alamb commented on code in PR #18329:
URL: https://github.com/apache/datafusion/pull/18329#discussion_r2477529977
##########
datafusion/physical-expr/src/expressions/case.rs:
##########
@@ -68,7 +70,104 @@ 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);
+ }
+
+ // Construct a mapping from the original column index to the projected
column index.
+ let column_index_map = used_column_indices
+ .iter()
+ .enumerate()
+ .map(|(projected, original)| (*original, projected))
+ .collect::<HashMap<usize, usize>>();
+
+ // Construct the projected body by rewriting each expression from the
original body
+ // using the column index mapping.
+ let project = |expr: &Arc<dyn PhysicalExpr>| -> Result<Arc<dyn
PhysicalExpr>> {
+ Arc::clone(expr)
+ .transform_down(|e| {
+ if let Some(column) = e.as_any().downcast_ref::<Column>() {
+ let original = column.index();
+ let projected =
*column_index_map.get(&original).unwrap();
+ if projected != original {
+ return Ok(Transformed::yes(Arc::new(Column::new(
+ column.name(),
+ projected,
+ ))));
+ }
+ }
+ Ok(Transformed::no(e))
+ })
+ .map(|t| t.data)
+ };
+
+ let projected_body = CaseBody {
+ expr: self.expr.as_ref().map(project).transpose()?,
+ when_then_expr: self
+ .when_then_expr
+ .iter()
+ .map(|(e, t)| Ok((project(e)?, project(t)?)))
+ .collect::<Result<Vec<_>>>()?,
+ else_expr: self.else_expr.as_ref().map(project).transpose()?,
+ };
+
+ // Construct the projection vector
+ let projection = column_index_map
+ .iter()
+ .sorted_by_key(|(_, v)| **v)
+ .map(|(k, _)| *k)
+ .collect::<Vec<_>>();
+
+ Ok(ProjectedCaseBody {
+ projection,
+ body: projected_body,
+ })
+ }
+}
+
+/// A derived case body that can be used to evaluate a case expression after
projecting
+/// record batches using a projection vector.
+#[derive(Debug, Hash, PartialEq, Eq)]
+struct ProjectedCaseBody {
+ projection: Vec<usize>,
+ body: CaseBody,
Review Comment:
I personally found the term `project` to make sense and be consistent with
the rest of the code
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