Create a "sort support" interface API for faster sorting.

[postgresql] / doc / src / sgml / ref / truncate.sgml

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doc/src/sgml/ref/truncate.sgml
PostgreSQL documentation
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<refentry id="SQL-TRUNCATE">
 <refmeta>
  <refentrytitle>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 ] [ ONLY ] <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.  If <literal>ONLY</> is specified, only that table is
      truncated.  If <literal>ONLY</> is not specified, the table and
      all its descendant tables (if any) are 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 <literal>ACCESS EXCLUSIVE</> lock on each
   table it operates on, which blocks all other concurrent operations
   on the table.  When <literal>RESTART IDENTITY</> is specified, any
   sequences that are to be restarted are likewise locked exclusively.
   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 and any sequences are reset.
   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>

  <para>
   When <literal>RESTART IDENTITY</> is specified, the implied
   <command>ALTER SEQUENCE RESTART</> operations are also done
   transactionally; that is, they will be rolled back if the surrounding
   transaction does not commit.  This is unlike the normal behavior of
   <command>ALTER SEQUENCE RESTART</>.  Be aware that if any additional
   sequence operations are done on the restarted sequences before the
   transaction rolls back, the effects of these operations on the sequences
   will be rolled back, but not their effects on <function>currval()</>;
   that is, after the transaction <function>currval()</> will continue to
   reflect the last sequence value obtained inside the failed transaction,
   even though the sequence itself may no longer be consistent with that.
   This is similar to the usual behavior of <function>currval()</> after
   a failed transaction.
  </para>
 </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 though 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>