ryan-johnson-databricks opened a new pull request, #40885:
URL: https://github.com/apache/spark/pull/40885
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### What changes were proposed in this pull request?
File-source constant metadata columns are often derived indirectly from
file-level metadata values rather than exposing those values directly. Add
support for metadata extractors, so that we can express such columns in a
generic way.
### Why are the changes needed?
Allows to express the existing file-source metadata columns in a generic way
(previously hard-wired), and also allows to lazily derive expensive metadata
values only if the query actually selects them.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
New unit test. Plus, existing file-source metadata unit tests pass.
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