<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       |       43
                | pg_depend_depender_index        |       40
                | pg_proc_proname_args_nsp_index  |       31
                | pg_description_o_c_o_index      |       21
                | pg_attribute_relid_attnam_index |       14
                | pg_proc_oid_index               |       10
                | pg_attribute_relid_attnum_index |        9
                | pg_amproc_fam_proc_index        |        5
                | pg_amop_opr_fam_index           |        5
                | pg_amop_fam_strat_index         |        5
(10 rows)
</screen>
      This example shows a session that performs verification of the
      10 largest catalog indexes in the database <quote>test</quote>.
      Verification of the presence of heap tuples as index tuples is
      requested for the subset that are unique indexes.  Since no
      error is raised, all indexes tested appear to be logically
      consistent.  Naturally, this query could easily be changed to
      call <function>bt_index_check</function> for every index in the
      database where verification is supported.
     </para>
     <para>
      <function>bt_index_check</function> acquires an <literal>AccessShareLock</literal>
      on the target index and the heap relation it belongs to. This lock mode
      is the same lock mode acquired on relations by simple
      <literal>SELECT</literal> statements.
      <function>bt_index_check</function> does not verify invariants
      that span child/parent relationships, but will verify the
      presence of all heap tuples as index tuples within the index
      when <parameter>heapallindexed</parameter> is
      <literal>true</literal>.  When <parameter>checkunique</parameter>
      is <literal>true</literal> <function>bt_index_check</function> will
      check that no more than one among duplicate entries in unique
      index is visible.  When a routine, lightweight test for
      corruption is required in a live production environment, using
      <function>bt_index_check</function> often provides the best
      trade-off between thoroughness of verification and limiting the
      impact on application performance and availability.
     </para>
    </listitem>
   </varlistentry>

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

    <listitem>
     <para>
      <function>bt_index_parent_check</function> tests that its
      target, a B-Tree index, respects a variety of invariants.
      Optionally, when the <parameter>heapallindexed</parameter>
      argument is <literal>true</literal>, the function verifies the
      presence of all heap tuples that should be found within the
      index.  When <parameter>checkunique</parameter>
      is <literal>true</literal> <function>bt_index_parent_check</function>