james-willis commented on code in PR #24865:
URL: https://github.com/apache/datafusion/pull/24865#discussion_r4392421544
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datafusion/common/src/metadata.rs:
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@@ -117,6 +117,36 @@ pub fn check_metadata_with_storage_equal(
Ok(())
}
+/// Compute the field metadata to propagate for an expression whose result
+/// may come from any one of several inputs coerced to a common type, such as
+/// `COALESCE`, `NVL2`, or `CASE WHEN`.
+///
+/// Returns the first non-empty metadata among the fields whose data type is
+/// not `Null` (an untyped NULL literal carries no metadata and cannot
+/// contribute a typed value), or empty metadata if there is none.
+///
+/// The inputs are deliberately not required to agree on their metadata:
+/// byte-wise comparison is too strict for extension types whose parameters
+/// are JSON-encoded, and unrelated metadata that arrived with the data (e.g.
+/// from an Arrow file or an embedded Arrow schema in a Parquet file) must not
+/// cause an extension type to be silently dropped from the result. Engines
+/// that want stricter behavior (e.g. rejecting conflicting extension types)
+/// can enforce it with an analyzer or optimizer rule, which can only observe
+/// the conflict if planning preserves the metadata in the first place.
+pub fn coerced_fields_metadata<'a>(
+ mut fields: impl Iterator<Item = &'a Field>,
Review Comment:
done
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