Disallow foreign-key references from temp tables to permanent tables.

Per recent discussion, this does not work because other backends can't
reliably see tuples in a temp table and so cannot run the RI checks
correctly.  Seems better to disallow this case than go back to accessing
temp tables through shared buffers.  Also, disallow FK references to
ON COMMIT DELETE ROWS tables.  We already caught this problem for normal
TRUNCATE, but the path used by ON COMMIT didn't check.

doc/src/sgml/ref/truncate.sgml

@@ -1,5 +1,5 @@
 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/truncate.sgml,v 1.13 2003/09/12 00:12:47 tgl Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/truncate.sgml,v 1.14 2003/09/19 21:04:19 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -50,6 +50,16 @@ TRUNCATE [ TABLE ] <replaceable class="PARAMETER">name</replaceable>
   </variablelist>
  </refsect1>
 
+ <refsect1>
+  <title>Notes</title>
+
+  <para>
+   <command>TRUNCATE</> cannot be used if there are foreign-key references
+   to the table from other tables.  Checking validity in such cases would
+   require table scans, and the whole point is not to do one.
+  </para>
+ </refsect1>
+
  <refsect1>
   <title>Examples</title>