wiedld commented on code in PR #17249:
URL: https://github.com/apache/datafusion/pull/17249#discussion_r2286497514
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datafusion/common/src/dfschema.rs:
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@@ -297,6 +297,19 @@ impl DFSchema {
/// Modify this schema by appending the fields from the supplied schema,
ignoring any
/// duplicate fields.
+ ///
+ /// ## Merge Precedence
+ ///
+ /// **Schema-level metadata**: Metadata from both schemas is merged.
+ /// If both schemas have the same metadata key, the value from the second
`other_schema` parameter takes precedence.
+ ///
+ /// **Field-level merging**: Only non-duplicate fields are added. This
means that the
+ /// first `self` fields will always take precedence over the second
`other_schema` fields.
+ /// Duplicate field detection is based on:
+ /// - For qualified fields: both qualifier and field name must match
+ /// - For unqualified fields: only field name needs to match
+ ///
+ /// Note: the merging operation prefers the first `self` fields, and the
second `other_schema` metadata.
Review Comment:
Note that this definition of schema merging behaves a differently in
precedence for fields (prefer self), vs metadata (prefer other).
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