sql/sql_select.cc:
  This assert is not relevant because:
  - the correct assert is DBUG_ASSERT(! (full && sl->uncacheable)) 
    (prevents freeing of uncacheable JOINs), it breaks view.test
  - it seems we can free internal JOINs, even if they are uncacheable:
    if the top level join is evaluated, we're not going to need  the
    internal joins any more