Individual sequence changes are not logged in the Oracle database log file. But changes to Oracle Sequence impact (update) the Oracle sys.seq$ table. These changes do not occur with each new sequence value generated. Instead, the sys.seq$ table is updated periodically, based on sequence caching refresh activity or other system changes. The value stored in the sys.seq$ table for a sequence is the next value to be assigned after the existing cache of values has been exhausted.As an example, a newly created sequence starts with a value of 1, increments by 1 and has a cache value of 20. These are all default values and can all be customized. The value stored in the sys.seq$ record for this new sequence is 21. This indicates that the “next” value to be used by the sequence, after the existing cache of 20 numbers is used, is 21. The record in sys.seq$ does not change until the sequence value hits 21. At that time, Oracle will cache the next 20 values for the sequence, and the sys.seq$ record will be updated to 41. It is this value (41) recorded in change to the sequences sys.seq$ record, that will be used for replication. The key point is recognizing that not every individual sequence update is recorded in the log, and is therefore not available for replication.
