Copilot commented on code in PR #25412:
URL: https://github.com/apache/datafusion/pull/25412#discussion_r4038058795
##########
datafusion/optimizer/src/extract_leaf_expressions.rs:
##########
@@ -702,27 +720,36 @@ fn build_extraction_projection_impl(
// than target_schema (the projection's output) because columns
produced
// by alias expressions (e.g., CSE's __common_expr_N) exist in the
output but
// not the input, and cannot be added as pass-through Column
references.
+ //
+ // Both sides of that check are read in the input's own spelling. A
column can
+ // arrive here unqualified while the input names it `t.c`, and the
other way round:
+ // a projection of bare names over a qualified input is what
eliminating one side
+ // of a union leaves behind. Resolving both sides lets a pass-through
the
+ // projection already carries match the one about to be added, and
pushes the one
+ // that is genuinely new under the name the input gives it. Left
unresolved, the
+ // merged projection would hold `t.c` and a bare `c` together, which
+ // `Projection::try_new` rejects as ambiguous.
+ let input_schema = existing.input.schema();
let existing_cols: IndexSet<Column> = existing
.expr
.iter()
.filter_map(|e| {
if let Expr::Column(c) = e {
- Some(c.clone())
+ resolve_against(input_schema, c)
Review Comment:
This set ignores trivial pass-through aliases such as `p.c AS c`, even
though `passthrough_column` explicitly treats them as pass-throughs. When
`columns_needed` contains `c`, the replacement map resolves it to `p.c`;
omitting the retained alias here then appends `p.c` beside the existing
unqualified `c`, recreating the same ambiguous schema. Normalize every
`passthrough_column(e)` here, and add a regression case with a same-name alias.
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