Can window function LAG reference the column which value is being calculated?

Question

I need to calculate value of some column X based on some other columns of the current record and the value of X for the previous record (using some partition and order). Bas

Answer 1

Naive recursive chain knitter:

---

        -- temp view to avoid nested CTE
CREATE TEMP VIEW drag AS
        SELECT e.type,e.time_stamp
        , ROW_NUMBER() OVER www as rn                   -- number the records
        , FIRST_VALUE(e.time_stamp) OVER www as fst     -- the "group leader"
        , EXISTS (SELECT * FROM event x
                WHERE x.type = e.type
                AND x.time_stamp < e.time_stamp) AS is_dup
        FROM event e
        WINDOW www AS (PARTITION BY type ORDER BY time_stamp)
        ;

WITH RECURSIVE ttt AS (
        SELECT d0.*
        FROM drag d0 WHERE d0.is_dup = False -- only the "group leaders"
    UNION ALL
        SELECT d1.type, d1.time_stamp, d1.rn
          , CASE WHEN d1.time_stamp - ttt.fst > 20 THEN d1.time_stamp
                 ELSE ttt.fst END AS fst   -- new "group leader"
          , CASE WHEN d1.time_stamp - ttt.fst > 20 THEN False
                 ELSE True END AS is_dup
        FROM drag d1
        JOIN ttt ON d1.type = ttt.type AND d1.rn = ttt.rn+1
        )
SELECT * FROM ttt
ORDER BY type, time_stamp
        ;

---

Results:

---

CREATE TABLE
INSERT 0 10
CREATE VIEW
 type | time_stamp | rn | fst | is_dup 
------+------------+----+-----+--------
    1 |          1 |  1 |   1 | f
    1 |          2 |  2 |   1 | t
    1 |          3 |  3 |   1 | t
    1 |         10 |  4 |   1 | t
    1 |         15 |  5 |   1 | t
    1 |         21 |  6 |   1 | t
    1 |         40 |  7 |  40 | f
    2 |          2 |  1 |   2 | f
    2 |         10 |  2 |   2 | t
    2 |         13 |  3 |   2 | t
(10 rows)