this call will trigger a subquery to fetch all the customer details
No, not unless you set the fetch mode to join or subquery.
As per the entity mapping definition you've mentioned below, it does not have a fetch mode. This means that the associations are loaded using proxies. This is the default fetch mode. Since CUST_ID is the identifier, only the id of the customer is loaded into the persistent context.
With your generated entity mapping, it should just work as you expect.