merge + order by (bug #515) temporary fix

properly fixed in 4.1

mysql-test/r/merge.result

@@ -607,4 +607,6 @@ x	y
 1	3
 select * from t3 where x = 1 and y < 5 order by y desc;
 x	y
+1	3
+1	2
 drop table t1,t2,t3;

sql/ha_myisammrg.h

@@ -40,7 +40,8 @@ class ha_myisammrg: public handler
   }
   ulong index_flags(uint inx) const
   {
-    ulong flags=(HA_READ_NEXT | HA_READ_PREV | HA_READ_ORDER);
+    ulong flags=(HA_READ_NEXT | HA_READ_PREV | HA_READ_ORDER |
+        HA_NOT_READ_PREFIX_LAST); // This - last - flag is ONLY for 4.0 !!!
     return (flags | ((table->key_info[inx].algorithm == HA_KEY_ALG_FULLTEXT) ?
 		     0 : HA_KEY_READ_ONLY));
   }

sql/sql_select.cc

@@ -5843,6 +5843,8 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
 	  */
 	  if (!select->quick->reverse_sorted())
 	  {
+            if (table->file->index_flags(ref_key) & HA_NOT_READ_PREFIX_LAST)
+              DBUG_RETURN(0);			// Use filesort
 	    // ORDER BY range_key DESC
 	    QUICK_SELECT_DESC *tmp=new QUICK_SELECT_DESC(select->quick,
 							 used_key_parts);