How Can I Generate Gap-Free Sequences in PostgreSQL?

How to Generate Gap-Free Sequences in PostgreSQL

In PostgreSQL, sequences are one way to generate unique numerical identifiers. However, by default, sequences may contain gaps, meaning that certain values may be skipped due to rollbacks or errors. In some cases, such as when generating unique invoice numbers, it is essential to have a gap-free sequence.

NOTE: PostgreSQL sequences do not natively support gap-free numerical generation. As such, alternative approaches are required to achieve this result.

Understanding the Issue

When using sequences for gap-free ID generation, it can be tempting to use a SQL query such as:

SELECT invoice_serial + 1 FROM invoice WHERE "type" = @type ORDER BY invoice_serial DESC LIMIT 1;

However, this approach does not guarantee gap-free generation, especially in isolation levels such as serialization. Rollbacks or errors can cause gaps in the sequence.

Alternative Solutions

While PostgreSQL does not provide a built-in solution for gap-free sequence generation, there are several alternative methods that can be employed.

Encapsulation in a Stored Procedure

One approach is to encapsulate the number generation logic in a stored procedure. This procedure can handle the following tasks:

1. Acquire a lock on the sequence.
2. Generate the next number in the sequence.
3. Insert a new row into the database with the generated number.
4. Commit the transaction and release the lock.

Trigger-Based Generation

Another option is to create a trigger on the table where the unique IDs are stored. This trigger is executed before each INSERT operation. The trigger can:

1. Generate the next number in the sequence.
2. Update the inserted row with the generated number.

Manual Sequence Management

In certain cases, it may be necessary to manually manage a sequence table. This involves tracking the current value and incrementing it manually as new records are inserted. While this approach provides the most control, it also requires careful coding to prevent gaps and concurrency issues.

Conclusion

Although PostgreSQL sequences do not natively support gap-free numerical generation, alternative methods can be implemented to achieve this result. Encapsulation in a stored procedure, trigger-based generation, and manual sequence management are all viable options, each with its own advantages and disadvantages. The choice of approach will depend on the specific requirements and application context.

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