CACHE FAQS

 1. What does the CACHE option do in Oracle sequences?

  • The CACHE option pre-allocates a set number of sequence values in memory. This allows Oracle to provide the next sequence number quickly without reading from disk each time.

2. What happens if the CACHE size is too large?

  • A larger CACHE size improves performance by reducing disk I/O but uses more memory. If memory is limited, it may lead to excessive memory consumption, potentially impacting system performance.

3. Can I change the cache size of an existing sequence?

  • No, the cache size cannot be altered once the sequence is created. To modify the cache size, you must drop and recreate the sequence.

4. What is the default cache size for a sequence?

  • If you don’t specify the CACHE value, the default cache size is typically 20. However, it can vary based on your Oracle version or system configuration.

5. Does the CACHE option guarantee that sequence values are contiguous?

  • No, sequence values may not be contiguous if the database crashes or the sequence is restarted before all cached values are used. However, the values remain unique.

6. What happens when a sequence with CACHE reaches its boundary?

  • If the sequence reaches its MAXVALUE or MINVALUE, it will either stop (with NOCYCLE) or wrap around (with CYCLE). Cached values are flushed, and the next value may not be contiguous with the previous one after a restart or crash.

7. Can I use CACHE with CYCLE and NOCYCLE?

  • Yes, you can use the CACHE option with both CYCLE and NOCYCLE. With CYCLE, the sequence will restart when it reaches the limit, and cached values may be reused.

8. How does the CACHE option affect performance?

  • Using CACHE improves performance by reducing the time spent accessing disk for each new sequence number. It is especially beneficial in systems where sequence values are generated frequently.

9. Can I use CACHE for sequences with high concurrency?

  • Yes, CACHE works well in high-concurrency environments because multiple sessions can quickly access pre-allocated values from memory without waiting for disk I/O.

10. What happens if a sequence with CACHE is lost due to a database crash?

  • If the database crashes or the sequence is restarted, any unused cached sequence numbers are lost. This may cause gaps in the sequence, but it does not affect the uniqueness of the values.

11. Can I see the current cache size of a sequence?

  • Yes, you can check the cache size by querying the USER_SEQUENCES view in Oracle:

SELECT sequence_name, cache_size

FROM user_sequences

WHERE sequence_name = 'your_sequence_name';

12. What happens if I don't use CACHE?

  • If you do not specify CACHE, Oracle will use the default cache size, typically 20. Alternatively, you can explicitly use NOCACHE to prevent any pre-allocation of sequence numbers.

 

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