alamb commented on code in PR #23807:
URL: https://github.com/apache/datafusion/pull/23807#discussion_r3647826426
##########
datafusion/expr/src/udf.rs:
##########
@@ -981,10 +986,44 @@ pub trait ScalarUDFImpl: Debug + DynEq + DynHash + Send +
Sync + Any {
/// the input ordering.
///
/// For example, `concat(a || b)` preserves lexicographical ordering, but
`abs(a)` does not.
+ ///
+ /// This is a *non-strict* (monotone) property: distinct inputs may map to
+ /// equal outputs. See [`Self::strictly_order_preserving`] for a related
+ /// strict property and [`ExprProperties::strictly_order_preserving`] for
+ /// an explanation of the difference.
fn preserves_lex_ordering(&self, _inputs: &[ExprProperties]) ->
Result<bool> {
Ok(false)
}
+ /// Returns true if the function is strictly order-preserving with respect
Review Comment:
How is this related to the output_ordering method? I am wondering if we can
extend that rather tan introduce a new method
https://docs.rs/datafusion/latest/datafusion/logical_expr/trait.ScalarUDFImpl.html#method.output_ordering
##########
datafusion/expr/src/udf.rs:
##########
@@ -981,10 +986,44 @@ pub trait ScalarUDFImpl: Debug + DynEq + DynHash + Send +
Sync + Any {
/// the input ordering.
///
/// For example, `concat(a || b)` preserves lexicographical ordering, but
`abs(a)` does not.
+ ///
+ /// This is a *non-strict* (monotone) property: distinct inputs may map to
+ /// equal outputs. See [`Self::strictly_order_preserving`] for a related
+ /// strict property and [`ExprProperties::strictly_order_preserving`] for
+ /// an explanation of the difference.
fn preserves_lex_ordering(&self, _inputs: &[ExprProperties]) ->
Result<bool> {
Ok(false)
}
+ /// Returns true if the function is strictly order-preserving with respect
+ /// to its `Ordered` inputs: assuming those inputs advance simultaneously
+ /// (component-wise, i.e. all of them are sorted in the data), the output
+ /// is ordered in the same direction, equal outputs can only result from
+ /// equal values of those inputs (i.e. the mapping is one-to-one), and
+ /// nulls map to nulls.
+ ///
+ /// i.e. setting this to true means that `a.cmp(b) == f(a).cmp(f(b))`
+ ///
+ /// This is not simply a stricter [`Self::preserves_lex_ordering`] — the
+ /// two properties also assume different input orderings, and with
+ /// multiple ordered inputs neither implies the other. Monotone but
Review Comment:
It would have helped me to move these examples up to the top of the docs
##########
datafusion/ffi/src/expr/expr_properties.rs:
##########
@@ -54,6 +54,9 @@ impl TryFrom<FFI_ExprProperties> for ExprProperties {
sort_properties,
range,
preserves_lex_ordering: value.preserves_lex_ordering,
+ // Not carried over the FFI boundary (it would break the ABI);
+ // stay conservative.
+ strictly_order_preserving: false,
Review Comment:
cc @timsaucer
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