findepi commented on code in PR #17139:
URL: https://github.com/apache/datafusion/pull/17139#discussion_r2270995227
##########
datafusion/optimizer/src/optimize_projections/mod.rs:
##########
@@ -153,23 +153,16 @@ fn optimize_projections(
// Only use the absolutely necessary aggregate expressions required
// by the parent:
- let mut new_aggr_expr =
aggregate_reqs.get_at_indices(&aggregate.aggr_expr);
-
- // Aggregations always need at least one aggregate expression.
- // With a nested count, we don't require any column as input, but
- // still need to create a correct aggregate, which may be optimized
- // out later. As an example, consider the following query:
- //
- // SELECT count(*) FROM (SELECT count(*) FROM [...])
- //
- // which always returns 1.
- if new_aggr_expr.is_empty()
- && new_group_bys.is_empty()
- && !aggregate.aggr_expr.is_empty()
- {
- // take the old, first aggregate expression
- new_aggr_expr = aggregate.aggr_expr;
- new_aggr_expr.resize_with(1, || unreachable!());
+ let new_aggr_expr =
aggregate_reqs.get_at_indices(&aggregate.aggr_expr);
+
+ if new_group_bys.is_empty() && new_aggr_expr.is_empty() {
+ // Global aggregation with no aggregate functions always
produces 1 row and no columns.
+ return Ok(Transformed::yes(LogicalPlan::EmptyRelation(
+ EmptyRelation {
+ produce_one_row: true,
+ schema: Arc::new(DFSchema::empty()),
Review Comment:
Here we don't produce any symbols, but maybe we use `EmptyRelation` also for
other cases?
For exmple `SELECT a, b, c FROM t WHERE false` could be replaced with
`EmptyRelation`, but something needs to project `a, b, c` symbols. It can be a
project above `EmptyRelation` or part of `EmptyRelation` itself.
A different question -- why do we have both, `EmptyRelation` and `Values`?
The latter is strictly more generic, without any visible downsides. It even
has a better name (`EmptyRelation` is only conditionally an empty relation).
Can we deprecate `EmptyRelation` and replace with `Values`?
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