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Don Burleson Blog 








Find gaps in Oracle sequences

Oracle Database Tips by Donald BurlesonOctober 26, 2015


Oracle sequences allow the DBA to insert unique sequence numbers for artificial keys and maintain an auto-numbering system.


While Oracle has tools to detect sequencing gaps in archived redo logs (the v$archive_gap view), we have to write own own SQL to detect gaps in user-defined Oracle sequences.


This blog has an interesting use of the Oracle LEAD and LAG functions for SQL to detect gaps in Oracle sequence numbers, very creative:


SQL> WITH aquery AS
2 (SELECT measurement_idafter_gap,
3 LAG(measurement_id,1,0) OVER (ORDER BY measurement_id) before_gap
4 FROM sensor_data)
6 before_gap, after_gap
8 aquery
10 before_gap != 0
11 AND
12 after_gap - before_gap > 1
14 before_gap;


---------- ----------
2 4
5 7




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