Method distinct_by

src/expr/src/relation.rs:1386–1388

Removes all but the first occurrence of each key. Columns not included in the `group_key` are discarded.

(self, group_key: Vec<usize>)

    /// Removes all but the first occurrence of each key. Columns not included
    /// in the `group_key` are discarded.
    pub fn distinct_by(self, group_key: Vec<usize>) -> Self {
        self.reduce(group_key, vec![], None)
    }

    /// Discards rows with a negative frequency.
    pub fn threshold(self) -> Self {