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$PostgreSQL: pgsql/doc/src/sgml/ref/truncate.sgml,v 1.31 2008/12/18 10:45:00 petere Exp $
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<refentry id="SQL-TRUNCATE">
 <refmeta>
  <refentrytitle id="SQL-TRUNCATE-TITLE">TRUNCATE</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>TRUNCATE</refname>
  <refpurpose>empty a table or set of tables</refpurpose>
 </refnamediv>

 <indexterm zone="sql-truncate">
  <primary>TRUNCATE</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
TRUNCATE [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ... ]
    [ RESTART IDENTITY | CONTINUE IDENTITY ] [ CASCADE | RESTRICT ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>TRUNCATE</command> quickly removes all rows from a set of
   tables. It has the same effect as an unqualified
   <command>DELETE</command> on each table, but since it does not actually
   scan the tables it is faster. Furthermore, it reclaims disk space
   immediately, rather than requiring a subsequent <command>VACUUM</command>
   operation. This is most useful on large tables.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="PARAMETER">name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of a table to be truncated.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>RESTART IDENTITY</literal></term>
    <listitem>
     <para>
      Automatically restart sequences owned by columns of
      the truncated table(s).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CONTINUE IDENTITY</literal></term>
    <listitem>
     <para>
      Do not change the values of sequences.  This is the default.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CASCADE</literal></term>
    <listitem>
     <para>
      Automatically truncate all tables that have foreign-key references
      to any of the named tables, or to any tables added to the group
      due to <literal>CASCADE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>RESTRICT</literal></term>
    <listitem>
     <para>
      Refuse to truncate if any of the tables have foreign-key references
      from tables that are not listed in the command.  This is the default.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   You must have the <literal>TRUNCATE</literal> privilege on a table
   to truncate it.
  </para>

  <para>
   <command>TRUNCATE</> acquires an access exclusive lock on the
   tables in operates on, which blocks all other concurrent operations
   on the table.  If concurrent access to a table is required, then
   the <command>DELETE</> command should be used instead.
  </para>

  <para>
   <command>TRUNCATE</> cannot be used on a table that has foreign-key
   references from other tables, unless all such tables are also truncated
   in the same command.  Checking validity in such cases would require table
   scans, and the whole point is not to do one.  The <literal>CASCADE</>
   option can be used to automatically include all dependent tables &mdash;
   but be very careful when using this option, or else you might lose data you
   did not intend to!
  </para>

  <para>
   <command>TRUNCATE</> will not fire any <literal>ON DELETE</literal>
   triggers that might exist for the tables.  But it will fire
   <literal>ON TRUNCATE</literal> triggers.
   If <literal>ON TRUNCATE</> triggers are defined for any of
   the tables, then all <literal>BEFORE TRUNCATE</literal> triggers are
   fired before any truncation happens, and all <literal>AFTER
   TRUNCATE</literal> triggers are fired after the last truncation is
   performed.  The triggers will fire in the order that the tables are
   to be processed (first those listed in the command, and then any
   that were added due to cascading).
  </para>

  <warning>
   <para>
    <command>TRUNCATE</> is not MVCC-safe (see <xref linkend="mvcc">
     for general information about MVCC).  After truncation, the table
     will appear empty to all concurrent transactions, even if they
     are using a snapshot taken before the truncation occurred.  This
     will only be an issue for a transaction that did not access the
     truncated table before the truncation happened &mdash; any
     transaction that has done so would hold at least an
     <literal>ACCESS SHARE</literal> lock, which would block
     <command>TRUNCATE</> until that transaction completes.  So
     truncation will not cause any apparent inconsistency in the table
     contents for successive queries on the same table, but it could
     cause visible inconsistency between the contents of the truncated
     table and other tables in the database.
   </para>
  </warning>

  <para>
   <command>TRUNCATE</> is transaction-safe with respect to the data
   in the tables: the truncation will be safely rolled back if the surrounding
   transaction does not commit.
  </para>

  <warning>
   <para>
    Any <command>ALTER SEQUENCE RESTART</> operations performed as a
    consequence of using the <literal>RESTART IDENTITY</> option are
    nontransactional and will not be rolled back on failure.  To minimize
    the risk, these operations are performed only after all the rest of
    <command>TRUNCATE</>'s work is done.  However, there is still a risk
    if <command>TRUNCATE</> is performed inside a transaction block that is
    aborted afterwards.  For example, consider

<programlisting>
BEGIN;
TRUNCATE TABLE foo RESTART IDENTITY;
COPY foo FROM ...;
COMMIT;
</programlisting>

    If the <command>COPY</> fails partway through, the table data
    rolls back correctly, but the sequences will be left with values
    that are probably smaller than they had before, possibly leading
    to duplicate-key failures or other problems in later transactions.
    If this is likely to be a problem, it's best to avoid using
    <literal>RESTART IDENTITY</>, and accept that the new contents of
    the table will have higher serial numbers than the old.
   </para>
  </warning>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Truncate the tables <literal>bigtable</literal> and
   <literal>fattable</literal>:

<programlisting>
TRUNCATE bigtable, fattable;
</programlisting>
  </para>

  <para>
   The same, and also reset any associated sequence generators:

<programlisting>
TRUNCATE bigtable, fattable RESTART IDENTITY;
</programlisting>
  </para>

  <para>
   Truncate the table <literal>othertable</literal>, and cascade to any tables
   that reference <literal>othertable</literal> via foreign-key
   constraints:

<programlisting>
TRUNCATE othertable CASCADE;
</programlisting>
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   The SQL:2008 standard includes a <command>TRUNCATE</command> command with the syntax
   <literal>TRUNCATE TABLE <replaceable>tablename</replaceable></literal>.
   The clauses <literal>CONTINUE IDENTITY</literal>/<literal>RESTART IDENTITY</literal>
   also appear in that standard but have slightly different but related meanings.
   Some of the concurrency behavior of this command is left implementation-defined
   by the standard, so the above notes should be considered and compared with
   other implementations if necessary.
  </para>
 </refsect1>
</refentry>
