zhuqi-lucas commented on code in PR #23288:
URL: https://github.com/apache/datafusion/pull/23288#discussion_r3518958417
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datafusion/physical-optimizer/src/ensure_requirements/enforce_sorting/sort_pushdown.rs:
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@@ -1042,6 +1054,77 @@ enum RequirementsCompatibility {
NonCompatible,
}
+/// Attempts to push parent ordering requirements through a [`ProjectionExec`].
+///
+/// This is safe when every required sort expression refers to a projected
output
+/// column that is backed by a simple input column. In that case, the
requirement
+/// can be remapped from the projection output schema to the projection input
+/// schema while preserving the original sort options.
+///
+/// For example, a parent requirement on `a@2` over:
+///
+/// ```text
+/// ProjectionExec: expr=[c@2 as c, b@1 as b, a@0 as a]
+/// ```
+///
+/// is remapped to a child requirement on `a@0`.
+///
+/// The implementation is intentionally conservative: computed projection
+/// expressions and non-column sort expressions are not pushed down. Returning
+/// `Ok(None)` leaves sorting above the projection, preserving correctness.
+fn handle_projection_pushdown(
Review Comment:
It seems `remap_requirement_through_projection` in #23199 does a similar
thing — can we deduplicate them?
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