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><H1
><A
NAME="SQL-BEGIN"
></A
>BEGIN</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN66368"
></A
><H2
>Name</H2
>BEGIN&nbsp;--&nbsp;start a transaction block</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN66373"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>BEGIN [ WORK | TRANSACTION ] [ <TT
CLASS="REPLACEABLE"
><I
>transaction_mode</I
></TT
> [, ...] ]

<SPAN
CLASS="phrase"
><SPAN
CLASS="PHRASE"
>where <TT
CLASS="REPLACEABLE"
><I
>transaction_mode</I
></TT
> is one of:</SPAN
></SPAN
>

    ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
    READ WRITE | READ ONLY
    [ NOT ] DEFERRABLE</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66378"
></A
><H2
>Description</H2
><P
>   <TT
CLASS="COMMAND"
>BEGIN</TT
> initiates a transaction block, that is,
   all statements after a <TT
CLASS="COMMAND"
>BEGIN</TT
> command will be
   executed in a single transaction until an explicit <A
HREF="sql-commit.html"
>COMMIT</A
> or <A
HREF="sql-rollback.html"
>ROLLBACK</A
> is given.
   By default (without <TT
CLASS="COMMAND"
>BEGIN</TT
>),
   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> executes
   transactions in <SPAN
CLASS="QUOTE"
>"autocommit"</SPAN
> 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).
  </P
><P
>   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.
  </P
><P
>   If the isolation level, read/write mode, or deferrable mode is specified, the new
   transaction has those characteristics, as if
   <A
HREF="sql-set-transaction.html"
>SET TRANSACTION</A
>
   was executed.
  </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66391"
></A
><H2
>Parameters</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>WORK</TT
><BR><TT
CLASS="LITERAL"
>TRANSACTION</TT
></DT
><DD
><P
>      Optional key words. They have no effect.
     </P
></DD
></DL
></DIV
><P
>   Refer to <A
HREF="sql-set-transaction.html"
>SET TRANSACTION</A
> for information on the meaning
   of the other parameters to this statement.
  </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66403"
></A
><H2
>Notes</H2
><P
>   <A
HREF="sql-start-transaction.html"
>START TRANSACTION</A
> has the same functionality
   as <TT
CLASS="COMMAND"
>BEGIN</TT
>.
  </P
><P
>   Use <A
HREF="sql-commit.html"
>COMMIT</A
> or
   <A
HREF="sql-rollback.html"
>ROLLBACK</A
>
   to terminate a transaction block.
  </P
><P
>   Issuing <TT
CLASS="COMMAND"
>BEGIN</TT
> 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 <A
HREF="sql-savepoint.html"
>SAVEPOINT</A
>).
  </P
><P
>   For reasons of backwards compatibility, the commas between successive
   <TT
CLASS="REPLACEABLE"
><I
>transaction_modes</I
></TT
> can be
   omitted.
  </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66416"
></A
><H2
>Examples</H2
><P
>   To begin a transaction block:

</P><PRE
CLASS="PROGRAMLISTING"
>BEGIN;</PRE
><P></P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66420"
></A
><H2
>Compatibility</H2
><P
>   <TT
CLASS="COMMAND"
>BEGIN</TT
> is a <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
   language extension.  It is equivalent to the SQL-standard command
   <A
HREF="sql-start-transaction.html"
>START TRANSACTION</A
>, whose reference page
   contains additional compatibility information.
  </P
><P
>   The <TT
CLASS="LITERAL"
>DEFERRABLE</TT
>
   <TT
CLASS="REPLACEABLE"
><I
>transaction_mode</I
></TT
>
   is a <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> language extension.
  </P
><P
>   Incidentally, the <TT
CLASS="LITERAL"
>BEGIN</TT
> 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.
  </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66432"
></A
><H2
>See Also</H2
><A
HREF="sql-commit.html"
>COMMIT</A
>, <A
HREF="sql-rollback.html"
>ROLLBACK</A
>, <A
HREF="sql-start-transaction.html"
>START TRANSACTION</A
>, <A
HREF="sql-savepoint.html"
>SAVEPOINT</A
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