Simulating Expiry Time for Data Deletion in PostgreSQL

The scenario:

• You want to store data in a PostgreSQL database with an "expiry time."
• After that expiry time, the data entry should be automatically deleted.

What PostgreSQL doesn't have:

• PostgreSQL itself doesn't offer a direct way to set an expiry time for automatic deletion.

How to achieve it:

There are two main approaches:

1. Using a timestamp and a scheduled task:

   • Add a column to your table to store the expiry time (e.g., "expiry_time").
   • Write a separate program (often a cron job or a function in pgAgent) that runs periodically (daily, hourly, etc.).
   • This program will query the table, find entries where the current time is past their "expiry_time," and delete them.

2. Using triggers (more advanced):

   • Create a trigger that fires whenever a new entry is inserted.
   • The trigger calculates the expiry time based on the current time and a pre-defined duration.
   • It then stores this expiry time in the new entry's "expiry_time" column.
   • You can optionally create another trigger that fires on every insert or update event and deletes entries whose "expiry_time" has passed.

Key terms:

• Database: A structured collection of data that allows for storage, retrieval, and management of information. PostgreSQL is a specific type of database.
• PostgreSQL: An open-source relational database management system (RDBMS).
• TTL (Time To Live): A concept used in computing to indicate the duration for which data remains valid before it's automatically removed.

Option 1: Using a timestamp and a scheduled task (cron job)

CREATE TABLE my_data (
  id SERIAL PRIMARY KEY,
  data TEXT,
  expiry_time TIMESTAMP NOT NULL
);

Scheduled task (example using cron):

This is just an example, the actual scheduling will depend on your system. You'll need to create a separate script or program to run this logic periodically.

# Run this script daily at midnight (0 0 * * *)
0 0 * * *  psql -h your_host -U your_user -d your_database -c "DELETE FROM my_data WHERE expiry_time < current_timestamp;"

This cron job will connect to PostgreSQL, delete entries from my_data where the expiry_time is earlier than the current timestamp.

Option 2: Using triggers (more advanced)

Table with expiry time:

CREATE TABLE my_data (
  id SERIAL PRIMARY KEY,
  data TEXT,
  expiry_time TIMESTAMP NOT NULL
);

Trigger on insert:

CREATE FUNCTION set_expiry() RETURNS TRIGGER AS $$
BEGIN
  NEW.expiry_time = current_timestamp + interval '1 hour';  -- Set expiry 1 hour from now
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER set_expiry_time
BEFORE INSERT ON my_data
FOR EACH ROW
EXECUTE PROCEDURE set_expiry();

This trigger calculates an expiry time one hour from the insert time and stores it in the expiry_time column.

Optional: Trigger on update/insert (for deletion):

CREATE FUNCTION delete_expired() RETURNS trigger AS $$
BEGIN
  DELETE FROM my_data WHERE expiry_time < current_timestamp;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER delete_on_expiry
AFTER INSERT OR UPDATE ON my_data
FOR EACH ROW
EXECUTE PROCEDURE delete_expired();

This trigger will fire on every insert or update event and will delete entries where the expiry_time has passed.

1. Using materialized views with cleanup jobs:

• Create a materialized view that reflects your desired data with an expiry filter (e.g., only shows entries not expired).
• Schedule a separate job (similar to the cron job in option 1) to periodically refresh the materialized view and potentially remove expired entries from the underlying tables.

1. External queueing systems with worker processes:

• Integrate your application with a message queue (e.g., RabbitMQ, Kafka).
• When data is inserted or updated, send a message to the queue containing the data and its expiry time.
• Have a separate worker process constantly listen to the queue and process messages.
  • The worker retrieves the data and expiry time from the message.
  • It checks if the expiry has passed. If so, it deletes the data from the database.

1. Application-level logic:

• Implement the expiry logic within your application code.
• When retrieving data from the database, check the "expiry_time" column and only use entries that are not expired.
• Optionally, your application can also handle deletion of expired data directly.

Choosing the right method depends on factors like:

• Frequency of expiry checks:
  • Scheduled tasks are good for periodic checks, while triggers offer real-time expiry handling.
• Performance: Triggers can impact insert/update performance, while background jobs might add complexity.
• Application integration: Queueing systems work well with distributed applications.