Github user lincoln-lil commented on the issue:
https://github.com/apache/flink/pull/3141
@fhueske agree with you that the check in validation phase of the Table API
is useful for failing fast( IMO, it seems no difference on user experience that
when an exception be thrown either in validation or translation phase ?), but
there may be other rules can match and generate alternative plans to execute an
outer join. For instance, we internal implemented a HbaseTableSource extends
TableSource that can be joined as a TableSourceTable. We created a new rule
that is used to match inner and outer joins. Though such special tableSource
probably may not be suitable to be merged into Flink
master, in the future there could be other TableSourceTables which can
support different join predicates depending on their different physical storage
characteristics.
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