PostgreSQL: Write report on `sequence` in postgresql

Report on Sequences in PostgreSQL

Introduction

 

In PostgreSQL, a sequence is a powerful and essential database object used to generate a unique series of numbers automatically. These sequences are primarily employed for purposes like creating unique identifiers, such as primary keys in tables, guaranteeing data integrity and avoiding duplication. They can also be used to generate unique numbers across multiple tables.

 

Key characteristics

Sequences offer several key characteristics:

• They generate unique values.
• The incremental value can be customized.
• Minimum and maximum bounds can be set.
• The CYCLE clause allows sequences to restart, but this should be used with caution. The default NO CYCLE behavior raises an error when the limit is reached.
• The CACHE setting can improve performance, but can also lead to gaps if not all cached values are used.

Usage and examples

• Creating a Sequence: Sequences are created using the CREATE SEQUENCE statement with options like START WITH, INCREMENT BY, MINVALUE, MAXVALUE, CYCLE, and CACHE.
• Retrieving Sequence Values:
  • nextval('sequence_name'): Gets the next value.
  • currval('sequence_name'): Gets the last value used in the current session.
  • setval('sequence_name', value, [is_called]): Resets the sequence value.

Integration with tables

• Sequences are often used as the DEFAULT value for SERIAL or BIGSERIAL columns for primary keys.
• The OWNED BY clause links a sequence to a table column, causing it to be dropped when the column or table is dropped.

Altering and dropping sequences

• ALTER SEQUENCE modifies existing sequences.
• DROP SEQUENCE removes a sequence.

Important considerations

• Sequences guarantee uniqueness but not strict sequential order; gaps can occur.
• Sequence operations are non-transactional and are not rolled back on transaction failure.

Conclusion

 

PostgreSQL sequences are valuable for generating unique identifiers and maintaining data integrity. Their customization makes them useful for various purposes, and understanding them is key to effectively using PostgreSQL.