Create a Foreign Key against a Postgres Sequence (but not a table primary key)

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Summary

The problem at hand is creating a foreign key in a PostgreSQL database that references a sequence, rather than a table’s primary key. This is necessary to group repeating events in an event table without creating a separate table for event batches. The goal is to enforce a foreign key constraint strictly in reference to a sequence, ensuring data consistency and preventing random integer values from being inserted into the field. Key considerations include data integrity, referential integrity, and adherence to best practices in database design.

Root Cause

The root cause of this issue is the need to establish a relationship between the event table and a unique identifier that is not a primary key of another table. The main challenge is that foreign keys in PostgreSQL typically reference the primary key of another table, not a sequence. The possible causes of this issue include:

  • The requirement to group repeating events without creating a separate table
  • The need to enforce data consistency and prevent random integer values
  • The limitation of foreign keys in PostgreSQL referencing only primary keys

Why This Happens in Real Systems

This issue occurs in real systems when:

  • There is a need to establish relationships between tables without creating additional tables
  • Data consistency and integrity are crucial, but the database design does not accommodate this
  • The database schema is not flexible enough to handle unique identifiers that are not primary keys
    Some common scenarios where this issue arises include:
  • Event tracking and logging
  • Data aggregation and grouping
  • Referential integrity in distributed databases

Real-World Impact

The real-world impact of this issue includes:

  • Data inconsistencies and errors due to random integer values
  • Difficulty in maintaining referential integrity and data relationships
  • Inability to scale the database design to accommodate changing requirements
    Some potential consequences of not addressing this issue include:
  • Data corruption and loss of integrity
  • System crashes and downtime
  • Increased maintenance and debugging efforts

Example or Code

CREATE TABLE events (
    id SERIAL PRIMARY KEY,
    event_id INTEGER NOT NULL,
    event_data TEXT
);

CREATE SEQUENCE event_batch_seq;

-- Create a trigger function to set the event_id to the next value in the sequence
CREATE OR REPLACE FUNCTION set_event_id()
RETURNS TRIGGER AS $$
BEGIN
    NEW.event_id = nextval('event_batch_seq');
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

-- Create a trigger to call the function before inserting a new event
CREATE TRIGGER set_event_id_trigger
BEFORE INSERT ON events
FOR EACH ROW
EXECUTE FUNCTION set_event_id();

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Re-evaluating the database design to accommodate unique identifiers that are not primary keys
  • Using triggers and functions to establish relationships between tables and sequences
  • Implementing data validation and constraints to ensure data consistency and integrity
    Some best practices for addressing this issue include:
  • Using sequences and triggers to generate unique identifiers
  • Establishing referential integrity through foreign keys and constraints
  • Regularly reviewing and refining the database design to accommodate changing requirements

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of experience with database design and referential integrity
  • Insufficient understanding of sequences and triggers in PostgreSQL
  • Inadequate testing and validation of database constraints and relationships
    Some common mistakes made by junior engineers include:
  • Not using sequences and triggers to generate unique identifiers
  • Not establishing referential integrity through foreign keys and constraints
  • Not regularly reviewing and refining the database design to accommodate changing requirements