrable mode is specified, the new
transaction has those characteristics, as if
<link linkend="sql-set-transaction"><command>SET TRANSACTION</command></link>
was executed.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><literal>WORK</literal></term>
<term><literal>TRANSACTION</literal></term>
<listitem>
<para>
Optional key words. They have no effect.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Refer to <xref linkend="sql-set-transaction"/> for information on the meaning
of the other parameters to this statement.
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
<link linkend="sql-start-transaction"><command>START TRANSACTION</command></link> has the same functionality
as <command>BEGIN</command>.
</para>
<para>
Use <link linkend="sql-commit"><command>COMMIT</command></link> or
<link linkend="sql-rollback"><command>ROLLBACK</command></link>
to terminate a transaction block.
</para>
<para>
Issuing <command>BEGIN</command> when already inside a transaction block will
provoke a warning message. The state of the transaction is not affected.
To nest transactions within a transaction block, use savepoints
(see <xref linkend="sql-savepoint"/>).
</para>
<para>
For reasons of backwards compatibility, the commas between successive
<replaceable class="parameter">transaction_modes</replaceable> can be
omitted.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
To begin a transaction block:
<programlisting>
BEGIN;
</programlisting></para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
<command>BEGIN</command> is a <productname>PostgreSQL</productname>
language extension. It is equivalent to the SQL-standard command
<link linkend="sql-start-transaction"><command>START TRANSACTION</command></link>, whose reference page
contains additional compatibility information.
</para>
<para>
The <literal>DEFERRABLE</literal>
<replaceable class="parameter">transaction_mode</replaceable>
is a <productname>PostgreSQL</productname> language extension.
</para>
<para>
Incidentally, the <literal>BEGIN</literal> key word is used for a
different purpose in embedded SQL. You are advised to be careful
about the transaction semantics when porting database applications.
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="sql-commit"/></member>
<member><xref linkend="sql-rollback"/></member>
<member><xref linkend="sql-start-transaction"/></member>
<member><xref linkend="sql-savepoint"/></member>
</simplelist>
</refsect1>
</refentry>
|
