Make ExecEvalFieldSelect throw a more intelligible error if it's asked to

extract a system column, and remove a couple of lines that are useless
in light of the fact that we aren't ever going to support this case.  There
isn't much point in trying to make this work because a tuple Datum does
not carry many of the system columns.  Per experimentation with a case
reported by Dean Rasheed; we'll have to fix his problem somewhere else.

src/backend/executor/execQual.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execQual.c,v 1.259 2010/01/02 16:57:41 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execQual.c,v 1.260 2010/01/09 20:46:19 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -3766,12 +3766,20 @@ ExecEvalFieldSelect(FieldSelectState *fstate,
 	tupDesc = get_cached_rowtype(tupType, tupTypmod,
 								 &fstate->argdesc, econtext);
 
-	/* Check for dropped column, and force a NULL result if so */
-	if (fieldnum <= 0 ||
-		fieldnum > tupDesc->natts)		/* should never happen */
+	/*
+	 * Find field's attr record.  Note we don't support system columns here:
+	 * a datum tuple doesn't have valid values for most of the interesting
+	 * system columns anyway.
+	 */
+	if (fieldnum <= 0)					/* should never happen */
+		elog(ERROR, "unsupported reference to system column %d in FieldSelect",
+			 fieldnum);
+	if (fieldnum > tupDesc->natts)		/* should never happen */
 		elog(ERROR, "attribute number %d exceeds number of columns %d",
 			 fieldnum, tupDesc->natts);
 	attr = tupDesc->attrs[fieldnum - 1];
+
+	/* Check for dropped column, and force a NULL result if so */
 	if (attr->attisdropped)
 	{
 		*isNull = true;
@@ -3787,14 +3795,8 @@ ExecEvalFieldSelect(FieldSelectState *fstate,
 						   format_type_be(attr->atttypid),
 						   format_type_be(fselect->resulttype))));
 
-	/*
-	 * heap_getattr needs a HeapTuple not a bare HeapTupleHeader.  We set all
-	 * the fields in the struct just in case user tries to inspect system
-	 * columns.
-	 */
+	/* heap_getattr needs a HeapTuple not a bare HeapTupleHeader */
 	tmptup.t_len = HeapTupleHeaderGetDatumLength(tuple);
-	ItemPointerSetInvalid(&(tmptup.t_self));
-	tmptup.t_tableOid = InvalidOid;
 	tmptup.t_data = tuple;
 
 	result = heap_getattr(&tmptup,