In addition to the JOIN operators, U-SQL optionally allows the query author to provide hints on the type of join to choose and what parallel join execution to use.
Join_Hints := [Parallel_Join_Hint] [Join_Algorithm_Hint].
Parallel_Join_Hint := 'SERIAL' | 'PAIR' | 'BROADCASTLEFT' | 'BROADCASTRIGHT' | 'FULLCROSS'.
Join_Algorithm_Hint := 'MERGE' | 'HASH' | 'LOOP' | 'INDEXLOOKUP'.
These hints impact the plan generation of the query optimizer. If the optimizer cannot find a succession of optimizer rules to satisfy the requested hints, for example because the hints are not applicable to a specific JOIN, a NO PLAN error will be raised.
These hints are best used with care, since specifying a certain hint will make it impossible for the optimizer to find a better plan. They should only be used if the default plan is not optimal.