BUG#35850 "Performance regression in 5.1.23/5.1.24"

- Disable the "prefer full scan on clustered primary key over full scan
  of any secondary key" rule introduced by BUG#35850.
- Update test results accordingly 
(bk trigger: file this for BUG#35850)

sql/sql_select.cc

@@ -6506,13 +6506,16 @@ make_join_readinfo(JOIN *join, ulonglong options)
 		   !(tab->select && tab->select->quick))
 	  {					// Only read index tree
 	    /*
-	      See bug #26447: "Using the clustered index for a table scan
-	      is always faster than using a secondary index".
-	    */
+            It has turned out that the below change, while speeding things
+            up for disk-bound loads, slows them down for cases when the data
+            is in disk cache (see BUG#35850):
+	    //  See bug #26447: "Using the clustered index for a table scan
+	    //  is always faster than using a secondary index".
             if (table->s->primary_key != MAX_KEY &&
                 table->file->primary_key_is_clustered())
               tab->index= table->s->primary_key;
             else
+	    */
               tab->index=find_shortest_key(table, & table->covering_keys);
 	    tab->read_first_record= join_read_first;
 	    tab->type=JT_NEXT;		// Read with index_first / index_next