Fix some ORDER BY ... DESC problems with InnoDB

sql/ha_heap.h:
  Move HA_NOT_READ_PREFIX_LAST to index_flags()
sql/ha_isammrg.h:
  Move HA_NOT_READ_PREFIX_LAST to index_flags()
sql/handler.h:
  Added comment
sql/sql_select.cc:
  Test HA_NOT_READ_PREFIX_LAST from index_flags() instead of table_flags().
  Should fix some ORDER BY ... DESC problems with InnoDB

sql/ha_heap.h

@@ -41,13 +41,14 @@ class ha_heap: public handler
   {
     return (HA_READ_RND_SAME | HA_NO_INDEX | HA_KEYPOS_TO_RNDPOS |
 	    HA_NO_BLOBS | HA_NULL_KEY | HA_REC_NOT_IN_SEQ |
-	    HA_NOT_READ_PREFIX_LAST | HA_NO_AUTO_INCREMENT);
+	    HA_NO_AUTO_INCREMENT);
   }
   ulong index_flags(uint inx) const
   {
     return ((table->key_info[inx].algorithm == HA_KEY_ALG_BTREE) ?
 	    (HA_READ_NEXT | HA_READ_PREV | HA_READ_ORDER) :
-	    (HA_ONLY_WHOLE_INDEX | HA_WRONG_ASCII_ORDER));
+	    (HA_ONLY_WHOLE_INDEX | HA_WRONG_ASCII_ORDER |
+	     HA_NOT_READ_PREFIX_LAST));
   }
   uint max_record_length() const { return HA_MAX_REC_LENGTH; }
   uint max_keys()          const { return MAX_KEY; }

sql/ha_isammrg.h

@@ -33,9 +33,9 @@ class ha_isammrg: public handler
   const char *table_type() const { return "MRG_ISAM"; }
   const char **bas_ext() const;
   ulong table_flags() const { return (HA_READ_RND_SAME | HA_KEYPOS_TO_RNDPOS |
-				      HA_NOT_READ_PREFIX_LAST |
 				      HA_REC_NOT_IN_SEQ); }
-  ulong index_flags(uint idx) const { return 0; }
+  ulong index_flags(uint idx) const { return HA_NOT_READ_PREFIX_LAST; }
+
   uint max_record_length() const { return HA_MAX_REC_LENGTH; }
   uint max_keys()          const { return 0; }
   uint max_key_parts()     const { return 0; }

sql/handler.h

@@ -81,7 +81,7 @@
 #define HA_READ_PREV		4	/* Read prev. record with same key */
 #define HA_READ_ORDER		8	/* Read through record-keys in order */
 #define HA_ONLY_WHOLE_INDEX	16	/* Can't use part key searches */
-#define HA_NOT_READ_PREFIX_LAST	32
+#define HA_NOT_READ_PREFIX_LAST	32	/* No support for index_read_last() */
 #define HA_KEY_READ_ONLY	64	/* Support HA_EXTRA_KEYREAD */
 
 /*

sql/sql_select.cc

@@ -5576,7 +5576,7 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
 	    Use a traversal function that starts by reading the last row
 	    with key part (A) and then traverse the index backwards.
 	  */
-	  if (table->file->table_flags() & HA_NOT_READ_PREFIX_LAST)
+	  if (table->file->index_flags(ref_key) & HA_NOT_READ_PREFIX_LAST)
 	    DBUG_RETURN(0);			// Use filesort
 	  tab->read_first_record=       join_read_last_key;
 	  tab->read_record.read_record= join_read_prev_same;