haohuaijin commented on code in PR #25227:
URL: https://github.com/apache/datafusion/pull/25227#discussion_r3999343833
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datafusion/physical-expr/src/expressions/cast.rs:
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@@ -249,8 +249,11 @@ impl CastExpr {
})
}
- /// Check if casting from the specified source type to the target type is a
- /// widening cast (e.g. from `Int8` to `Int16`).
+ /// Check if casting from the source type to the target type is known to be
+ /// lossless and strictly order-preserving for all source values,
preserving nulls.
+ /// This includes widening casts (e.g. `Int8` to `Int16`) and
representation
+ /// conversions such as `Int32` to `Date32`, which interprets the same
integer
+ /// as days since the epoch.
pub fn check_bigger_cast(cast_type: &DataType, src: &DataType) -> bool {
if cast_type.eq(src) {
return true;
Review Comment:
We can extend this helper in a follow-up to recognize more safe conversions,
such as `UInt32` → `Int64`, allowing statistics to be preserved without
requiring exact bounds on both ends.
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