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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/begin.sgml,v 1.20 2002/09/21 18:32:54 petere Exp $
PostgreSQL documentation
-->
<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>
<refsynopsisdivinfo>
<date>1999-07-20</date>
</refsynopsisdivinfo>
<synopsis>
BEGIN [ WORK | TRANSACTION ]
</synopsis>
<refsect2 id="R2-SQL-BEGIN-1">
<refsect2info>
<date>1999-06-11</date>
</refsect2info>
<title>
Inputs
</title>
<para>
<variablelist>
<varlistentry>
<term>WORK</term>
<term>TRANSACTION</term>
<listitem>
<para>
Optional keywords. They have no effect.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
<refsect2 id="R2-SQL-BEGIN-2">
<refsect2info>
<date>1999-06-11</date>
</refsect2info>
<title>
Outputs
</title>
<para>
<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>
</para>
</refsect2>
</refsynopsisdiv>
<refsect1 id="R1-SQL-BEGIN-1">
<refsect1info>
<date>1999-06-11</date>
</refsect1info>
<title>
Description
</title>
<para>
By default, <productname>PostgreSQL</productname> executes transactions
in <firstterm>unchained mode</firstterm>
(also known as <quote>autocommit</quote> in other database
systems).
In other words, each user 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 user transaction in chained mode,
i.e., all user 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 chained mode,
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 clients will be unable to see the intermediate
states wherein not all the related updates have been done.
</para>
<para>
The default transaction isolation level in
<productname>PostgreSQL</productname>
is READ COMMITTED, wherein each query inside the transaction sees changes
committed before that query begins execution. So, you have to use
<command>SET TRANSACTION ISOLATION LEVEL SERIALIZABLE</command>
just after <command>BEGIN</command> if you need more rigorous transaction
isolation. (Alternatively, you can change the default transaction
isolation level; see the <citetitle>PostgreSQL Administrator's
Guide</citetitle> for details.)
In SERIALIZABLE mode queries will see only changes committed before
the entire
transaction began (actually, before execution of the first <acronym>DML</> statement
in the transaction).
</para>
<para>
Transactions have the standard <acronym>ACID</acronym>
(atomic, consistent, isolatable, and durable) properties.
</para>
<refsect2 id="R2-SQL-BEGIN-3">
<refsect2info>
<date>1999-06-11</date>
</refsect2info>
<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>
Refer to <xref linkend="sql-lock" endterm="sql-lock-title">
for further information
about locking tables inside a transaction.
</para>
<para>
If you turn <literal>autocommit</> mode off, then <command>BEGIN</>
is not required: any SQL command automatically starts a transaction.
</para>
</refsect2>
</refsect1>
<refsect1 id="R1-SQL-BEGIN-2">
<title>
Usage
</title>
<para>
To begin a user transaction:
<programlisting>
BEGIN WORK;
</programlisting>
</para>
</refsect1>
<refsect1 id="R1-SQL-BEGIN-3">
<title>
Compatibility
</title>
<refsect2 id="R2-SQL-BEGIN-4">
<refsect2info>
<date>1999-06-11</date>
</refsect2info>
<title>
SQL92
</title>
<para>
<command>BEGIN</command>
is a <productname>PostgreSQL</productname> language extension.
There is no explicit <command>BEGIN</command>
command in <acronym>SQL92</acronym>;
transaction initiation is always implicit and it terminates either
with a <command>COMMIT</command> or <command>ROLLBACK</command> statement.
<note>
<para>
Many relational database systems offer an autocommit feature as a
convenience.
</para>
</note>
</para>
<para>
Incidentally, the <literal>BEGIN</literal> keyword is used for a different
purpose in embedded SQL. You are advised to be careful about the transaction
semantics when porting database applications.
</para>
<para>
<acronym>SQL92</acronym> also requires SERIALIZABLE to be the default
transaction isolation level.
</para>
</refsect2>
</refsect1>
</refentry>
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