Method reduce

src/expr/src/relation.rs:1327–1340

Perform a key-wise reduction / aggregation. The `group_key` argument indicates columns in the input collection that should be grouped, and `aggregates` lists aggregation functions each of which produces one output column in addition to the keys.

(
        self,
        group_key: Vec<usize>,
        aggregates: Vec<AggregateExpr>,
        expected_group_size: Option<u64>,
    )

    /// be grouped, and `aggregates` lists aggregation functions each of which produces
    /// one output column in addition to the keys.
    pub fn reduce(
        self,
        group_key: Vec<usize>,
        aggregates: Vec<AggregateExpr>,
        expected_group_size: Option<u64>,
    ) -> Self {
        MirRelationExpr::Reduce {
            input: Box::new(self),
            group_key: group_key.into_iter().map(MirScalarExpr::column).collect(),
            aggregates,
            monotonic: false,
            expected_group_size,
        }
    }

    /// Perform a key-wise reduction order by and limit.
    ///