postgresql/doc/src/sgml/pageinspect.sgml

<sect1 id="pageinspect">
 <title>pageinspect</title>
 
 <indexterm zone="pageinspect">
  <primary>pageinspect</primary>
 </indexterm>

 <para>
  The functions in this module allow you to inspect the contents of data pages
  at a low level, for debugging purposes.
 </para>

 <sect2>
  <title>Functions included</title>

  <itemizedlist>
   <listitem>
    <para>
     <literal>get_raw_page</literal> reads one block of the named table and returns a copy as a
     bytea field. This allows a single time-consistent copy of the block to be
     made. Use of this functions is restricted to superusers.
    </para>
   </listitem>

   <listitem>
    <para>
     <literal>page_header</literal> shows fields which are common to all PostgreSQL heap and index
     pages. Use of this function is restricted to superusers.
    </para>
    <para>
     A page image obtained with <literal>get_raw_page</literal> should be passed as argument:
    </para>
    <programlisting>
regression=# 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 PageHeaderData struct.
     See src/include/storage/bufpage.h for more details.
    </para>
   </listitem>

   <listitem>
    <para>
     <literal>heap_page_items</literal> 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. Use of this function is restricted to
     superusers.
    </para>
    <para>
     A heap page image obtained with <literal>get_raw_page</literal> should be passed as argument:
    </para>
    <programlisting>
        test=# SELECT * FROM heap_page_items(get_raw_page('pg_class',0));
    </programlisting>
    <para>
     See src/include/storage/itemid.h and src/include/access/htup.h for
     explanations of the fields returned.
    </para>
   </listitem>

   <listitem>
    <para>
     <literal>bt_metap()</literal> returns information about the btree index metapage:
    </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>

   <listitem>
    <para>
     <literal>bt_page_stats()</literal> shows information about single btree pages:
    </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>

   <listitem>
    <para>
     <literal>bt_page_items()</literal> returns information about specific items on btree pages:
    </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>
  </itemizedlist>
 </sect2>
</sect1>