Ecto.Query.WindowAPI.row_number

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

Returns number of the current row within its partition, counting from 1.

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

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