Function first_value 

pub fn first_value<'a, V, E>(expr: E) -> WindowFnExpr<'a, V, E::SQLType, Null>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,

FIRST_VALUE(expr) — value of expr from the first row of the frame.

Always nullable (frame may be empty for some edge cases).