Ecto.Query.WindowAPI.percent_rank

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

Returns relative rank of the current row: (rank - 1) / (total rows - 1).

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

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