doc/src/sgml/pgsurgery.sgml

   1 <!-- doc/src/sgml/pgsurgery.sgml -->
   2
   3 <sect1 id="pgsurgery" xreflabel="pg_surgery">
   4  <title>pg_surgery &mdash; perform low-level surgery on relation data</title>
   5
   6  <indexterm zone="pgsurgery">
   7   <primary>pg_surgery</primary>
   8  </indexterm>
   9
  10  <para>
  11   The <filename>pg_surgery</filename> module provides various functions to
  12   perform surgery on a damaged relation. These functions are unsafe by design
  13   and using them may corrupt (or further corrupt) your database. For example,
  14   these functions can easily be used to make a table inconsistent with its
  15   own indexes, to cause <literal>UNIQUE</literal> or
  16   <literal>FOREIGN KEY</literal> constraint violations, or even to make
  17   tuples visible which, when read, will cause a database server crash.
  18   They should be used with great caution and only as a last resort.
  19  </para>
  20
  21  <sect2 id="pgsurgery-funcs">
  22   <title>Functions</title>
  23
  24   <variablelist>
  25    <varlistentry>
  26     <term>
  27      <function>heap_force_kill(regclass, tid[]) returns void</function>
  28     </term>
  29
  30     <listitem>
  31      <para>
  32       <function>heap_force_kill</function> marks <quote>used</quote> line
  33       pointers as <quote>dead</quote> without examining the tuples. The
  34       intended use of this function is to forcibly remove tuples that are not
  35       otherwise accessible. For example:
  36 <programlisting>
  37 test=&gt; select * from t1 where ctid = '(0, 1)';
  38 ERROR:  could not access status of transaction 4007513275
  39 DETAIL:  Could not open file "pg_xact/0EED": No such file or directory.
  40
  41 test=# select heap_force_kill('t1'::regclass, ARRAY['(0, 1)']::tid[]);
  42  heap_force_kill
  43 -----------------
  44
  45 (1 row)
  46
  47 test=# select * from t1 where ctid = '(0, 1)';
  48 (0 rows)
  49
  50 </programlisting>
  51     </para>
  52     </listitem>
  53    </varlistentry>
  54
  55    <varlistentry>
  56     <term>
  57      <function>heap_force_freeze(regclass, tid[]) returns void</function>
  58     </term>
  59
  60     <listitem>
  61      <para>
  62       <function>heap_force_freeze</function> marks tuples as frozen without
  63       examining the tuple data. The intended use of this function is to
  64       make accessible tuples which are inaccessible due to corrupted
  65       visibility information, or which prevent the table from being
  66       successfully vacuumed due to corrupted visibility information.
  67       For example:
  68 <programlisting>
  69 test=&gt; vacuum t1;
  70 ERROR:  found xmin 507 from before relfrozenxid 515
  71 CONTEXT:  while scanning block 0 of relation "public.t1"
  72
  73 test=# select ctid from t1 where xmin = 507;
  74  ctid
  75 -------
  76  (0,3)
  77 (1 row)
  78
  79 test=# select heap_force_freeze('t1'::regclass, ARRAY['(0, 3)']::tid[]);
  80  heap_force_freeze
  81 -------------------
  82
  83 (1 row)
  84
  85 test=# select ctid from t1 where xmin = 2;
  86  ctid
  87 -------
  88  (0,3)
  89 (1 row)
  90
  91 </programlisting>
  92     </para>
  93    </listitem>
  94   </varlistentry>
  95
  96   </variablelist>
  97  </sect2>
  98
  99  <sect2 id="pgsurgery-authors">
 100   <title>Authors</title>
 101
 102   <para>
 103    Ashutosh Sharma <email>ashu.coek88@gmail.com</email>
 104   </para>
 105  </sect2>
 106
 107 </sect1>