why outer join supports only simple equijoin and is not supported with aggregate

[xcl961717684]

I read the paper "Efficient_Maintenance_of_Materialized_Outer-Join_Views" and your code, it is theoretically possible to support more join conditions， but you only support simple equivalence conditions. Do you take efficiency into consideration or do you have any special reasons？

And your inner join with aggregate worked out perfectly，but outer join is not supported with aggregate. Is there any technical difficulty here? Can you tell me your consideration？