SQL row trimming

Question

I have a select query that will return something like the following table:

start | stop | id
------------------
0 | 100 | 1
1 | 101 | 1
2 | 102 | 1
2 | 102 | 2
5 | 105 | 1
7 | 107 | 2
...
300 | 400 | 1
370 | 470 | 1
450 | 550 | 1

Where stop = start + n; n = 100 in this case.

I would like to combine overlays for each id:

start | stop | id
------------------
0 | 105 | 1
2 | 107 | 2
...
300 | 550 | 1

id 1 does not give 0 - 550, because the beginning of 300 after stopping 105.

There will be hundreds of thousands of records returned by the first request, and n can go up to tens of thousands, so the faster it is processed, the better.

Using PostgreSQL by the way.

Answer 1

WITH    bounds AS
        (
        SELECT  *, ROW_NUMBER() OVER (PARTITION BY id ORDER BY start) AS rn
        FROM    (
                SELECT  id, LAG(stop) OVER (PARTITION BY id ORDER BY start) AS pstop, start
                FROM    q
                UNION ALL
                SELECT  id, MAX(stop), NULL
                FROM    q
                GROUP BY
                        id
                ) q2
        WHERE   start > pstop OR pstop IS NULL OR start IS NULL
        )
SELECT  b2.start, b1.pstop
FROM    bounds b1
JOIN    bounds b2
ON      b1.id = b2.id
        AND b1.rn = b2.rn + 1