drin commented on code in PR #20008:
URL: https://github.com/apache/datafusion/pull/20008#discussion_r2729394863
##########
datafusion/expr/src/udf.rs:
##########
@@ -709,20 +709,49 @@ pub trait ScalarUDFImpl: Debug + DynEq + DynHash + Send +
Sync {
Ok(ExprSimplifyResult::Original(args))
}
- /// Returns the [preimage] for this function and the specified scalar
value, if any.
+ /// Returns a single contiguous preimage for this function and the
specified
+ /// scalar expression, if any.
///
- /// A preimage is a single contiguous [`Interval`] of values where the
function
- /// will always return `lit_value`
+ /// # Return Value
///
- /// Implementations should return intervals with an inclusive lower bound
and
- /// exclusive upper bound.
+ /// Implementations should return a half-open interval: inclusive lower
+ /// bound and exclusive upper bound. Note that this is slightly different
+ /// from normal [`Interval`] semantics where the upper bound is closed. The
+ /// upper endpoint should be adjusted to the next value.
///
- /// This rewrite is described in the [ClickHouse Paper] and is particularly
- /// useful for simplifying expressions `date_part` or equivalent
functions. The
- /// idea is that if you have an expression like `date_part(YEAR, k) =
2024` and you
- /// can find a [preimage] for `date_part(YEAR, k)`, which is the range of
dates
- /// covering the entire year of 2024. Thus, you can rewrite the expression
to `k
- /// >= '2024-01-01' AND k < '2025-01-01' which is often more optimizable.
+ /// # Background
+ ///
+ /// A [preimage] here is a single contiguous [`Interval`] of the function's
+ /// argument(s) where the function will return a single literal (constant)
+ /// value. This can also be thought of as a form of interval containment.
+ ///
+ /// Using a preimage to rewrite predicates is described in the [ClickHouse
+ /// Paper]:
Review Comment:
```suggestion
/// Inspired by the [ClickHouse Paper], a "preimage rewrite" transforms
/// a predicate containing a function call into a predicate containing an
/// equivalent set of input literal (constant) values. The resulting
predicate
/// is then in a form that can be further optimized by other rewrites.
/// From the paper:
```
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