Improve C comment about why we return "0 0" for some tags.

src/backend/tcop/pquery.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/tcop/pquery.c,v 1.134 2010/01/02 16:57:52 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/tcop/pquery.c,v 1.135 2010/02/13 22:45:41 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1318,9 +1318,14 @@ PortalRunMulti(Portal portal, bool isTopLevel,
 	 * If a command completion tag was supplied, use it.  Otherwise use the
 	 * portal's commandTag as the default completion tag.
 	 *
-	 * Exception: clients will expect INSERT/UPDATE/DELETE tags to have
-	 * counts, so fake something up if necessary.  (This could happen if the
-	 * original query was replaced by a DO INSTEAD rule.)
+	 * Exception: Clients expect INSERT/UPDATE/DELETE tags to have
+	 * counts, so fake them with zeros.  This can happen with DO INSTEAD
+	 * rules if there is no replacement query of the same type as the
+	 * original.  We print "0 0" here because technically there is no
+	 * query of the matching tag type, and printing a non-zero count for
+	 * a different query type seems wrong, e.g.  an INSERT that does
+	 * an UPDATE instead should not print "0 1" if one row
+	 * was updated.  See QueryRewrite(), step 3, for details.
 	 */
 	if (completionTag && completionTag[0] == '\0')
 	{