Ecto.Query.WindowAPI.first_value

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

Returns value evaluated at the row that is the first row of the window frame.

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

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