Method reduce

src/transform/src/analysis.rs:1805–1823

(
            &self,
            group_key: &Vec<MirScalarExpr>,
            expected_group_size: &Option<u64>,
            input: CardinalityEstimate,
        )

        }

        fn reduce(
            &self,
            group_key: &Vec<MirScalarExpr>,
            expected_group_size: &Option<u64>,
            input: CardinalityEstimate,
        ) -> CardinalityEstimate {
            // TODO(mgree): if no `group_key` is present, we can do way better

            if let Some(expected_group_size) = expected_group_size {
                // if expected group size is 0, treat it as 1 (to avoid DBZ/+inf estimates)
                let group_size = u64::max(*expected_group_size, 1);
                input / f64::cast_lossy(group_size)
            } else if group_key.is_empty() {
                CardinalityEstimate::from(1.0)
            } else {
                // in the worst case, every row is its own group
                input
            }
        }

        fn topk(
            &self,