adriangb commented on PR #20038:
URL: https://github.com/apache/datafusion/pull/20038#issuecomment-3847797343

   This is a really big PR doing a lot of things. The PR description is super 
helpful in getting a high level view, thank you for including that.
   
   ## High level concern
   
   My main high-level concern is that we have an XY problem here: what we (the 
community, users) really want is for `PhysicalExprAdapter` to properly handle 
structs and other casts. I don't think it matters if it internally uses 
`CastColumnExpr` or `CastExpr`. But more broadly: we want to be able to use 
casts in other places (SQL for example) and have them "just work". I think this 
requires a single expression that is able to handle all casts. So our real goal 
should be to make this expression. I think `cast()` is the right expression 
because (1) it predates `CastColumnExpr` and (2) it's standard SQL, etc.
   
   Can we merge `CastColumnExpr` into `cast()` so that `cast()` has all of the 
functionality of `CastColumnExpr`? If so that solves the `PhysicalExprAdapter` 
case as well right?
   
   ## This PR specifically
   
   Regardless I think this PR should be split up into smaller pieces. There's a 
lot of valuable work here that could be merged much quicker if it was part of a 
more focused smaller PR. In particular can some of`Schema rewriter cleanup` be 
pulled out into its own PR?


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