Function apply_nth_value

nodedb-query/src/window/offset.rs:83–99

PostgreSQL `nth_value(expr, n)` — value of `expr` at the n'th row of the window frame, NULL if the frame doesn't yet contain n rows. Default frame is RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, so the first n-1 rows of each partition return NULL and rows from the n'th onward return the value of `expr` at the n'th row.

(
    rows: &mut [(String, serde_json::Value)],
    indices: &[usize],
    spec: &WindowFuncSpec,
)

/// rows of each partition return NULL and rows from the n'th onward return
/// the value of `expr` at the n'th row.
pub(super) fn apply_nth_value(
    rows: &mut [(String, serde_json::Value)],
    indices: &[usize],
    spec: &WindowFuncSpec,
) {
    let field = col_arg(spec, 0);
    let n = usize_arg(spec, 1, 1).max(1);

    for (pos, &i) in indices.iter().enumerate() {
        let val = if pos + 1 >= n {
            get_field(&rows[indices[n - 1]].1, field)
        } else {
            serde_json::Value::Null
        };
        set_window_col(&mut rows[i].1, &spec.alias, val);
    }
}