kosiew commented on code in PR #24490:
URL: https://github.com/apache/datafusion/pull/24490#discussion_r3940334233
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datafusion/optimizer/src/simplify_expressions/expr_simplifier.rs:
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@@ -747,6 +747,105 @@ impl ConstEvaluator {
}
}
+/// Position of the single pushdown-eligible CASE argument; every other
+/// argument must be a metadata-free literal.
+fn case_pushdown_target(args: &[Expr]) -> Option<usize> {
+ let mut case_position = None;
+ for (position, arg) in args.iter().enumerate() {
+ match arg {
+ Expr::Literal(_, None) => {}
+ Expr::Case(case) if case_pushdown_eligible(case) => {
+ if case_position.replace(position).is_some() {
+ return None;
+ }
+ }
+ _ => return None,
+ }
+ }
+ case_position
+}
+
+fn case_pushdown_eligible(case: &Case) -> bool {
+ if case.expr.is_some() || case.when_then_expr.is_empty() {
+ return false;
+ }
+ let mut output_type: Option<DataType> = None;
+ let mut literal_of_common_type = |e: &Expr| -> bool {
+ match e {
+ Expr::Literal(s, None) => {
+ let data_type = s.data_type();
+ if data_type == DataType::Null {
+ return true;
+ }
+ match &output_type {
+ Some(t) => *t == data_type,
+ None => {
+ output_type = Some(data_type);
+ true
+ }
+ }
+ }
Review Comment:
One small suggestion: this guard checks the data type, metadata, and
nullability, but intentionally doesn't check the field name because
`SimplifyExpressions` restores the original name through `NamePreserver`. Could
we add a regression at that boundary for `f(CASE ...)` without an explicit
alias? I think that would make this ownership clear and protect the behavior if
the surrounding rewrite machinery changes later.
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