Få forskellige på hinanden følgende datointervaller fra overlappende datointervaller

Logikken er:

• i begyndelsen af et interval føje dets værdi til en kumulativ sum
• til slutningen af et interval trække dets værdi fra denne sum
• men for at feje datolinjen, bliver vi nødt til at indsamle alle de (unikke) dato/tidsstempler, enten start eller stop.

Så pointen er:konverter data fra en række intervaller til en række (start/stop) begivenheder , og aggregér over disse.

-- \i tmp.sql

create table coinsonperiod(
  id serial,
  startdate date,
  enddate date,
  coins integer
);
insert into coinsonperiod (startdate, enddate, coins) values
  ('2018-01-01','2018-01-31', 80)
, ('2018-01-07','2018-01-10', 10)
, ('2018-01-07','2018-01-31', 10)
, ('2018-01-11','2018-01-31', 5)
, ('2018-01-25','2018-01-27', 5)
, ('2018-02-02','2018-02-23', 100)
        ;

WITH changes AS (
    SELECT startdate AS tickdate , coins
            , 1 AS cover
    FROM coinsonperiod
    UNION ALL
    -- add 1 day to convert to half-open intervals
    SELECT 1+enddate AS tickdate, -1* coins
            , -1 AS cover
    FROM coinsonperiod
    )
, sumchanges  AS (
        SELECT tickdate, SUM(coins) AS change, SUM(cover) AS cover
        FROM changes
        GROUP BY tickdate
        )
, aggregated AS (
        SELECT
        tickdate AS startdate
        , lead(tickdate) over www AS enddate
        , sum(change) OVER www AS cash
          -- number of covered intervals
        , sum(cover) OVER www AS cover
        FROM sumchanges
        WINDOW www AS (ORDER BY tickdate)
        )
             -- substract one day from enddate to correct back to closed intervals
SELECT startdate, enddate-1 AS enddate, cash, cover
FROM aggregated
WHERE cover > 0
ORDER BY startdate
        ;