CACHE

The CACHE option in Oracle sequences determines how the sequence values are stored in memory, allowing faster access to the next sequence numbers. When you define a sequence with the CACHE option, Oracle pre-allocates a set of sequence values in memory rather than generating the next value from disk each time it’s requested. This can improve performance by reducing disk I/O operations.

What is the CACHE Option?

• CACHE tells Oracle to store a specified number of sequence values in memory at once.
• This pre-allocation of sequence values reduces the overhead of accessing disk for each NEXTVAL call, which can significantly improve performance, especially in systems where sequence values are generated frequently.

Syntax for CACHE in a Sequence

CREATE SEQUENCE sequence_name

  START WITH starting_value

  INCREMENT BY increment_value

  CACHE number_of_values;

You specify the number of sequence values Oracle should pre-allocate into memory by using the CACHE option.

Example: Creating a Sequence with CACHE

CREATE SEQUENCE seq_example

  START WITH 1

  INCREMENT BY 1

  CACHE 20;

In this example:

• The sequence starts at 1 and increments by 1.
• Oracle will pre-allocate 20 sequence values in memory. These 20 values will be available for fast retrieval without additional disk I/O.

Key Concepts of CACHE

1. Pre-allocating Sequence Numbers

• With CACHE, Oracle pre-allocates a set of sequence numbers in memory, so when a NEXTVAL is requested, Oracle can return a value from memory.
• This improves performance because Oracle doesn't need to read from the disk every time it generates a sequence number.

2. Performance Benefits

• The performance benefit of CACHE comes from reducing the disk I/O that would otherwise be required to fetch the next sequence value. This is particularly useful in systems that generate large numbers of sequence values, such as auto-incremented IDs for tables in high-transaction environments.
• It can result in faster sequence number generation and reduced contention for sequence numbers in concurrent environments.

3. Cache Size

• The number specified in the CACHE clause determines how many sequence values will be pre-allocated.
• For example, if you specify CACHE 20, Oracle will allocate 20 sequence numbers in memory for each cycle of sequence number retrieval.
• Larger cache sizes typically provide better performance, but they also use more memory.

4. Default Cache Value

• If you don't specify a CACHE value, Oracle uses the default cache size. In most cases, the default value is 20, but it can vary based on the Oracle version or system configuration.

5. No Cache (using NOCACHE)

• If you don't want any pre-allocation of sequence values, you can use NOCACHE. This will make Oracle generate each sequence value one at a time from disk.
• NOCACHE can be used in situations where conservation of memory is more important than performance or where the sequence will be used only occasionally.

6. Automatic Cache Flushing

• Oracle automatically flushes the entire cache if the sequence is restarted or the database crashes. This means if a crash occurs before all cached sequence numbers are used, the unused cached numbers are lost.
• As a result, the next sequence value after a crash or reset may not be contiguous with the previous one, as some numbers will have been skipped.

For example:

• If your sequence is cached with CACHE 10 and the database crashes after the first 5 numbers are used, the next value will be 6 (not 11) when the sequence is restarted.

7. Impact of Sequence Caching on Unique Values

• Sequence caching does not compromise the uniqueness of the sequence values. Even if some sequence values are skipped due to a crash, the sequence remains unique within the context of the database.
• Cached values are stored in memory, and Oracle guarantees that each value retrieved with NEXTVAL is unique.

Performance Considerations

1. Improved Performance with CACHE

• High-throughput systems with frequent NEXTVAL calls benefit the most from CACHE because it minimizes disk I/O.
• For example, in applications generating large numbers of unique IDs for rows in a table (e.g., transactions, logs), caching can significantly reduce delays caused by sequence number retrieval.

2. Memory Usage

• Larger cache sizes result in more sequence values being stored in memory. This increases the amount of memory required to maintain the cache.
• If memory is limited or if you have many sequences with large cache sizes, the overall memory usage could become a concern.

3. Concurrency

• In environments with high concurrency, multiple processes or threads accessing the sequence will be able to fetch values faster since the sequence values are readily available in memory.

4. Cache Size Recommendations

• For most systems, a cache size of 20 to 100 is typically sufficient. However, in extremely high-transaction environments, it may make sense to increase the cache size further, especially if many sequence numbers are needed in a short period.
• Oracle automatically manages the cache to ensure the values are still unique, so you can adjust the cache size based on your performance needs.

Example Use Case for CACHE

If you are generating unique invoice numbers for a large e-commerce platform where new orders are placed every second, using CACHE will allow faster retrieval of the next invoice number.

CREATE SEQUENCE invoice_seq

  START WITH 1000

  INCREMENT BY 1

  CACHE 1000;

In this case, Oracle will pre-allocate 1000 sequence values in memory, ensuring that invoice numbers are generated efficiently and quickly.

 

Key Considerations

1. CACHE vs NOCACHE:
• Use CACHE when performance is critical and you need fast sequence generation.
• Use NOCACHE when you need to minimize memory usage or if the sequence will not be used frequently.
2. No Guarantee of Contiguous Values After a Crash:
• After a database crash, the sequence values may not be contiguous due to the loss of cached numbers, but this does not affect the uniqueness of the sequence.
3. Choosing the Cache Size:
• Consider the transaction volume and system memory when choosing the cache size. If your system generates many sequence values quickly, a larger cache size will provide better performance.
• If the system has limited memory, use a smaller cache or avoid caching entirely.
4. Impact of Cache Size on Performance:
• A larger cache improves performance but consumes more memory. Balance the need for speed with available system resources.

FAQs on CACHE Option

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

• No, you cannot alter the cache size of an existing sequence. To change the cache size, you must drop and recreate the sequence with the desired cache setting.

2. Does CACHE guarantee that sequence numbers are contiguous?

• No, sequence numbers may not be contiguous in cases where the database is restarted or crashes. The values in memory may be lost, but Oracle ensures unique values.

3. How can I see the current cache size of a sequence?

• You can query the USER_SEQUENCES view to check the cache size of a sequence:

·        SELECT sequence_name, cache_size

·        FROM user_sequences

·        WHERE sequence_name = 'your_sequence_name';

4. Is there any risk in using a large CACHE size?

• The main risk of using a large CACHE size is higher memory usage. Ensure your system has enough memory to support the cache size you choose.

5. What happens if I don't specify CACHE?

• If you don’t specify CACHE, Oracle will use the default cache size, which is typically 20. You can always check the system documentation for your Oracle version to confirm the default.

Conclusion

The CACHE option in Oracle sequences is a powerful tool for improving performance when generating large numbers of sequence values. It allows Oracle to pre-allocate sequence values in memory, reducing disk I/O and improving overall performance, particularly in high-transaction environments. However, it’s important to balance performance with memory usage, especially in systems with limited resources.

Let me know if you need further clarification or examples!