Note that the example aggregate array_accum is comparable to the now

built-in array_agg.  Per suggestion from Robert Haas.

doc/src/sgml/xaggr.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/xaggr.sgml,v 1.35 2007/02/01 00:28:18 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/xaggr.sgml,v 1.36 2008/11/20 21:10:44 tgl Exp $ -->
 
  <sect1 id="xaggr">
   <title>User-Defined Aggregates</title>
@@ -132,7 +132,10 @@ CREATE AGGREGATE array_accum (anyelement)
 </programlisting>
 
    Here, the actual state type for any aggregate call is the array type
-   having the actual input type as elements.
+   having the actual input type as elements.  The behavior of the aggregate
+   is to concatenate all the inputs into an array of that type.
+   (Note: the built-in aggregate <function>array_agg</> provides similar
+   functionality, with better performance than this definition would have.)
   </para>
 
   <para>
@@ -149,14 +152,14 @@ SELECT attrelid::regclass, array_accum(attname)
  pg_tablespace | {spcname,spcowner,spclocation,spcacl}
 (1 row)
 
-SELECT attrelid::regclass, array_accum(atttypid)
+SELECT attrelid::regclass, array_accum(atttypid::regtype)
     FROM pg_attribute
     WHERE attnum &gt; 0 AND attrelid = 'pg_tablespace'::regclass
     GROUP BY attrelid;
 
    attrelid    |        array_accum        
----------------+-----------------
- pg_tablespace | {19,26,25,1034}
+---------------+---------------------------
+ pg_tablespace | {name,oid,text,aclitem[]}
 (1 row)
 </programlisting>
   </para>