<!-- doc/src/sgml/pgsurgery.sgml -->

<sect1 id="pgsurgery" xreflabel="pg_surgery">
 <title>pg_surgery &mdash; perform low-level surgery on relation data</title>

 <indexterm zone="pgsurgery">
  <primary>pg_surgery</primary>
 </indexterm>

 <para>
  The <filename>pg_surgery</filename> module provides various functions to
  perform surgery on a damaged relation. These functions are unsafe by design
  and using them may corrupt (or further corrupt) your database. For example,
  these functions can easily be used to make a table inconsistent with its
  own indexes, to cause <literal>UNIQUE</literal> or
  <literal>FOREIGN KEY</literal> constraint violations, or even to make
  tuples visible which, when read, will cause a database server crash.
  They should be used with great caution and only as a last resort.
 </para>

 <sect2 id="pgsurgery-funcs">
  <title>Functions</title>

  <variablelist>
   <varlistentry>
    <term>
     <function>heap_force_kill(regclass, tid[]) returns void</function>
    </term>

    <listitem>
     <para>
      <function>heap_force_kill</function> marks <quote>used</quote> line
      pointers as <quote>dead</quote> without examining the tuples. The
      intended use of this function is to forcibly remove tuples that are not
      otherwise accessible. For example:
<programlisting>
test=&gt; select * from t1 where ctid = '(0, 1)';
ERROR:  could not access status of transaction 4007513275
DETAIL:  Could not open file "pg_xact/0EED": No such file or directory.

test=# select heap_force_kill('t1'::regclass, ARRAY['(0, 1)']::tid[]);
 heap_force_kill
-----------------

(1 row)

test=# select * from t1 where ctid = '(0, 1)';
(0 rows)

</programlisting>
    </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>heap_force_freeze(regclass, tid[]) returns void</function>
    </term>

    <listitem>
     <para>
      <function>heap_force_freeze</function> marks tuples as frozen without
      examining the tuple data. The intended use of this function is to
      make accessible tuples which are inaccessible due to corrupted
      visibility information, or which prevent the table from being
      successfully vacuumed due to corrupted visibility information.
      For example:
<programlisting>
test=&gt; vacuum t1;
ERROR:  found xmin 507 from before relfrozenxid 515
CONTEXT:  while scanning block 0 of relation "public.t1"

test=# select ctid from t1 where xmin = 507;
 ctid
-------
 (0,3)
(1 row)

test=# select heap_force_freeze('t1'::regclass, ARRAY['(0, 3)']::tid[]);
 heap_force_freeze
-------------------

(1 row)

test=# select ctid from t1 where xmin = 2;
 ctid
-------
 (0,3)
(1 row)

</programlisting>
    </para>
   </listitem>
  </varlistentry>

  </variablelist>
 </sect2>

 <sect2 id="pgsurgery-authors">
  <title>Authors</title>

  <para>
   Ashutosh Sharma <email>ashu.coek88@gmail.com</email>
  </para>
 </sect2>

</sect1>
