As we know, "A “semi-join” between two tables returns rows from the first table where one or more matches are found in the second table. The difference between a semi-join and a conventional join is that rows in the first table will be returned at most once. Even if the second table contains two matches for a row in the first table, only one copy of the row will be returned." (ref http://dbspecialists.com/speeding-queries-semi-joins-anti-joins-oracle-evaluates-exists-not-exists-not)
I wondered why must SemiJoin extend from EquiJoin and have only equi join condition in Calcite? e.g. select * from l where l.a in (select r.c from r where l.b > r.d) According to the definition, the above SQL can be converted to semi-join. There is an equi join condition: l.a = r.c, which can be used as shuffle key on distributed environment. However, it can not be converted to SemiJoin in Calcite now. Because there is a non-equi join conditions: lb. > r.d.
