viirya commented on code in PR #8459:
URL: https://github.com/apache/arrow-datafusion/pull/8459#discussion_r1419514312
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datafusion/optimizer/src/optimize_projections.rs:
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@@ -213,6 +213,16 @@ fn optimize_projections(
let (aggregate_input, _is_added) =
add_projection_on_top_if_helpful(aggregate_input,
necessary_exprs, true)?;
+ // Aggregate always needs 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
+ // The aggregate may be optimized out later (select count(*) from
(select count(*) from [...]) always returns 1
+ if new_aggr_expr.is_empty()
+ && new_group_bys.is_empty()
+ && !aggregate.aggr_expr.is_empty()
+ {
+ new_aggr_expr = vec![aggregate.aggr_expr[0].clone()];
Review Comment:
Hmm, why only taking the agg expr at index 0? If there are multiple agg
expr? Is because they are not necessary by the parent query?
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