adriangb commented on code in PR #18329:
URL: https://github.com/apache/datafusion/pull/18329#discussion_r2470471026
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datafusion/physical-expr/src/expressions/case.rs:
##########
@@ -68,7 +70,106 @@ enum EvalMethod {
/// if there is just one when/then pair and both the `then` and `else` are
expressions
///
/// CASE WHEN condition THEN expression ELSE expression END
- ExpressionOrExpression,
+ ExpressionOrExpression(ProjectedCaseBody),
+}
+
+/// The body of a CASE expression which consists of an optional base
expression, the "when/then"
+/// branches and an optional "else" branch.
+#[derive(Debug, Hash, PartialEq, Eq)]
+struct CaseBody {
+ /// Optional base expression that can be compared to literal values in the
"when" expressions
+ expr: Option<Arc<dyn PhysicalExpr>>,
+ /// One or more when/then expressions
+ when_then_expr: Vec<WhenThen>,
+ /// Optional "else" expression
+ else_expr: Option<Arc<dyn PhysicalExpr>>,
+}
+
+impl CaseBody {
+ /// Derives a [ProjectedCaseBody] from this [CaseBody].
+ fn project(&self) -> Result<ProjectedCaseBody> {
+ // Determine the set of columns that are used in all the expressions
of the case body.
+ let mut used_column_indices = HashSet::<usize>::new();
+ let mut collect_column_indices = |expr: &Arc<dyn PhysicalExpr>| {
+ expr.apply(|expr| {
+ if let Some(column) = expr.as_any().downcast_ref::<Column>() {
+ used_column_indices.insert(column.index());
+ }
+ Ok(TreeNodeRecursion::Continue)
+ })
+ .expect("Closure cannot fail");
+ };
+
+ if let Some(e) = &self.expr {
+ collect_column_indices(e);
+ }
+ self.when_then_expr.iter().for_each(|(w, t)| {
+ collect_column_indices(w);
+ collect_column_indices(t);
+ });
+ if let Some(e) = &self.else_expr {
+ collect_column_indices(e);
+ }
Review Comment:
There's `ProjectionExprs::column_indices` which is pub and similar to the
non-pub `collect_column_indices` referenced above. I haven't reviewed this PR
in detail but there may be other helper bits that you can use and generally it
would be nice if we coalesce projection manipulation into `ProjectionExprs`
because I feel like there's a lot of duplicate code in random places right now
(obviously needs to be balanced with keeping the API surface area on
`ProjectionExprs` reasonable).
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