<!-- $PostgreSQL: pgsql/doc/src/sgml/pageinspect.sgml,v 1.6 2009/06/08 16:22:44 tgl Exp $ -->

<sect1 id="pageinspect">
 <title>pageinspect</title>

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

 <para>
  The <filename>pageinspect</> module provides functions that allow you to
  inspect the contents of database pages at a low level, which is useful for
  debugging purposes.  All of these functions may be used only by superusers.
 </para>

 <sect2>
  <title>Functions</title>

  <variablelist>
   <varlistentry>
    <term>
     <function>get_raw_page(relname text, fork text, blkno int) returns bytea</function>
    </term>

    <listitem>
     <para>
      <function>get_raw_page</function> reads the specified block of the named
      table and returns a copy as a <type>bytea</> value.  This allows a
      single time-consistent copy of the block to be obtained.
      <replaceable>fork</replaceable> should be <literal>'main'</literal> for
      the main data fork, or <literal>'fsm'</literal> for the free space map,
      or <literal>'vm'</literal> for the visibility map.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>get_raw_page(relname text, blkno int) returns bytea</function>
    </term>

    <listitem>
     <para>
      A shorthand version of <function>get_raw_page</function>, for reading
      from the main fork.  Equivalent to
      <literal>get_raw_page(relname, 'main', blkno)</literal>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>page_header(page bytea) returns record</function>
    </term>

    <listitem>
     <para>
      <function>page_header</function> shows fields that are common to all
      <productname>PostgreSQL</> heap and index pages.
     </para>

     <para>
      A page image obtained with <function>get_raw_page</function> should be
      passed as argument.  For example:
     </para>
     <programlisting>
test=# SELECT * FROM page_header(get_raw_page('pg_class', 0));
    lsn    | tli | flags | lower | upper | special | pagesize | version | prune_xid
-----------+-----+-------+-------+-------+---------+----------+---------+-----------
 0/24A1B50 |   1 |     1 |   232 |   368 |    8192 |     8192 |       4 |         0
     </programlisting>

     <para>
      The returned columns correspond to the fields in the
      <structname>PageHeaderData</> struct.
      See <filename>src/include/storage/bufpage.h</> for details.
    </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>heap_page_items(page bytea) returns setof record</function>
    </term>

    <listitem>
     <para>
      <function>heap_page_items</function> shows all line pointers on a heap
      page.  For those line pointers that are in use, tuple headers are also
      shown. All tuples are shown, whether or not the tuples were visible to
      an MVCC snapshot at the time the raw page was copied.
     </para>
     <para>
      A heap page image obtained with <function>get_raw_page</function> should
      be passed as argument.  For example:
     </para>
     <programlisting>
test=# SELECT * FROM heap_page_items(get_raw_page('pg_class', 0));
     </programlisting>
     <para>
      See <filename>src/include/storage/itemid.h</> and
      <filename>src/include/access/htup.h</> for explanations of the fields
      returned.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>bt_metap(relname text) returns record</function>
    </term>

    <listitem>
     <para>
      <function>bt_metap</function> returns information about a btree
      index's metapage.  For example:
     </para>
     <programlisting>
test=# SELECT * FROM bt_metap('pg_cast_oid_index');
-[ RECORD 1 ]-----
magic     | 340322
version   | 2
root      | 1
level     | 0
fastroot  | 1
fastlevel | 0
     </programlisting>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>bt_page_stats(relname text, blkno int) returns record</function>
    </term>

    <listitem>
     <para>
      <function>bt_page_stats</function> returns summary information about
      single pages of btree indexes.  For example:
     </para>
     <programlisting>
test=# SELECT * FROM bt_page_stats('pg_cast_oid_index', 1);
-[ RECORD 1 ]-+-----
blkno         | 1
type          | l
live_items    | 256
dead_items    | 0
avg_item_size | 12
page_size     | 8192
free_size     | 4056
btpo_prev     | 0
btpo_next     | 0
btpo          | 0
btpo_flags    | 3
     </programlisting>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>bt_page_items(relname text, blkno int) returns setof record</function>
    </term>

    <listitem>
     <para>
      <function>bt_page_items</function> returns detailed information about
      all of the items on a btree index page.  For example:
     </para>
     <programlisting>
test=# SELECT * FROM bt_page_items('pg_cast_oid_index', 1);
 itemoffset |  ctid   | itemlen | nulls | vars |    data
------------+---------+---------+-------+------+-------------
          1 | (0,1)   |      12 | f     | f    | 23 27 00 00
          2 | (0,2)   |      12 | f     | f    | 24 27 00 00
          3 | (0,3)   |      12 | f     | f    | 25 27 00 00
          4 | (0,4)   |      12 | f     | f    | 26 27 00 00
          5 | (0,5)   |      12 | f     | f    | 27 27 00 00
          6 | (0,6)   |      12 | f     | f    | 28 27 00 00
          7 | (0,7)   |      12 | f     | f    | 29 27 00 00
          8 | (0,8)   |      12 | f     | f    | 2a 27 00 00
     </programlisting>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>fsm_page_contents(page bytea) returns text</function>
    </term>

    <listitem>
     <para>
      <function>fsm_page_contents</function> shows the internal node structure
      of a FSM page. The output is a multi-line string, with one line per
      node in the binary tree within the page. Only those nodes that are not
      zero are printed. The so-called "next" pointer, which points to the
      next slot to be returned from the page, is also printed.
     </para>
     <para>
      See <filename>src/backend/storage/freespace/README</> for more
      information on the structure of an FSM page.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </sect2>

</sect1>
