Ecto.Query.WindowAPI.cume_dist

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

Returns relative rank of the current row: (number of rows preceding or peer with current row) / (total rows).

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

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