<entry><filename>pg_serial</filename></entry>
 <entry>Subdirectory containing information about committed serializable transactions</entry>
</row>

<row>
 <entry><filename>pg_snapshots</filename></entry>
 <entry>Subdirectory containing exported snapshots</entry>
</row>

<row>
 <entry><filename>pg_stat</filename></entry>
 <entry>Subdirectory containing permanent files for the statistics
  subsystem</entry>
</row>

<row>
 <entry><filename>pg_stat_tmp</filename></entry>
 <entry>Subdirectory containing temporary files for the statistics
  subsystem</entry>
</row>

<row>
 <entry><filename>pg_subtrans</filename></entry>
 <entry>Subdirectory containing subtransaction status data</entry>
</row>

<row>
 <entry><filename>pg_tblspc</filename></entry>
 <entry>Subdirectory containing symbolic links to tablespaces</entry>
</row>

<row>
 <entry><filename>pg_twophase</filename></entry>
 <entry>Subdirectory containing state files for prepared transactions</entry>
</row>

<row>
 <entry><filename>pg_wal</filename></entry>
 <entry>Subdirectory containing WAL (Write Ahead Log) files</entry>
</row>

<row>
 <entry><filename>pg_xact</filename></entry>
 <entry>Subdirectory containing transaction commit status data</entry>
</row>

<row>
 <entry><filename>postgresql.auto.conf</filename></entry>
 <entry>A file used for storing configuration parameters that are set by
<command>ALTER SYSTEM</command></entry>
</row>

<row>
 <entry><filename>postmaster.opts</filename></entry>
 <entry>A file recording the command-line options the server was
last started with</entry>
</row>

<row>
 <entry><filename>postmaster.pid</filename></entry>
 <entry>A lock file recording the current postmaster process ID (PID),
  cluster data directory path,
  postmaster start timestamp,
  port number,
  Unix-domain socket directory path (could be empty),
  first valid listen_address (IP address or <literal>*</literal>, or empty if
  not listening on TCP),
  and shared memory segment ID
  (this file is not present after server shutdown)</entry>
</row>

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

<para>
For each database in the cluster there is a subdirectory within
<varname>PGDATA</varname><filename>/base</filename>, named after the database's OID in
<structname>pg_database</structname>.  This subdirectory is the default location
for the database's files; in particular, its system catalogs are stored
there.
</para>

<para>
 Note that the following sections describe the behavior of the builtin
 <literal>heap</literal> <link linkend="tableam">table access method</link>,
 and the builtin <link linkend="indexam">index access methods</link>. Due
 to the extensible nature of <productname>PostgreSQL</productname>, other
 access methods might work differently.
</para>

<para>
Each table and index is stored in a separate file.  For ordinary relations,
these files are named after the table or index's <firstterm>filenode</firstterm> number,
which can be found in <structname>pg_class</structname>.<structfield>relfilenode</structfield>. But
for temporary relations, the file name is of the form
<literal>t<replaceable>BBB</replaceable>_<replaceable>FFF</replaceable></literal>, where <replaceable>BBB</replaceable>
is the process number of the backend which created the file, and <replaceable>FFF</replaceable>
is the filenode number.  In either case, in addition to the main file (a/k/a
main fork), each table and index has a <firstterm>free space map</firstterm> (see <xref
linkend="storage-fsm"/>), which stores information about free space available in
the relation.  The free space map is stored in a file named with the filenode
number plus the suffix <literal>_fsm</literal>.  Tables also have a
<firstterm>visibility map</firstterm>, stored in a fork with the suffix <literal>_vm</literal>,
to track which pages are known to have no dead tuples.  The visibility map is
described further in <xref linkend="storage-vm"/>.  Unlogged tables and indexes
have a third fork, known as the initialization fork, which is stored in a fork