I work on query to identify gaps/islands in ordered sequence of numbers in table. Of course there is lot of solutions for that problem but i am looking for most efficient way how to identify most recent gap. In other words just need to get top 1 island.
Following works good (note that i have to use PARTITION BY as each id in table has its own number sequence)
select /*+ PARALLEL (4) */ * FROM ( WITH tt AS (SELECT id, seq, seq - DENSE_RANK () OVER (PARTITION BY id ORDER BY seq) AS col2 FROM DPD_tst WHERE ID =1) SELECT MIN (seq) AS first_island_elemnt, MAX (seq) AS last_island_element, RANK () OVER (PARTITION BY id ORDER BY MIN (seq) desc ) AS rank, id FROM tt GROUP BY id, col2 ORDER BY first_island_elemnt ) where rank=1;
But seems like this query can not utilize TOP-N search. Is there more efficient way how to identify TOP island in sequence ?
Table DPD_tst can have up to 8000 000 000 rows. That is why i want to leverage fact that i need only first island.
thanks for help !
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