How Can We Generate Gapless Sequences in PostgreSQL While Handling Concurrent Processes?

Gapless Sequence Generation in PostgreSQL

In the realm of database management, the creation of unique identifiers is a crucial aspect, especially in scenarios where the IDs need to maintain a continuous, non-repeating sequence. This requirement arises in various applications, including invoice numbering and other situations where gaps in the ID sequence can lead to complications.

In PostgreSQL, sequences serve as a common approach for generating unique identifiers. However, it's important to note that sequences do not naturally generate gap-free sets of numbers. This stems from the fact that database transactions can roll back or encounter errors, leading to the "use" of sequence numbers but not necessarily the execution of the associated actions.

Understanding the Challenges

The primary challenge in achieving gap-free sequences lies in the interplay of multiple processes concurrently creating entities that require these unique identifiers. In such scenarios, it becomes paramount that the numbers are generated at the moment the entities are created. However, this can introduce a point of serialization within the application, as each process or thread must contend to obtain the next available sequence number.

Alternative Approaches

If the requirement for gap-free sequences can be relaxed to allow for gaps, the use of Oracle sequences presents a viable option. These sequences are highly performant, and the occurrence of gaps can be contained by minimizing the likelihood of failures between number generation and transaction commit.

Another approach, suitable for scenarios where multiple processes are not involved and instant generation is not a strict requirement, is to batch generate numbers separately from the entity creation process. This can be achieved by first determining the current maximum value and incrementally assigning numbers to each entity.

Addressing the Trifecta

In situations where the trifecta of instant generation, gap-free sequences, and multiple processes come into play, the best course of action is to minimize the serialization period. Several strategies can be employed:

• Store current values in a dedicated table rather than using a sequence
• Employ a function or procedure to encapsulate new number generation
• Implement serialization with DBMS_Lock, dedicating specific locks to each series
• Delay number generation to the latest possible moment
• Address potential rollback scenarios by establishing mechanisms to return used numbers to the pool
• Explore encapsulation of the entire process within a trigger or automated insert mechanism

By understanding the challenges and exploring alternative approaches, it is possible to implement gap-free sequences in PostgreSQL, ensuring the integrity of unique identifiers in scenarios requiring continuous non-repeating values.

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