Trigger for audit logging in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a trigger in PostgreSQL?

A trigger is a special function that automatically runs when certain events happen on a table, like insert, update, or delete.

beginner

Why use triggers for audit logging?

Triggers help record changes automatically in an audit table whenever data changes, so you can track who changed what and when.

beginner

What event types can a trigger listen to for audit logging?

Triggers can listen to INSERT, UPDATE, and DELETE events to capture data changes for auditing.

beginner

What is the purpose of an audit table in PostgreSQL?

An audit table stores records of changes made to data, including old and new values, who made the change, and when it happened.

intermediate

How do you create a trigger function for audit logging in PostgreSQL?

You write a PL/pgSQL function that inserts a record into the audit table whenever a data change event occurs.

Which PostgreSQL event can a trigger NOT listen to for audit logging?

A DELETE

B UPDATE

C SELECT

D INSERT

Triggers cannot be set on SELECT statements; they only work on data-changing events like INSERT, UPDATE, and DELETE.

What language is commonly used to write trigger functions in PostgreSQL?

A PL/pgSQL

B Python

C JavaScript

D Bash

PL/pgSQL is the procedural language used inside PostgreSQL to write trigger functions.

What is the main purpose of an audit trigger?

A To delete old data

B To automatically backup the database

C To speed up queries

D To log changes made to data

Audit triggers log changes made to data for tracking and security purposes.

Which of these is NOT typically stored in an audit log?

A Database server uptime

B New data values

C Old data values

D User who made the change

Database server uptime is not related to audit logs, which focus on data changes.

How do you attach a trigger function to a table in PostgreSQL?

A Using CREATE FUNCTION statement

B Using CREATE TRIGGER statement

C Using ALTER TABLE statement

D Using INSERT INTO statement

CREATE TRIGGER is used to link a trigger function to a table event.

Explain how a trigger can be used to implement audit logging in PostgreSQL.

Think about what happens when data changes and how to capture that automatically.

Describe the steps to create an audit logging system using triggers in PostgreSQL.

Start with where to store logs, then how to capture changes, and finally how to connect them.

Practice

1. What is the main purpose of a trigger in PostgreSQL for audit logging?

easy

A. To backup the database periodically

B. To automatically record changes made to data in a table

C. To create new tables automatically

D. To speed up query execution

Solution

1. Step 1: Understand what triggers do

   Triggers run code automatically when data changes occur in a table.

2. Step 2: Connect triggers to audit logging

   Audit logging means recording who changed what and when, which triggers help automate.

3. Final Answer:

   To automatically record changes made to data in a table -> Option B

4. Quick Check:

   Trigger = automatic audit record [OK]

Hint: Triggers run code on data changes to log audits [OK]

Common Mistakes:

• Thinking triggers speed up queries
• Confusing triggers with backups
• Assuming triggers create tables

2. Which of the following is the correct syntax to create a trigger function for audit logging in PostgreSQL?

easy

A. CREATE TRIGGER audit_log BEFORE INSERT ON audit_table EXECUTE FUNCTION log_changes();

B. CREATE FUNCTION audit_log() RETURNS void AS $$ BEGIN UPDATE audit_table SET changed = TRUE; END; $$ LANGUAGE sql;

C. CREATE FUNCTION audit_log() RETURNS trigger AS $$ BEGIN INSERT INTO audit_table VALUES (OLD.*); RETURN NEW; END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION audit_log() RETURNS trigger AS $$ BEGIN INSERT INTO audit_table VALUES (NEW.*); RETURN OLD; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Check function return type and language

   Trigger functions must return type 'trigger' and use 'plpgsql' language.

2. Step 2: Verify correct use of OLD and NEW

   For audit logging on updates/deletes, OLD.* is used to capture previous data; function returns NEW to continue operation.

3. Final Answer:

   CREATE FUNCTION audit_log() RETURNS trigger AS $$ BEGIN INSERT INTO audit_table VALUES (OLD.*); RETURN NEW; END; $$ LANGUAGE plpgsql; -> Option C

4. Quick Check:

   Trigger function syntax = CREATE FUNCTION audit_log() RETURNS trigger AS $$ BEGIN INSERT INTO audit_table VALUES (OLD.*); RETURN NEW; END; $$ LANGUAGE plpgsql; [OK]

Hint: Trigger functions return 'trigger' and use plpgsql [OK]

Common Mistakes:

• Using RETURNS void instead of RETURNS trigger
• Returning OLD instead of NEW
• Wrong language like SQL instead of plpgsql

3. Given this trigger function and trigger creation:

CREATE FUNCTION audit_func() RETURNS trigger AS $$ BEGIN INSERT INTO audit_log(user_name, action_time) VALUES (current_user, now()); RETURN NEW; END; $$ LANGUAGE plpgsql;
CREATE TRIGGER audit_trigger AFTER INSERT ON employees FOR EACH ROW EXECUTE FUNCTION audit_func();

What happens when a new row is inserted into employees?

medium

A. A new row is added to audit_log with current user and timestamp

B. The insert into employees fails with an error

C. No action occurs because the trigger is AFTER INSERT

D. The employees row is deleted immediately

Solution

1. Step 1: Understand AFTER INSERT trigger behavior

   AFTER INSERT triggers run after a new row is added, so the insert succeeds first.

2. Step 2: Analyze trigger function actions

   The function inserts a row into audit_log with current user and timestamp, logging the event.

3. Final Answer:

   A new row is added to audit_log with current user and timestamp -> Option A

4. Quick Check:

   AFTER INSERT triggers log data after insert [OK]

Hint: AFTER INSERT triggers run after data is inserted [OK]

Common Mistakes:

• Thinking AFTER INSERT prevents insert
• Assuming trigger deletes data
• Believing no action happens after insert

4. You wrote this trigger function:

CREATE FUNCTION audit_changes() RETURNS trigger AS $$ BEGIN INSERT INTO audit_log VALUES (NEW.*); RETURN NEW; END; $$ LANGUAGE plpgsql;

But when the trigger fires (e.g., on INSERT or UPDATE to the table), you get an error. What is the likely cause?

medium

A. Triggers cannot insert into tables

B. Trigger functions cannot use RETURN NEW

C. The function must be written in SQL, not plpgsql

D. The audit_log table does not match the NEW record structure

Solution

1. Step 1: Check compatibility of NEW.* with audit_log table

   NEW.* expands to all columns of the triggering table, which must match audit_log columns exactly.

2. Step 2: Identify mismatch causes error

   If audit_log has different columns or order, the insert fails when the trigger fires.

3. Final Answer:

   The audit_log table does not match the NEW record structure -> Option D

4. Quick Check:

   Column mismatch causes insert error [OK]

Hint: Ensure audit_log columns match NEW record exactly [OK]

Common Mistakes:

• Thinking RETURN NEW is invalid
• Assuming language must be SQL
• Believing triggers cannot insert data

5. You want to create an audit log that records old and new values on UPDATE for a products table. Which trigger function code correctly captures both old and new data for audit logging?

hard

A. CREATE FUNCTION audit_update() RETURNS trigger AS $$ BEGIN INSERT INTO audit_log(old_name, new_name) VALUES (OLD.name, NEW.name); RETURN NEW; END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION audit_update() RETURNS trigger AS $$ BEGIN INSERT INTO audit_log(name) VALUES (NEW.name); RETURN OLD; END; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION audit_update() RETURNS trigger AS $$ BEGIN INSERT INTO audit_log(old_name, new_name) VALUES (NEW.name, OLD.name); RETURN NEW; END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION audit_update() RETURNS trigger AS $$ BEGIN UPDATE audit_log SET name = NEW.name WHERE name = OLD.name; RETURN NEW; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Identify correct use of OLD and NEW in UPDATE triggers

   OLD contains previous row data, NEW contains updated data; audit log needs both.

2. Step 2: Check function logic and return value

   Insert old and new names correctly, then return NEW to allow update to proceed.

3. Final Answer:

   CREATE FUNCTION audit_update() RETURNS trigger AS $$ BEGIN INSERT INTO audit_log(old_name, new_name) VALUES (OLD.name, NEW.name); RETURN NEW; END; $$ LANGUAGE plpgsql; -> Option A

4. Quick Check:

   OLD before, NEW after update [OK]

Hint: Use OLD for old data, NEW for new data in audit triggers [OK]

Common Mistakes:

• Swapping OLD and NEW values
• Returning OLD instead of NEW
• Using UPDATE instead of INSERT in audit log