[GitHub] [arrow] alamb commented on pull request #8785: ARROW-10729: [Rust] [DataFusion] Add SQL support for JOIN using implicit syntax [WIP]

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alamb commented on pull request #8785:
URL: https://github.com/apache/arrow/pull/8785#issuecomment-735225696


   > @jorgecarleitao This code is basically working but I think it has exposed a bug in the filter push down logic where it tries to push a filter down through the join where the expression is left_column = right_column so it is not valid to push this through the join because it depends on the output from the join.
   
   I think another way to think about this is that "the expression refers to two relations and thus can only be evaluated when they have already been joined"
   
   One way to represent this is to track which relation each `Column` reference is to (aka with multiple tables in a SQL query, a single column name is insufficient to identify what column is being referred to).
   
   Until we add "to what what relation is this column" into the column reference itself, this kind of plan will be tricky to pull off


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