Which three statements are true about sequences in a single instance Oracle database?
A.
A sequence's unallocated cached values are lost if the instance shuts down.
B.
Two or more tables cannot have keys generated from the same sequence.
C.
A sequence number that was allocated can be rolled back if a transaction fails.
D.
A sequence can issue duplicate values.
E.
Sequences can always have gaps.
F.
A sequence can only be dropped by a DBA.
The Answer Is:
A, C, E
This question includes an explanation.
Explanation:
Sequences are database objects used to generate unique numeric identifiers. Here's the correct understanding of sequences in Oracle:
A: Correct. Cached sequence values that are not yet used are lost if the database instance shuts down, as the cache is held in memory.
B: Incorrect. The same sequence can be used to generate keys for more than one table. There is no such limitation.
C: Correct. If a transaction using a sequence number is rolled back, the sequence number that was used or allocated is not reused.
D: Incorrect. By their nature and configuration, sequences are designed to avoid issuing duplicate values unless explicitly designed to cycle.
E: Correct. Sequences may have gaps, which can occur due to caching, sequence increment settings, or if a transaction using a sequence number is rolled back.
F: Incorrect. A sequence can be dropped by any user with adequate privileges, not just a DBA.
[Reference: Oracle Database Concepts 12c and SQL Language Reference 12c documentation on creating, using, and managing sequences., ]
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