path: root/doc/src/sgml/amcheck.sgml

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

<sect1 id="amcheck" xreflabel="amcheck">
 <title>amcheck</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.  If the
  structure appears to be valid, no error is raised.
 </para>

 <para>
  The 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.
 </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>
  <filename>amcheck</filename> functions may only be used by superusers.
 </para>

 <sect2>
  <title>Functions</title>

  <variablelist>
   <varlistentry>
    <term>
     <function>bt_index_check(index regclass, heapallindexed 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       |       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 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) 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, and that there are no missing downlinks in the index
      structure.  When the optional <parameter>rootdescend</parameter>
      argument is <literal>true</literal>, verification re-finds
      tuples on the leaf level by performing a new search from the
      root page for each tuple.  The checks that can be performed by
      <function>bt_index_parent_check</function> are a superset of the
      checks that can be performed by <function>bt_index_check</function>.
      <function>bt_index_parent_check</function> can be thought of as
      a more thorough variant of <function>bt_index_check</function>:
      unlike <function>bt_index_check</function>,
      <function>bt_index_parent_check</function> also checks
      invariants that span parent/child relationships.
      <function>bt_index_parent_check</function> follows the general
      convention of raising an error if it finds a logical
      inconsistency or other problem.
     </para>
     <para>
      A <literal>ShareLock</literal> is required on the target index by
      <function>bt_index_parent_check</function> (a
      <literal>ShareLock</literal> is also acquired on the heap relation).
      These locks prevent concurrent data modification from
      <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command>
      commands.  The locks also prevent the underlying relation from
      being concurrently processed by <command>VACUUM</command>, as well as
      all other utility commands.  Note that the function holds locks
      only while running, not for the entire transaction.
     </para>
     <para>
      <function>bt_index_parent_check</function>'s additional
      verification is more likely to detect various pathological
      cases.  These cases may involve an incorrectly implemented
      B-Tree operator class used by the index that is checked, or,
      hypothetically, undiscovered bugs in the underlying B-Tree index
      access method code.  Note that
      <function>bt_index_parent_check</function> cannot be used when
      Hot Standby mode is enabled (i.e., on read-only physical
      replicas), unlike <function>bt_index_check</function>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </sect2>

 <sect2>
  <title>Optional <parameter>heapallindexed</parameter> Verification</title>
 <para>
  When the <parameter>heapallindexed</parameter> argument to
  verification functions is <literal>true</literal>, an additional
  phase of verification is performed against the table associated with
  the target index relation.  This consists of a <quote>dummy</quote>
  <command>CREATE INDEX</command> operation, which checks for the
  presence of all hypothetical new index tuples against a temporary,
  in-memory summarizing structure (this is built when needed during
  the basic first phase of verification).  The summarizing structure
  <quote>fingerprints</quote> every tuple found within the target
  index.  The high level principle behind
  <parameter>heapallindexed</parameter> verification is that a new
  index that is equivalent to the existing, target index must only
  have entries that can be found in the existing structure.
 </para>
 <para>
  The additional <parameter>heapallindexed</parameter> phase adds
  significant overhead: verification will typically take several times
  longer.  However, there is no change to the relation-level locks
  acquired when <parameter>heapallindexed</parameter> verification is
  performed.
 </para>
 <para>
  The summarizing structure is bound in size by
  <varname>maintenance_work_mem</varname>.  In order to ensure that
  there is no more than a 2% probability of failure to detect an
  inconsistency for each heap tuple that should be represented in the
  index, approximately 2 bytes of memory are needed per tuple.  As
  less memory is made available per tuple, the probability of missing
  an inconsistency slowly increases.  This approach limits the
  overhead of verification significantly, while only slightly reducing
  the probability of detecting a problem, especially for installations
  where verification is treated as a routine maintenance task.  Any
  single absent or malformed tuple has a new opportunity to be
  detected with each new verification attempt.
 </para>

 </sect2>

 <sect2>
  <title>Using <filename>amcheck</filename> Effectively</title>

 <para>
  <filename>amcheck</filename> can be effective at detecting various types of
  failure modes that <link
  linkend="app-initdb-data-checksums"><application>data page
  checksums</application></link> will always fail to catch.  These include:

  <itemizedlist>
   <listitem>
    <para>
     Structural inconsistencies caused by incorrect operator class
     implementations.
    </para>
    <para>
     This includes issues caused by the comparison rules of operating
     system collations changing. Comparisons of datums of a collatable
     type like <type>text</type> must be immutable (just as all
     comparisons used for B-Tree index scans must be immutable), which
     implies that operating system collation rules must never change.
     Though rare, updates to operating system collation rules can
     cause these issues. More commonly, an inconsistency in the
     collation order between a master server and a standby server is
     implicated, possibly because the <emphasis>major</emphasis> operating
     system version in use is inconsistent.  Such inconsistencies will
     generally only arise on standby servers, and so can generally
     only be detected on standby servers.
    </para>
    <para>
     If a problem like this arises, it may not affect each individual
     index that is ordered using an affected collation, simply because
     <emphasis>indexed</emphasis> values might happen to have the same
     absolute ordering regardless of the behavioral inconsistency. See
     <xref linkend="locale"/> and <xref linkend="collation"/> for
     further details about how <productname>PostgreSQL</productname> uses
     operating system locales and collations.
    </para>
   </listitem>
   <listitem>
    <para>
     Structural inconsistencies between indexes and the heap relations
     that are indexed (when <parameter>heapallindexed</parameter>
     verification is performed).
    </para>
    <para>
     There is no cross-checking of indexes against their heap relation
     during normal operation.  Symptoms of heap corruption can be subtle.
    </para>
   </listitem>
   <listitem>
    <para>
     Corruption caused by hypothetical undiscovered bugs in the
     underlying <productname>PostgreSQL</productname> access method
     code, sort code, or transaction management code.
    </para>
    <para>
     Automatic verification of the structural integrity of indexes
     plays a role in the general testing of new or proposed
     <productname>PostgreSQL</productname> features that could plausibly allow a
     logical inconsistency to be introduced.  Verification of table
     structure and associated visibility and transaction status
     information plays a similar role.  One obvious testing strategy
     is to call <filename>amcheck</filename> functions continuously
     when running the standard regression tests.  See <xref
     linkend="regress-run"/> for details on running the tests.
    </para>
   </listitem>
   <listitem>
    <para>
     File system or storage subsystem faults where checksums happen to
     simply not be enabled.
    </para>
    <para>
     Note that <filename>amcheck</filename> examines a page as represented in some
     shared memory buffer at the time of verification if there is only a
     shared buffer hit when accessing the block. Consequently,
     <filename>amcheck</filename> does not necessarily examine data read from the
     file system at the time of verification. Note that when checksums are
     enabled, <filename>amcheck</filename> may raise an error due to a checksum
     failure when a corrupt block is read into a buffer.
    </para>
   </listitem>
   <listitem>
    <para>
     Corruption caused by faulty RAM, or the broader memory subsystem.
    </para>
    <para>
     <productname>PostgreSQL</productname> does not protect against correctable
     memory errors and it is assumed you will operate using RAM that
     uses industry standard Error Correcting Codes (ECC) or better
     protection.  However, ECC memory is typically only immune to
     single-bit errors, and should not be assumed to provide
     <emphasis>absolute</emphasis> protection against failures that
     result in memory corruption.
    </para>
    <para>
     When <parameter>heapallindexed</parameter> verification is
     performed, there is generally a greatly increased chance of
     detecting single-bit errors, since strict binary equality is
     tested, and the indexed attributes within the heap are tested.
    </para>
   </listitem>
  </itemizedlist>
  In general, <filename>amcheck</filename> can only prove the presence of
  corruption; it cannot prove its absence.
 </para>

 </sect2>
 <sect2>
  <title>Repairing Corruption</title>
 <para>
  No error concerning corruption raised by <filename>amcheck</filename> should
  ever be a false positive.  <filename>amcheck</filename> raises
  errors in the event of conditions that, by definition, should never
  happen, and so careful analysis of <filename>amcheck</filename>
  errors is often required.
 </para>
 <para>
  There is no general method of repairing problems that
  <filename>amcheck</filename> detects.  An explanation for the root cause of
  an invariant violation should be sought.  <xref
  linkend="pageinspect"/> may play a useful role in diagnosing
  corruption that <filename>amcheck</filename> detects.  A <command>REINDEX</command>
  may not be effective in repairing corruption.
 </para>

 </sect2>

</sect1>