unallocated space. New item identifiers are allocated from
the start of this area, new items from the end.</entry>
</row>

<row>
<entry>Items</entry>
<entry>The actual items themselves.</entry>
</row>

<row>
<entry>Special space</entry>
<entry>Index access method specific data. Different methods store different
data. Empty in ordinary tables.</entry>
</row>

</tbody>
</tgroup>
</table>

 <para>

  The first 24 bytes of each page consists of a page header
  (<structname>PageHeaderData</structname>). Its format is detailed in <xref
  linkend="pageheaderdata-table"/>. The first field tracks the most
  recent WAL entry related to this page. The second field contains
  the page checksum if <xref linkend="app-initdb-data-checksums"/> are
  enabled.  Next is a 2-byte field containing flag bits. This is followed
  by three 2-byte integer fields (<structfield>pd_lower</structfield>,
  <structfield>pd_upper</structfield>, and
  <structfield>pd_special</structfield>).  These contain byte offsets
  from the page start to the start of unallocated space, to the end of
  unallocated space, and to the start of the special space.  The next 2
  bytes of the page header, <structfield>pd_pagesize_version</structfield>,
  store both the page size and a version indicator.  Beginning with
  <productname>PostgreSQL</productname> 8.3 the version number is 4;
  <productname>PostgreSQL</productname> 8.1 and 8.2 used version number 3;
  <productname>PostgreSQL</productname> 8.0 used version number 2;
  <productname>PostgreSQL</productname> 7.3 and 7.4 used version number 1;
  prior releases used version number 0.
  (The basic page layout and header format has not changed in most of these
  versions, but the layout of heap row headers has.)  The page size
  is basically only present as a cross-check; there is no support for having
  more than one page size in an installation.
  The last field is a hint that shows whether pruning the page is likely
  to be profitable: it tracks the oldest un-pruned XMAX on the page.

 </para>

 <table tocentry="1" id="pageheaderdata-table">
 <title>PageHeaderData Layout</title>
 <titleabbrev>PageHeaderData Layout</titleabbrev>
 <tgroup cols="4">
 <thead>
  <row>
   <entry>Field</entry>
   <entry>Type</entry>
   <entry>Length</entry>
   <entry>Description</entry>
  </row>
 </thead>
 <tbody>
  <row>
   <entry>pd_lsn</entry>
   <entry>PageXLogRecPtr</entry>
   <entry>8 bytes</entry>
   <entry>LSN: next byte after last byte of WAL record for last change
   to this page</entry>
  </row>
  <row>
   <entry>pd_checksum</entry>
   <entry>uint16</entry>
   <entry>2 bytes</entry>
   <entry>Page checksum</entry>
  </row>
  <row>
   <entry>pd_flags</entry>
   <entry>uint16</entry>
   <entry>2 bytes</entry>
   <entry>Flag bits</entry>
  </row>
  <row>
   <entry>pd_lower</entry>
   <entry>LocationIndex</entry>
   <entry>2 bytes</entry>
   <entry>Offset to start of free space</entry>
  </row>
  <row>
   <entry>pd_upper</entry>
   <entry>LocationIndex</entry>
   <entry>2 bytes</entry>
   <entry>Offset to end of free space</entry>
  </row>
  <row>
   <entry>pd_special</entry>
   <entry>LocationIndex</entry>
   <entry>2 bytes</entry>
   <entry>Offset to start of special space</entry>
  </row>
  <row>
   <entry>pd_pagesize_version</entry>
   <entry>uint16</entry>
   <entry>2 bytes</entry>
   <entry>Page size and layout version number information</entry>
  </row>
  <row>
   <entry>pd_prune_xid</entry>
   <entry>TransactionId</entry>
   <entry>4 bytes</entry>
   <entry>Oldest unpruned XMAX on page, or zero if none</entry>
  </row>
 </tbody>
 </tgroup>
 </table>

 <para>
  All the details can be found in
  <filename>src/include/storage/bufpage.h</filename>.
 </para>

 <para>
  Following the page header are item identifiers
  (<type>ItemIdData</type>), each requiring four bytes.
  An item identifier contains a byte-offset to
  the start of an item, its length in bytes, and a few attribute bits
  which affect its interpretation.
  New item identifiers