- create_ref_for_key() has the code that walks KEYUSE array and tries to use
  maximum number of keyparts for ref (and eq_ref and ref_or_null) access.
  When one constructs ref access for table that is inside a SJ-Materialization
  nest, it is not possible to use tables that are ouside the nest (because 
  materialization is performed before they have any "current value").
  The bug was caused by this function not taking this into account.