Managing Sequence Resetting in Oracle Databases

Understanding Oracle Sequences

Oracle sequences are database objects that generate unique, sequential numbers. They are frequently used as primary keys or to generate unique identifiers for records. While sequences automatically increment, there are scenarios where you might need to reset or restart them, such as during testing, data migration, or to address issues with gaps in the sequence. This tutorial explores various methods for managing sequence resets in Oracle databases.

Basic Sequence Creation

Before diving into resetting, let’s quickly review how to create a sequence:

CREATE SEQUENCE my_sequence
MINVALUE 1
MAXVALUE 999999999999999999999
START WITH 1
INCREMENT BY 1
CACHE 20;

This creates a sequence named my_sequence starting at 1, incrementing by 1, with a cache size of 20. MINVALUE and MAXVALUE define the range of numbers the sequence can generate.

Resetting Sequences: Methods and Considerations

Oracle offers several approaches to resetting or restarting sequences. The best method depends on your Oracle version and specific requirements.

1. Using ALTER SEQUENCE ... RESTART START WITH (Oracle 18c and later)

From Oracle 18c onwards, the simplest and most direct method is to use the RESTART clause:

ALTER SEQUENCE my_sequence RESTART START WITH 1;

This immediately resets the sequence to start with the specified value (1 in this example). This is the preferred approach when available due to its clarity and simplicity.

Important Note: While this functionality appears in documentation from Oracle 18c onwards, it was available (though undocumented) from Oracle 12.1. Using it in older versions is possible but carries the risk of unsupported functionality.

2. Modifying the Increment and Then Correcting (General Approach)

This method is more versatile and works across various Oracle versions. It involves temporarily changing the sequence’s increment value, advancing it to the desired starting point, and then restoring the original increment.

DECLARE
  l_current NUMBER;
  l_difference NUMBER;
  l_minvalue NUMBER;
BEGIN
  -- Get the current sequence value
  EXECUTE IMMEDIATE 'SELECT :seq.nextval FROM dual' INTO l_current;

  -- Get the minimum value of the sequence
  SELECT min_value INTO l_minvalue FROM user_sequences WHERE sequence_name = 'MY_SEQUENCE';

  -- Calculate the difference needed to reach the desired start value
  l_difference := 1 - l_current;

  -- Alter the sequence to temporarily change the increment
  EXECUTE IMMEDIATE 'ALTER SEQUENCE my_sequence INCREMENT BY ' || l_difference || ' MINVALUE ' || l_minvalue;

  -- Get the new sequence value (should be 1)
  EXECUTE IMMEDIATE 'SELECT my_sequence.nextval FROM dual' INTO l_difference;

  -- Restore the original increment (usually 1)
  EXECUTE IMMEDIATE 'ALTER SEQUENCE my_sequence INCREMENT BY 1 MINVALUE ' || l_minvalue;
END;
/

This example resets the sequence to 1. Adapt the calculations and start with value to achieve the desired result.

3. Deleting and Recreating the Sequence

The most drastic but sometimes necessary approach is to drop and recreate the sequence. This is suitable when you need to ensure a completely clean reset, but be cautious as it will disrupt any applications relying on the sequence.

DROP SEQUENCE my_sequence;

CREATE SEQUENCE my_sequence
MINVALUE 1
MAXVALUE 999999999999999999999
START WITH 1
INCREMENT BY 1
CACHE 20;

4. Programmatic Resetting with Procedures

For more complex scenarios involving multiple sequences or dynamic reset values, consider using stored procedures. This approach promotes code reusability and maintainability.

Here’s an example procedure to reset a sequence to a specified value:

CREATE OR REPLACE PROCEDURE Reset_Sequence (
    p_seq_name IN VARCHAR2,
    p_val IN NUMBER DEFAULT 0
) IS
    l_current NUMBER := 0;
    l_difference NUMBER := 0;
    l_minvalue NUMBER;
BEGIN
    -- Get minimum value for the sequence
    SELECT min_value INTO l_minvalue FROM user_sequences WHERE sequence_name = p_seq_name;

    -- Get the current sequence value
    EXECUTE IMMEDIATE 'SELECT ' || p_seq_name || '.nextval FROM dual' INTO l_current;
    
    -- Calculate the difference
    l_difference := p_val - l_current;

    -- Alter sequence increment to push to target value
    EXECUTE IMMEDIATE 'ALTER SEQUENCE ' || p_seq_name || ' INCREMENT BY ' || l_difference || ' MINVALUE ' || l_minvalue;

    -- Get the new value (now at p_val)
    EXECUTE IMMEDIATE 'SELECT ' || p_seq_name || '.nextval FROM dual' INTO l_difference;

    -- Reset increment back to 1
    EXECUTE IMMEDIATE 'ALTER SEQUENCE ' || p_seq_name || ' INCREMENT BY 1 MINVALUE ' || l_minvalue;
END Reset_Sequence;
/

Best Practices and Considerations

• Backup: Before resetting any sequence, especially in a production environment, create a backup of your database.
• Dependencies: Analyze the impact of resetting a sequence on any applications or processes that rely on it.
• Testing: Thoroughly test any sequence reset operation in a non-production environment before applying it to production.
• Documentation: Document any sequence reset procedures for future reference.
• Concurrency: Be mindful of concurrency issues when resetting sequences in a multi-user environment.