Oracle 11g Release 2

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WITH yourTable AS (
    SELECT 100000001 AS "timestamp", 5 AS step FROM dual UNION ALL
    SELECT 100000002, 1 FROM dual UNION ALL
    SELECT 100000003, 1 FROM dual UNION ALL
    SELECT 100000004, 2 FROM dual UNION ALL
    SELECT 100000005, 2 FROM dual UNION ALL
    SELECT 100000006, 4 FROM dual UNION ALL
    SELECT 100000007, 5 FROM dual UNION ALL
    SELECT 100000008, 3 FROM dual UNION ALL
    SELECT 100000009, 4 FROM dual UNION ALL
    SELECT 100000010, 2 FROM dual UNION ALL
    SELECT 100000011, 5 FROM dual UNION ALL
    SELECT 100000012, 7 FROM dual 
),
cte AS (
    SELECT t1.*, (SELECT COUNT(DISTINCT t2.step) FROM yourTable t2
                 WHERE t2."timestamp" < t1."timestamp" AND t2.step < t1.step) AS cnt
    FROM yourTable t1
),
cte2 AS (
    SELECT t.*, ROW_NUMBER() OVER (PARTITION BY cnt ORDER BY step, "timestamp") rn
    FROM cte t
)
SELECT t1."timestamp", t1.step, t1.rn, t1.cnt
FROM cte2 t1
WHERE rn = 1 AND (step = 1 OR EXISTS (SELECT 1 FROM yourTable t2
                                      WHERE t2.step = t1.step - 1))
ORDER BY "timestamp";

timestamp	STEP	RN	CNT
100000002	1	1	0
100000004	2	1	1
100000008	3	1	2
100000009	4	1	3
100000011	5	1	4