How to Reset Sequence in PostgreSQL

In PostgreSQL, sequences are used to generate unique numbers, often for primary keys in tables with auto-incremented columns. However, over time, you might find that the sequence has fallen out of sync with the values in your table. This tutorial will show you how to reset a sequence to ensure it generates the correct next value.

Step 1: Identify the Sequence Name

First, you need to know the name of the sequence. If you're using a serial column, PostgreSQL will automatically create a sequence for it. The sequence name typically follows the pattern tablename_columnname_seq.

SELECT pg_get_serial_sequence('your_table', 'your_column');

Step 2: Reset the Sequence

To reset the sequence, you can use the setval function in PostgreSQL. The syntax for this function is as follows:

SELECT setval('your_sequence_name', new_value, true);

Where:

• your_sequence_name is the name of the sequence you identified in Step 1.
• new_value is the value you want the sequence to start from. Typically, this would be the highest value currently in the table plus one.
• true indicates that the sequence should be set to the new value and the sequence should return the next value on the next call.

For example, to reset the sequence for a table employees and column id:

SELECT setval('employees_id_seq', (SELECT MAX(id) FROM employees), true);

Step 3: Verify the Reset

To verify that the sequence has been reset correctly, you can use the following query to check the current value of the sequence:

SELECT last_value FROM your_sequence_name;

Conclusion

Resetting a sequence in PostgreSQL is a simple process that can help maintain the integrity of your database. Be sure to use this method when you encounter issues with auto-incrementing values that fall out of sync with the actual data in your table.