I have tried the Orders.CustomerId+0=Customers.CustomerId proposal but
regrettably I get the same natural plan: PLAN JOIN (CUSTOMERS NATURAL,
ORDERS INDEX (IDX_ORDER_DATE)). in fact, it is worst because now I
never get the optimal plan, including the initial statement:

select sum(Orders.units) from Orders inner join Customers on
Orders.CustomerId=Customers.CustomerId
where Orders.OrderDate between `2007/1/1' and `2007/12/31'

returns a optimal plan, but


select sum(Orders.units) from Orders inner join Customers on
Orders.CustomerId+0=Customers.CustomerId
where Orders.OrderDate between `2007/1/1' and `2007/12/31'