WITH ms AS

    (SELECT DISTINCT DATE_TRUNC('month', created_at)::date AS mnth

        , SUM(costs) OVER (PARTITION BY DATE_TRUNC('month', created_at)::date

                          ORDER BY DATE_TRUNC('month', created_at)::date) AS sum_month --numeric

    FROM tools_shop.costs

    ORDER BY mnth)

   

SELECT mnth

    , sum_month

    --, LAG(sum_month, 1, 0) OVER () -- так не работает? ошибка sum_month: function lag(numeric, integer, integer) does not exist

    , LAG(CAST(sum_month as int), 1, 0) OVER ()

    , sum_month - LAG(CAST(sum_month as int), 1, 0) OVER () AS month_diff

 

FROM ms

ORDER BY mnth