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