CYCLE

The CYCLE option in Oracle sequences allows a sequence to restart when it reaches its maximum (MAXVALUE) or minimum (MINVALUE) value. This option is useful when you want the sequence to wrap around instead of throwing an error or stopping once the limit is reached.

Let's dive deep into the behavior of the CYCLE option and its usage in Oracle sequences.

What is the CYCLE Option?

When creating a sequence, you can specify the CYCLE option to instruct Oracle to automatically restart the sequence from its initial START WITH value once it hits the MAXVALUE (for ascending sequences) or MINVALUE (for descending sequences).

If CYCLE is not specified, the default behavior is NOCYCLE, which means the sequence will stop generating values once it reaches the MAXVALUE (ascending sequences) or MINVALUE (descending sequences).

Syntax for CYCLE in a Sequence

CREATE SEQUENCE sequence_name

  START WITH starting_value

  INCREMENT BY increment_value

  [MINVALUE min_value]

  [MAXVALUE max_value]

  CYCLE;

Example: Creating a Sequence with CYCLE

CREATE SEQUENCE seq_example

  START WITH 1

  INCREMENT BY 1

  MAXVALUE 5

  CYCLE;

In this example:

• The sequence starts at 1 and increments by 1.
• The maximum value (MAXVALUE) is set to 5.
• Once the sequence reaches 5, it will restart from 1 due to the CYCLE option.

Key Concepts

1. Wrapping Around:

• When the sequence reaches the MAXVALUE (for an ascending sequence) or MINVALUE (for a descending sequence), it will automatically wrap around to the START WITH value (or to the MINVALUE for descending sequences). This allows for continuous cycling without interruptions.

2. Ascending Sequences:

• For an ascending sequence, once the sequence reaches the MAXVALUE, it will return to the START WITH value and continue incrementing from there.
• Example:

CREATE SEQUENCE seq_example

  START WITH 1

  INCREMENT BY 1

  MAXVALUE 5

  CYCLE;

The sequence will generate:

1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, ...

3. Descending Sequences:

• For a descending sequence, once the sequence reaches the MINVALUE, it will restart from the START WITH value (or the MAXVALUE for descending sequences).
• Example:

CREATE SEQUENCE seq_example

  START WITH 10

  INCREMENT BY -1

  MINVALUE 1

  CYCLE;

The sequence will generate:

10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 10, 9, 8, ...

4. Handling Boundary Limits with CYCLE:

• If CYCLE is specified, once the sequence reaches either MAXVALUE (ascending) or MINVALUE (descending), it wraps around to the START WITH value.
• Without CYCLE, once the sequence reaches the limit, it will stop, and an error will occur on subsequent NEXTVAL calls.

How Does CYCLE Affect Sequence Behavior?

1. Max Value Reached (Ascending Sequences):
• When the sequence reaches the MAXVALUE (for ascending sequences) and CYCLE is enabled, it will reset back to START WITH and continue generating values.
2. Min Value Reached (Descending Sequences):
• When the sequence reaches the MINVALUE (for descending sequences) and CYCLE is enabled, it will reset back to the START WITH value (or MAXVALUE if you are using a descending sequence).
3. CYCLE with NOCYCLE:
• If NOCYCLE is used instead of CYCLE, once the sequence reaches the limit, it will stop generating values and will raise an error when a subsequent NEXTVAL is called.
4. Cycle Behavior with INCREMENT BY:
• The cycle will continue indefinitely, with the sequence value increasing or decreasing according to the value defined in INCREMENT BY. After hitting the limit, it will start from the START WITH value and keep going.

 

Use Cases for CYCLE

1. Repeating Number Sequences:

You might want to generate a series of numbers repeatedly for something like a round-robin allocation or limited set of values. The CYCLE option will allow you to generate numbers in a repeating cycle.

• Example:

CREATE SEQUENCE round_robin_seq

  START WITH 1

  INCREMENT BY 1

  MAXVALUE 3

  CYCLE;

This sequence will generate 1, 2, 3, 1, 2, 3, 1, 2, 3, ..., making it ideal for cases like cyclic tasks or allocations.

2. Generating Repeating Codes or Identifiers:

If you want to generate a sequence of identifiers that repeats periodically, you can use CYCLE. For example, if you are generating user IDs for a group of users and need to reset IDs after reaching a maximum, CYCLE helps in restarting the sequence automatically.

• Example:

CREATE SEQUENCE user_id_seq

  START WITH 1001

  INCREMENT BY 1

  MAXVALUE 1010

  CYCLE;

3. Resource Allocation:

In a scenario where a fixed set of resources (e.g., servers, workers, etc.) are involved in a round-robin fashion, CYCLE allows you to cycle through the resources continuously.

• Example: Assigning resources to tasks.

CREATE SEQUENCE resource_seq

  START WITH 1

  INCREMENT BY 1

  MAXVALUE 5

  CYCLE;

 

Important Notes on CYCLE

1. Cyclic Sequences and Performance:
• Be cautious when using CYCLE for sequences that are used in high-volume applications. If the sequence is frequently used and the cycle is long, this can cause unexpected behavior if not handled properly in your application logic.
2. Behavior with START WITH and INCREMENT BY:
• The START WITH value is where the sequence starts, and INCREMENT BY determines how the sequence moves. When CYCLE is enabled, the sequence will always wrap back to START WITH after reaching the upper (MAXVALUE) or lower (MINVALUE) boundary.
3. CYCLE with Non-Numeric Sequences:
• CYCLE only works with numeric sequences. It does not apply to sequences where you need non-numeric values.
4. Cycle Reset:
• Once the cycle wraps, the sequence will continue from the START WITH value. If you want a different behavior (such as restarting from a different value), you would need to manually handle that logic in your application.

 

FAQs on CYCLE

1. Can I change the CYCLE setting after creating the sequence?
• No, you cannot alter the CYCLE option once the sequence is created. You would need to drop and recreate the sequence with the new setting.
2. What happens if a sequence with CYCLE exceeds the MAXVALUE or MINVALUE?
• If CYCLE is used and the sequence exceeds the MAXVALUE (ascending sequence) or MINVALUE (descending sequence), it will wrap around and continue from the START WITH value.
3. Can I use CYCLE with NOCYCLE?
• No, CYCLE and NOCYCLE are mutually exclusive options. You must choose one or the other.

 

Conclusion

The CYCLE option in Oracle sequences is an important feature that allows sequences to repeat once they hit their boundaries, making it ideal for generating cyclical data such as round-robin identifiers, repetitive codes, and recurring numbers. It's crucial to understand how CYCLE interacts with sequence limits, and how it can be leveraged for applications that require repeating or looping number sequences.