Problem:-
In derived tables implementation, We only get to use indexes on derived tables 
on the first execution of a procedure and  in later execution we are not able 
to use indexes.


Analysis:-
In case of derived tables, we call add_key_field() to add new fake keys for 
range optimizer. To choose the best key to read data. After creating the actual 
keys from this fake keys. 
We set the variable in TABLE_LIST::derived_keys_ready, to show that actual 
keys are created.
This table_list is maintained in the memory after our exection is completed.                       
So when we call our procedure second time, this TABLE_LIST::derived_keys_ready 
is already set and we assume that actual keys are already being created, but we 
will not find any key, which cause change in query execution plan.

Solution:-
Reset the TABLE_LIST::derived_keys_ready at the time JOIN_TAB::cleanup.