pepijnve commented on code in PR #17973:
URL: https://github.com/apache/datafusion/pull/17973#discussion_r2418438428
##########
datafusion/physical-expr/src/expressions/case.rs:
##########
@@ -431,6 +428,15 @@ impl CaseExpr {
_ => Cow::Owned(prep_null_mask_filter(when_value)),
};
+ let true_count = when_value.true_count();
+ if true_count == batch.num_rows() {
+ // Avoid evaluate_selection when all rows are true
+ return self.when_then_expr[0].1.evaluate(batch);
+ } else if true_count == 0 {
+ // Avoid evaluate_selection when all rows are false/null
+ return self.else_expr.as_ref().unwrap().evaluate(batch);
+ }
+
Review Comment:
I woke up early this morning to the realisation that the actual bug was a
subtlety in the implementation of `evaluate_selection`. There's a difference
between calling it with the empty set vs a non-empty set and a false selection
vector. The implementation was actually treating both cases identically which
can cause a spurious row to get materialised. I've pushed a correction for this
and tweaked the comments in the code a bit.
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