LiaCastaneda commented on code in PR #24162:
URL: https://github.com/apache/datafusion/pull/24162#discussion_r3756483013
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datafusion/physical-expr/src/expressions/lambda.rs:
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@@ -43,6 +43,15 @@ pub struct LambdaExpr {
body: Arc<dyn PhysicalExpr>,
projected_body: Arc<dyn PhysicalExpr>,
projection: Vec<usize>,
+ /// Subset of `params` (by name) that the body actually references,
+ /// computed with nested-lambda shadow tracking. Empty when no parameter
Review Comment:
when a lambda has nested lambdas like `(k, v) -> func(col, (k, v2) -> k + v2
+ v)`, the innermost `k` "shadows"/"overrides" the outermost `k` bc even though
they share a name, the innermost `k` is its own separate parameter, not a
reference to the outer one. So when collecting which parameters a lambda's body
references n a nested lambda, we need to take this possible name conflict into
account, otherwise we'd wrongly count the outer `k` as used just because the
name appears in the body of the nested lambda.
Its true its a bit unclear by just reading it in the param description, I
moved the explanation to `CollectUsedVisitor` that actually handles the logic
for "shadowed" params
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