Copilot commented on code in PR #24063:
URL: https://github.com/apache/datafusion/pull/24063#discussion_r3705084739
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datafusion/physical-expr/src/equivalence/properties/mod.rs:
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@@ -1165,8 +1165,27 @@ impl EquivalenceProperties {
self.constraints.project(&indices)
}
- /// Projects the equivalences within according to `mapping` and
- /// `output_schema`.
+ /// Projects these equivalence properties onto `output_schema` according to
+ /// `mapping`, returning the properties that still hold for the projected
+ /// output.
+ ///
+ /// `mapping` maps each source expression (evaluated against the current
+ /// schema) to the output column(s) it produces. This is more than a
+ /// column-index remap: the orderings, the equivalence group and the
+ /// constraints are all carried through `mapping`, keeping only what the
+ /// projected schema can still express.
+ ///
+ /// - Orderings: an existing ordering is carried to a target only when the
+ /// mapping expression is order-preserving for it, as determined from the
+ /// expression's [`SortProperties`]. For example, an ordering on `c` is
+ /// preserved through `c + 1` but dropped through `abs(c)`. Orderings
+ /// implied by the mapping are also derived, e.g. an ordering on `a + b`
+ /// yields one on the projected `a_new + b_new`.
Review Comment:
The example “an ordering on `a + b` yields one on the projected `a_new +
b_new`” is ambiguous because `a + b` is not an ordering *on a column* but an
ordering on an expression, and it’s unclear whether the intent is (1) the input
already had an ordering on the expression `a + b`, or (2) an ordering on `(a,
b)` implies ordering on `a + b` (which is not generally true). Consider
rephrasing to explicitly say “an ordering on the expression `a + b` …” and, if
helpful, clarify that this only applies when that expression exists in the
input properties and remains expressible after projection.
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