Ecto.Query.WindowAPI.dense_rank

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

Returns rank of the current row without gaps; this function counts peer groups.

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

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