friendlymatthew opened a new pull request, #18253:
URL: https://github.com/apache/datafusion/pull/18253

   ## Which issue does this PR close?
   
   - Related to https://github.com/apache/datafusion/issues/14993
   
   ## Rationale for this change
   
   To enable expression pushdown to file sources, we need to plumb expressions 
through the `FileScanConfig` layer. Currently, `FileScanConfig` only tracks 
column indices for projection, which limits us to simple and naive column 
selection.
   
   This PR begins expression pushdown implementation by having `FileScanConfig` 
own a list of `ProjectionExpr`s, instead of column indices. This allows file 
sources to eventually receive and optimize based on the actual expressions 
being projected. 
   
   
   ## Notes about this PR
   - The first commit is based off of 
https://github.com/apache/datafusion/pull/18231
   - To avoid a super large diff and a harder review, I've decided to break 
(#14993) into 2 tasks:
       - Have the `DataSource` (`FileScanConfig`) actually hold projection 
expressions (this PR)
       - Flow the projection expressions from `DataSourceExec` all the way to 
the `FileSource`


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