Help!!! Mutating table error !!!!!!

 We created two tables, table1 and table2, and table1 is a parent table of  table2.

  Table1 has three fileds,
  obj_name, obj_type and total_number

  Table2 has three fields
  obj_name, obj_type and obj_values

  We also created a database trigger which will update the total_number in   table1 every time the record in table2 was deleted or inserted. The   trigger look like this: (it may have some buggs )

   Create or replace trigger tri_name

     after
     INSERT or DELETE on table2 for each row
     declare
       total  number;
     begin
       select count(*) into total from table2
         where obj_name = :old.obj_name
         group by obj_name;

       update table1
       set total_number = total
       where obj_name = :old.obj_name;
     end;

     We are getting ORA-04091 table table2 is mutating, trigger/function may
     not see it.

     How can we write a trigger to update table1 when the total number of
     the obj is changed.

     Please e-mail to joe.kau_at_lmco.com

     Any help will be appreciated.....