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$Header: /cvsroot/pgsql/doc/src/sgml/ref/begin.sgml,v 1.23 2003/04/15 13:25:08 petere Exp $
PostgreSQL documentation
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<refentry id="SQL-BEGIN">
 <refmeta>
  <refentrytitle id="SQL-BEGIN-TITLE">BEGIN</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

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

 <refsynopsisdiv>
<synopsis>
BEGIN [ WORK | TRANSACTION ]
</synopsis>
 </refsynopsisdiv>
 
 <refsect1>
  <title>Description</title>

  <para>
   By default, <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).
   <command>BEGIN</command> initiates a transaction block, that is,
   all statements after <command>BEGIN</command> command will be
   executed in a single transaction until an explicit <xref
   linkend="sql-commit" endterm="sql-commit-title"> or <xref
   linkend="sql-rollback" endterm="sql-rollback-title">.  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 changing several related
   tables: other sessions will be unable to see the intermediate states
   wherein not all the related updates have been done.
  </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>
 </refsect1>
  
 <refsect1>
  <title>Diagnostics</title>

  <variablelist>
   <varlistentry>
    <term><computeroutput>BEGIN</computeroutput></term>
    <listitem>
     <para>
      This signifies that a new transaction has been started.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><computeroutput>WARNING:  BEGIN: already a transaction in progress</computeroutput></term>
    <listitem>
     <para>
      This indicates that a transaction was already in progress.  The
      current transaction is not affected.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>
  
 <refsect1>
  <title>Notes</title>

  <para>
   <xref linkend="sql-start-transaction"
   endterm="sql-start-transaction-title"> has the same functionality
   as <command>BEGIN</>.
  </para>
   
  <para>
   Use <xref linkend="SQL-COMMIT" endterm="SQL-COMMIT-TITLE"> or
   <xref linkend="SQL-ROLLBACK" endterm="SQL-ROLLBACK-TITLE">
   to terminate a transaction.
  </para>
   
  <para>
   If you turn the configuration parameter <varname>autocommit</> off,
   then <command>BEGIN</> is not required: any SQL command
   automatically starts a transaction.
  </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.  There is no explicit <command>BEGIN</command>
   command in the SQL standard; transaction initiation is
   always implicit and it terminates either with a
   <command>COMMIT</command> or <command>ROLLBACK</command> statement.
  </para>

  <para>
   Other relational database systems may offer an autocommit feature
   as a convenience.
  </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>
</refentry>
 
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