Transaction independence guarantees that each subsequent sequence value is incremented (positively or negatively) beyond its previous value, but does not guarantee the extent of the increment. In other words, with sequences you can have incremental gaps in the sequential values actually used in your application. These gaps result when sequences are incremented during a transaction that is subsequently undone, leaving the sequences set to their latest values. If the transaction restarts, it uses these latest values, not those generated for the previously undone transaction.

The following figure shows how two overlapping transactions with rollback can create records and commit consecutive sequence values with gaps.

Both transactions start out with the sequence set to 1, which has already been used in the database. Transaction T1 increments the sequence first, assigns, and then rolls back leaving the sequence value at 2. Then transaction T2 increments, assigns, and commits the sequence value 3 to a database field. Transaction T1 then increments, assigns, and rolls back a second time, leaving the sequence value at 4. Finally, transaction T1 increments, assigns, and commits the sequence value 5 to the same database field in a different record. Thus, the sequence values 2 and 4 are skipped and never used in the database. All subsequent transactions increment and commit sequence values greater than 5.

Control tables, however, obey the same transaction rules as any other table, and can ensure that their field values are reset for undone transactions. If it is essential to generate sequence values without incremental gaps, you must use a control table rather than a sequence.