begin.sgml

<!--
doc/src/sgml/ref/begin.sgml
PostgreSQL documentation
-->

<refentry id="SQL-BEGIN">
 <refmeta>
  <refentrytitle>BEGIN</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>BEGIN</refname>
  <refpurpose>start a transaction block</refpurpose>
 </refnamediv>

 <indexterm zone="sql-begin">
  <primary>BEGIN</primary>
 </indexterm>

 <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
</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 <xref
   linkend="sql-commit"> or <xref
   linkend="sql-rollback"> 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 or read/write mode is specified, the new
   transaction has those characteristics, as if
   <xref linkend="sql-set-transaction"> 
   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>
   <xref linkend="sql-start-transaction"> has the same functionality
   as <command>BEGIN</>.
  </para>
   
  <para>
   Use <xref linkend="SQL-COMMIT"> or
   <xref linkend="SQL-ROLLBACK">
   to terminate a transaction block.
  </para>

  <para>
   Issuing <command>BEGIN</> 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
   <xref linkend="sql-start-transaction">, whose reference page
   contains additional compatibility information.
  </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>