[GitHub] [arrow-datafusion] houqp commented on pull request #605: fix join column handling logic for `On` and `Using` constraints

[GitBox <gi...@apache.org>]

houqp commented on pull request #605:
URL: https://github.com/apache/arrow-datafusion/pull/605#issuecomment-867081932


   Good catch on the regressions, I will get them fixed tonight with unit tests.
   
   > It seems like a core challenge is in the semantics of the * expansion in select * from ... type queries which varies depending on type of join.
   
   Yeah, that's exactly the problem.
   
   Initially, I tried the approach of keeping the join columns from both relations (i.e. `f1.foo` and `f2.foo`) in the logical schema. Then I ran into the problem of it breaking the [physical planning invariants](https://github.com/apache/arrow-datafusion/blob/master/docs/specification/invariants.md#the-physical-schema-is-invariant-under-planning). Because for `Using` join, we are only producing a single join column in the output, the physical plan schema can only has a single field for the join column. The schema from the logical plan will need to match that as well in order to honor that invariant.


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