Ecto.Query.WindowAPI.rank

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

Returns rank of the current row with gaps; same as row_number/0 of its first peer.

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

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