Improve COPY documentation to clarify that it doesn't copy data to or from

child tables.  Per gripe from Jaime Casanova.

doc/src/sgml/ref/copy.sgml

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-$PostgreSQL: pgsql/doc/src/sgml/ref/copy.sgml,v 1.97 2010/04/23 23:21:43 rhaas Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/copy.sgml,v 1.98 2010/04/30 19:49:06 tgl Exp $
 PostgreSQL documentation
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@@ -308,6 +308,16 @@ COPY <replaceable class="parameter">count</replaceable>
     <replaceable class="parameter">viewname</replaceable>) TO ...</literal>.
    </para>
 
+   <para>
+    <command>COPY</command> only deals with the specific table named;
+    it does not copy data to or from child tables.  Thus for example
+    <literal>COPY <replaceable class="parameter">table</> TO</literal>
+    shows the same data as <literal>SELECT * FROM ONLY <replaceable
+    class="parameter">table</></literal>.  But <literal>COPY
+    (SELECT * FROM <replaceable class="parameter">table</>) TO ...</literal>
+    can be used to dump all of the data in an inheritance hierarchy.
+   </para>
+
    <para>
     You must have select privilege on the table
     whose values are read by <command>COPY TO</command>, and