postgresql/doc/src/sgml/pageinspect.sgml

<!-- $PostgreSQL: pgsql/doc/src/sgml/pageinspect.sgml,v 1.3 2007/12/10 05:32:51 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(text, 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.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>page_header(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(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(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(text, 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(text, 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>
  </variablelist>
 </sect2>

</sect1>