Add section on showing disk usage.

doc/src/sgml/maintenance.sgml

 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/maintenance.sgml,v 1.12 2002/03/06 06:44:31 momjian Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/maintenance.sgml,v 1.13 2002/06/13 04:36:50 momjian Exp $
 -->
 
 <chapter id="maintenance">
@@ -366,6 +366,92 @@ VACUUM
   </sect2>
  </sect1>
 
+ <sect1 id="diskspace-maintenance">
+  <title>Disk Space Maintenance</title>
+
+  <indexterm zone="diskspace-maintenance">
+   <primary>disk space</primary>
+  </indexterm>
+
+  <para>
+   Each table has a primary heap disk file where most of the data is
+   stored. To store long column values, there is also a
+   <acronym>TOAST</> file associated with the table, named based on the
+   table's oid (actually pg_class.relfilenode), and an index on the
+   <acronym>TOAST</> table. There also may be indexes associated with
+   the base table.
+  </para>
+
+  <para>
+   You can monitor disk space from two places; from inside
+   <application>psql</> and from the command line using
+   <application>contrib/oid2name</>. Using <application>psql</> you can
+   issue queries to see the disk usage for any table:
+<programlisting>
+play=# SELECT relfilenode, relpages
+play-# FROM pg_class
+play-# WHERE relname = 'customer';
+ relfilenode | relpages 
+-------------+----------
+       16806 |       60
+(1 row)
+</programlisting>
+  </para>
+
+  <para>   
+   Each page is typically 8 kilobytes. <literal>relpages</> is only
+   updated by <command>VACUUM</> and <command>ANALYZE</>. To show the
+   space used by <acronym>TOAST</> tables, use a query based on the heap
+   relfilenode:
+<programlisting>
+play=# SELECT relname, relpages
+play-# FROM pg_class
+play-# WHERE relname = 'pg_toast_16806' or
+play-#       relname = 'pg_toast_16806_index'
+play-# ORDER BY relname;
+       relname        | relpages 
+----------------------+----------
+ pg_toast_16806       |        0
+ pg_toast_16806_index |        1
+</programlisting>
+  </para>
+
+  <para>
+   You can easily display index usage too:
+<programlisting>
+play=# SELECT c2.relname, c2.relpages
+play-# FROM pg_class c, pg_class c2, pg_index i
+play-# WHERE c.relname = 'customer' AND
+play-#       c.oid = i.indrelid AND
+play-#       c2.oid = i.indexrelid
+play-# ORDER BY c2.relname;
+       relname        | relpages 
+----------------------+----------
+ customer_id_indexdex |       26
+</programlisting>
+  </para>
+
+  <para>
+   It is easy to find your largest files using <application>psql</>:
+<programlisting>
+play=# SELECT relname, relpages
+play-# FROM pg_class
+play-# ORDER BY relpages DESC;
+       relname        | relpages 
+----------------------+----------
+ bigtable             |     3290
+ customer             |     3144
+</programlisting>
+  </para>
+
+  <para>
+   You can also use <application>oid2name</> to show disk usage. See
+   <filename>README.oid2name</> for examples. It includes a script
+   shows disk usage for each database.
+  </para>
+ </sect1>
+
+
  <sect1 id="logfile-maintenance">
   <title>Log File Maintenance</title>