lyne7-sc opened a new pull request, #22905:
URL: https://github.com/apache/datafusion/pull/22905
## Which issue does this PR close?
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- Related to #19458
- Related to #20935
## Rationale for this change
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Why are you proposing this change? If this is already explained clearly in
the issue then this section is not needed.
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When a `Dictionary(K, Utf8)` column is passed to a string scalar function,
the type-coercion layer currently materializes it to flat Utf8/Utf8View before
the function runs, so the operation is applied to every row instead of just the
unique dictionary values, and the dictionary encoding is lost on the output.
See #19458 for the underlying coercion behavior and #20935 for the
string-function-specific impact.
This is wasteful for low-cardinality columns and inflates Arrow IPC/Flight
message sizes downstream.
## What changes are included in this PR?
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- Add `EncodingPreservation { None, Dictionary }` and opt-in constructors on
`Coercion` (`new_exact_preserving_encoding`, `with_encoding_preservation`).
- In `get_valid_types`, when a `Coercible` arg requests `Dictionary`
preservation, run coercion against the dictionary's value type and re-wrap the
result as `Dictionary(K, V')`, so the function receives a `DictionaryArray`.
- Make `lower/upper` opt in and handle dictionary inputs (array + scalar) by
converting only the dictionary values and re-wrapping with the original keys.
## Are these changes tested?
Yes
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## Are there any user-facing changes?
Yes. upper/lower now return Dictionary(...) for dictionary inputs instead of
the previously materialized Utf8View. New public API (EncodingPreservation, new
Coercion constructors) is added — please add the api change label.
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