Why Row-level triggers for audit logs in Supabase? - Purpose & Use Cases

The Big Idea

What if your database could remember every change for you, without any extra work?

The Scenario

Imagine you have a busy database where many users add, change, or delete data every day. You want to keep track of every change to know who did what and when. Doing this by hand means checking every row and writing notes manually.

The Problem

Manually tracking changes is slow and easy to forget. You might miss important updates or make mistakes. It's like trying to watch every move in a crowded room without any help -- exhausting and unreliable.

The Solution

Row-level triggers automatically watch each change in your database. They create audit logs instantly without you lifting a finger. This means every insert, update, or delete is recorded perfectly and on time.

Before vs After

✗ Before

Manually query changes and write logs after each update.

✓ After

CREATE TRIGGER audit_trigger AFTER INSERT OR UPDATE OR DELETE ON table_name FOR EACH ROW EXECUTE FUNCTION audit_log_function();

What It Enables

This lets you trust your data history fully and focus on building features instead of chasing changes.

Real Life Example

A company uses row-level triggers to track every change in customer orders. If a mistake happens, they quickly see who changed what and fix it fast.

Key Takeaways

Manual tracking is slow and error-prone.

Row-level triggers automate audit logging perfectly.

This builds trust and saves time in managing data changes.

Practice

1. What is the main purpose of a row-level trigger in Supabase for audit logs?

easy

A. To automatically record changes made to individual rows in a table

B. To speed up database queries by caching results

C. To create new tables automatically when data is inserted

D. To delete old data from the database periodically

Solution

1. Step 1: Understand what row-level triggers do

   Row-level triggers activate for each row affected by an insert, update, or delete operation.

2. Step 2: Connect triggers to audit logs

   They are used to record changes to rows, capturing who changed what and when.

3. Final Answer:

   To automatically record changes made to individual rows in a table -> Option A

4. Quick Check:

   Row-level trigger purpose = record row changes [OK]

Hint: Triggers log row changes automatically for audit [OK]

Common Mistakes:

• Thinking triggers speed up queries
• Confusing triggers with table creation
• Assuming triggers delete old data

2. Which of the following is the correct syntax to create a row-level trigger in Supabase that fires AFTER an UPDATE on a table named users?

easy

A. CREATE TRIGGER user_update AFTER INSERT ON users FOR EACH ROW EXECUTE FUNCTION audit_log();

B. CREATE TRIGGER user_update BEFORE UPDATE ON users FOR EACH STATEMENT EXECUTE FUNCTION audit_log();

C. CREATE TRIGGER user_update AFTER UPDATE ON users FOR EACH ROW EXECUTE FUNCTION audit_log();

D. CREATE TRIGGER user_update AFTER UPDATE ON users FOR EACH STATEMENT EXECUTE FUNCTION audit_log();

Solution

1. Step 1: Identify trigger timing and event

   The question asks for AFTER UPDATE trigger on table users.

2. Step 2: Confirm row-level execution

   Row-level triggers use FOR EACH ROW, not FOR EACH STATEMENT.

3. Final Answer:

   CREATE TRIGGER user_update AFTER UPDATE ON users FOR EACH ROW EXECUTE FUNCTION audit_log(); -> Option C

4. Quick Check:

   AFTER UPDATE + FOR EACH ROW = CREATE TRIGGER user_update AFTER UPDATE ON users FOR EACH ROW EXECUTE FUNCTION audit_log(); [OK]

Hint: Use AFTER UPDATE and FOR EACH ROW for row-level update triggers [OK]

Common Mistakes:

• Using BEFORE instead of AFTER
• Using FOR EACH STATEMENT instead of FOR EACH ROW
• Using INSERT event instead of UPDATE

3. Given this trigger function snippet in Supabase SQL:

CREATE OR REPLACE FUNCTION audit_log() RETURNS trigger AS $$
BEGIN
  INSERT INTO audit_table(user_id, action, changed_at)
  VALUES (NEW.id, TG_OP, NOW());
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

What will happen when a row in the users table is updated?

medium

A. A new row is inserted into audit_table with the user's id, action type, and timestamp

B. The updated row in users is deleted

C. The trigger function will cause a syntax error and fail

D. Nothing happens because OLD is not used

Solution

1. Step 1: Analyze the trigger function logic

   The function inserts a row into audit_table with NEW.id, operation type TG_OP, and current time.

2. Step 2: Understand trigger behavior on update

   On update, NEW holds the new row data, so the insert logs the change.

3. Final Answer:

   A new row is inserted into audit_table with the user's id, action type, and timestamp -> Option A

4. Quick Check:

   Trigger inserts audit record on update = A new row is inserted into audit_table with the user's id, action type, and timestamp [OK]

Hint: Trigger inserts audit row using NEW and TG_OP on update [OK]

Common Mistakes:

• Thinking OLD is required for insert
• Assuming trigger deletes rows
• Confusing syntax causing errors

4. You wrote this trigger function to log deletes:

CREATE OR REPLACE FUNCTION audit_log_delete() RETURNS trigger AS $$
BEGIN
  INSERT INTO audit_table(user_id, action, changed_at)
  VALUES (NEW.id, TG_OP, NOW());
  RETURN OLD;
END;
$$ LANGUAGE plpgsql;

But it fails when deleting rows. What is the error?

medium

A. TG_OP cannot be used in trigger functions

B. RETURN OLD is invalid in DELETE triggers

C. NOW() is not allowed in trigger functions

D. Using NEW.id on DELETE triggers causes an error because NEW is NULL

Solution

1. Step 1: Understand DELETE trigger context

   On DELETE, NEW is NULL because the row is removed; OLD holds the deleted row.

2. Step 2: Identify the error in the function

   The function uses NEW.id which is NULL on DELETE, causing failure.

3. Final Answer:

   Using NEW.id on DELETE triggers causes an error because NEW is NULL -> Option D

4. Quick Check:

   DELETE triggers use OLD, not NEW [OK]

Hint: Use OLD, not NEW, in DELETE triggers to avoid errors [OK]

Common Mistakes:

• Using NEW in DELETE triggers
• Thinking RETURN OLD is invalid
• Believing TG_OP or NOW() cause errors

5. You want to create an audit log that records who updated a row in the orders table, including old and new values. Which approach correctly combines row-level triggers and audit logging in Supabase?

hard

A. Create a trigger function that only logs NEW data and create a BEFORE INSERT trigger on orders

B. Create a trigger function that inserts into audit_orders using both OLD and NEW row data, then create AFTER UPDATE trigger FOR EACH ROW on orders

C. Create a trigger function that deletes old rows from orders and logs the deletion

D. Create a trigger function that logs changes only when rows are inserted, ignoring updates

Solution

1. Step 1: Identify audit log needs

   You want to log who updated rows and capture old and new values for changes.

2. Step 2: Choose correct trigger timing and data

   Use AFTER UPDATE trigger FOR EACH ROW to access OLD and NEW data and insert audit record.

3. Step 3: Confirm audit table insert logic

   The trigger function should insert both OLD and NEW values into audit_orders for full change history.

4. Final Answer:

   Create a trigger function that inserts into audit_orders using both OLD and NEW row data, then create AFTER UPDATE trigger FOR EACH ROW on orders -> Option B

5. Quick Check:

   Use AFTER UPDATE + OLD/NEW + FOR EACH ROW for full audit [OK]

Hint: Use AFTER UPDATE trigger with OLD and NEW for full audit logs [OK]

Common Mistakes:

• Logging only NEW data misses old values
• Using BEFORE INSERT triggers for updates
• Deleting rows instead of logging changes