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

<sect1 id="amcheck" xreflabel="amcheck">
 <title>amcheck &mdash; tools to verify table and index consistency</title>

 <indexterm zone="amcheck">
  <primary>amcheck</primary>
 </indexterm>

 <para>
  The <filename>amcheck</filename> module provides functions that allow you to
  verify the logical consistency of the structure of relations.
 </para>

 <para>
  The B-Tree checking functions verify various <emphasis>invariants</emphasis> in the
  structure of the representation of particular relations.  The
  correctness of the access method functions behind index scans and
  other important operations relies on these invariants always
  holding.  For example, certain functions verify, among other things,
  that all B-Tree pages have items in <quote>logical</quote> order (e.g.,
  for B-Tree indexes on <type>text</type>, index tuples should be in
  collated lexical order).  If that particular invariant somehow fails
  to hold, we can expect binary searches on the affected page to
  incorrectly guide index scans, resulting in wrong answers to SQL
  queries.  If the structure appears to be valid, no error is raised.
  While these checking functions are run, the <xref
  linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
  pg_temp</literal>.
 </para>
 <para>
  Verification is performed using the same procedures as those used by
  index scans themselves, which may be user-defined operator class
  code.  For example, B-Tree index verification relies on comparisons
  made with one or more B-Tree support function 1 routines.  See <xref
  linkend="xindex-support"/> for details of operator class support
  functions.
 </para>
 <para>
  Unlike the B-Tree checking functions which report corruption by raising
  errors, the heap checking function <function>verify_heapam</function> checks
  a table and attempts to return a set of rows, one row per corruption
  detected.  Despite this, if facilities that
  <function>verify_heapam</function> relies upon are themselves corrupted, the
  function may be unable to continue and may instead raise an error.
 </para>
 <para>
  Permission to execute <filename>amcheck</filename> functions may be granted
  to non-superusers, but before granting such permissions careful consideration
  should be given to data security and privacy concerns.  Although the
  corruption reports generated by these functions do not focus on the contents
  of the corrupted data so much as on the structure of that data and the nature
  of the corruptions found, an attacker who gains permission to execute these
  functions, particularly if the attacker can also induce corruption, might be
  able to infer something of the data itself from such messages.
 </para>

 <sect2 id="amcheck-functions">
  <title>Functions</title>

  <variablelist>
   <varlistentry>
    <term>
     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
     <indexterm>
      <primary>bt_index_check</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>bt_index_check</function> tests that its target, a
      B-Tree index, respects a variety of invariants.  Example usage:
<screen>
test=# SELECT bt_index_check(index =&gt; c.oid, heapallindexed =&gt; i.indisunique),
               c.relname,
               c.relpages
FROM pg_index i
JOIN pg_opclass op ON i.indclass[0] = op.oid
JOIN pg_am am ON op.opcmethod = am.oid
JOIN pg_class c ON i.indexrelid = c.oid
JOIN pg_namespace n ON c.relnamespace = n.oid
WHERE am.amname = 'btree' AND n.nspname = 'pg_catalog'
-- Don't check temp tables, which may be from another session:
AND c.relpersistence != 't'
-- Function may throw an error when this is omitted:
AND c.relkind = 'i' AND i.indisready AND i.indisvalid
ORDER BY c.relpages DESC LIMIT 10;
 bt_index_check |             relname             | relpages
----------------+---------------------------------+----------
                | pg_depend_reference_index