[GitHub] [arrow] jorgecarleitao commented on pull request #8839: ARROW-10732: [Rust] [DataFusion] Integrate DFSchema as a step towards supporting qualified column names

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jorgecarleitao commented on pull request #8839:
URL: https://github.com/apache/arrow/pull/8839#issuecomment-739448512


   Hey @andygrove . Thanks a lot for this!
   
   I would benefit from understanding the use-case for `DFSchema` at the physical plan. Note that this is primarily for my own understanding, as I am only familiar with qualifier names in SQL to disambiguate columns in expressions concerning more than one table - not in the representation of a statement at the logical and physical plan. Maybe you could give an example of where `arrow::Schema` is not sufficient at the physical level?
   
   My current understanding is that, without qualifiers, we can't write things like `(table1.a + 1) >= (table2.b - 1)`.
   
   What I am trying to understand is when do we need such an expression at the physical level. Typically, these plans require some form of join and are mapped to `filter(join(a, b))`, in which case I do not see how a qualifier is used: before the join there are two input nodes that are joined on a key (i.e. always an equality relationship between columns); after the join, there is a single node, and thus qualifiers are not needed.
   
   One use case case I see for this is when the join is itself over an expression, e.g. `JOIN ON (table1.a + 1) == (table2.b - 1)`. However, in this case, at the physical level, this can always be mapped to `join(projection())`. I.e. it seems to me that it is more of a convenience at building a logical statement than a necessity for executing such a statement.
   
   If the goal is that we can add the qualifier to the column name after the join, to desambiguate `table1.a` from `table2.a`, wouldn't it be easier to do that at the logical plan?


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