Function dict_agg

src/expr/src/relation/func.rs:285–320

(datums: I, temp_storage: &'a RowArena, order_by: &[ColumnOrder])

}

fn dict_agg<'a, I>(datums: I, temp_storage: &'a RowArena, order_by: &[ColumnOrder]) -> Datum<'a>
where
    I: IntoIterator<Item = Datum<'a>>,
{
    let datums = order_aggregate_datums(datums, order_by);
    temp_storage.make_datum(|packer| {
        let mut datums: Vec<_> = datums
            .into_iter()
            .filter_map(|d| {
                if d.is_null() {
                    return None;
                }
                let mut list = d.unwrap_list().iter();
                let key = list.next().unwrap();
                let val = list.next().unwrap();
                if key.is_null() {
                    // TODO(benesch): this should produce an error, but
                    // aggregate functions cannot presently produce errors.
                    None
                } else {
                    Some((key.unwrap_str(), val))
                }
            })
            .collect();
        // datums are ordered by any ORDER BY clause now, and we want to preserve
        // the last entry for each key, but we also need to present unique and sorted
        // keys to push_dict. Use sort_by here, which is stable, and so will preserve
        // the ORDER BY order. Then reverse and dedup to retain the last of each
        // key. Reverse again so we're back in push_dict order.
        datums.sort_by_key(|(k, _v)| *k);
        datums.reverse();
        datums.dedup_by_key(|(k, _v)| *k);
        datums.reverse();
        packer.push_dict(datums);
    })
}

/// Assuming datums is a List, sort them by the 2nd through Nth elements
/// corresponding to order_by, then return the 1st element.