elixir scrivener handle slow couting example

jsonapi_paginator.ex

# Paginate
@spec handle_pagination(conn :: Plug.Conn.t(), page :: number | atom, page_size :: number | atom, query :: module) ::
        Plug.Conn.t()
defp handle_pagination(conn, page, page_size, query) do
  config = %Scrivener.Config{
    module: Repo,
    page_number: page,
    page_size: page_size,
    options: [total_entries: total_entries(query)]
  }

  query |> Scrivener.paginate(config)
end

# Retrieves the estimated record count
@spec total_entries(query :: module) :: number
defp total_entries(query) do
  table_name = table_name(query)

  {:ok, result} =
    Ecto.Adapters.SQL.query(Repo, "SELECT reltuples::BIGINT AS estimate FROM pg_class WHERE relname=$1", [table_name])

  result.rows |> List.first() |> List.first()
end

# Determines the table name based on a given query
@spec table_name(query :: module) :: module
defp table_name(query) do
  module_from_query(query).__schema__(:source)
end

# Retrieves the model module from a given query
@spec module_from_query(module :: module) :: module
defp module_from_query(%{from: {_table, module}}), do: module
defp module_from_query(module), do: module