Ecto.Query.WindowAPI.nth_value

You're seeing just the function nth_value, go back to Ecto.Query.WindowAPI module for more information.

Returns value evaluated at the row that is the nth row of the window frame (counting from 1); nil if no such row.

from p in Post,
     select: nth_value(p.id, 4) |> over(partition_by: p.category_id, order_by: p.date)

Note that this function must be invoked using window function syntax.