Fixed some comments , plus introduced some tests so that I do not

make some new mistakes.

mysql-test/r/merge.result

@@ -470,4 +470,7 @@ CREATE TABLE t (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0
 select max(b) from t where a = 2;
 max(b)
 NULL
+select max(b) from t1 where a = 2;
+max(b)
+1
 drop table if exists t,t1,t2;

mysql-test/t/merge.test

@@ -174,4 +174,5 @@ CREATE TABLE t2 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '
 INSERT INTO t2 VALUES (1,2), (2,2);
 CREATE TABLE t (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  KEY a (a,b)) TYPE=MRG_MyISAM UNION=(t1,t2);
 select max(b) from t where a = 2;
+select max(b) from t1 where a = 2;
 drop table if exists t,t1,t2;

sql/sql_update.cc

@@ -506,7 +506,7 @@ multi_update::prepare(List<Item> &values)
 void
 multi_update::initialize_tables(JOIN *join)
 {
-/* 
+#ifdef NOT_YET
    We skip it as it only makes a mess ...........
   TABLE_LIST *walk;
   table_map tables_to_update_from=0;
@@ -522,12 +522,12 @@ multi_update::initialize_tables(JOIN *join)
     {
        We are going to update from this table 
        TABLE *tbl=walk->table=tab->table;
-       Don't use KEYREAD optimization on this table 
+       /* Don't use KEYREAD optimization on this table */
        tbl->no_keyread=1;
        walk=walk->next;
     }
   }
-*/
+#endif
 }