xudong963 commented on code in PR #17419:
URL: https://github.com/apache/datafusion/pull/17419#discussion_r2385828068
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datafusion/physical-expr/src/equivalence/projection.rs:
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@@ -156,6 +157,97 @@ impl FromIterator<(Arc<dyn PhysicalExpr>,
ProjectionTargets)> for ProjectionMapp
}
}
+/// Projects a slice of [LexOrdering]s onto the given schema.
+///
+/// This is a convenience wrapper that applies [project_ordering] to each
+/// input ordering and collects the successful projections:
+/// - For each input ordering, the result of [project_ordering] is appended to
+/// the output if it is `Some(...)`.
+/// - Order is preserved and no deduplication is attempted.
+/// - If none of the input orderings can be projected, an empty `Vec` is
+/// returned.
+///
+/// See [project_ordering] for the semantics of projecting a single
+/// [LexOrdering].
+pub fn project_orderings(
+ orderings: &[LexOrdering],
+ schema: &SchemaRef,
+) -> Vec<LexOrdering> {
+ let mut projected_orderings = vec![];
+
+ for ordering in orderings {
+ projected_orderings.extend(project_ordering(ordering, schema));
+ }
+
+ projected_orderings
+}
+
+/// Projects a single [LexOrdering] onto the given schema.
+///
+/// This function attempts to rewrite every [PhysicalSortExpr] in the provided
+/// [LexOrdering] so that any [Column] expressions point at the correct field
+/// indices in `schema`.
+///
+/// Key details:
+/// - Columns are matched by name, not by index. The index of each matched
+/// column is looked up with
[Schema::column_with_name](arrow::datatypes::Schema::column_with_name) and a new
+/// [Column] with the correct [index](Column::index) is substituted.
+/// - If an expression references a column name that does not exist in
+/// `schema`, projection of the current ordering stops and only the already
+/// rewritten prefix is kept. This models the fact that a lexicographical
+/// ordering remains valid for any leading prefix whose expressions are
+/// present in the projected schema.
+/// - If no expressions can be projected (i.e. the first one is missing), the
+/// function returns `None`.
+///
+/// Return value:
+/// - `Some(LexOrdering)` if at least one sort expression could be projected.
+/// The returned ordering may be a strict prefix of the input ordering.
+/// - `None` if no part of the ordering can be projected onto `schema`.
+///
+/// Example
+///
+/// Suppose we have an input ordering `[col("a@0"), col("b@1")]` but the
projected
+/// schema only contains b and not a. The result will be `Some([col("a@0")])`.
In other
+/// words, the column reference is reindexed to match the projected schema.
+/// If neither a nor b is present, the result will be None.
+pub fn project_ordering(
+ ordering: &LexOrdering,
+ schema: &SchemaRef,
+) -> Option<LexOrdering> {
+ let mut projected_exprs = vec![];
+ for PhysicalSortExpr { expr, options } in ordering.iter() {
+ let transformed = Arc::clone(expr).transform_up(|expr| {
+ let Some(col) = expr.as_any().downcast_ref::<Column>() else {
+ return Ok(Transformed::no(expr));
+ };
+
+ let name = col.name();
+ if let Some((idx, _)) = schema.column_with_name(name) {
+ // Compute the new column expression (with correct index)
after projection:
+ Ok(Transformed::yes(Arc::new(Column::new(name, idx))))
+ } else {
+ // Cannot find expression in the projected_schema,
+ // signal this using an Err result
+ plan_err!("")
+ }
+ });
+
+ match transformed {
+ Ok(transformed) => {
+ projected_exprs.push(PhysicalSortExpr::new(transformed.data,
*options));
+ }
+ Err(_) => {
+ // Err result indicates an expression could not be found in the
+ // projected_schema, stop iterating since rest of the
orderings are violated
+ break;
Review Comment:
Should we give a `warn!()` here?
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