Trigger function creation in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this trigger function execution?

Consider the following PostgreSQL trigger function and trigger on a table employees:

CREATE OR REPLACE FUNCTION update_last_modified()
RETURNS TRIGGER AS $$
BEGIN
  NEW.last_modified := NOW();
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_update_last_modified
BEFORE UPDATE ON employees
FOR EACH ROW
EXECUTE FUNCTION update_last_modified();

If a row in employees is updated, what will be the value of last_modified column after the update?

A The <code>last_modified</code> column will remain unchanged from its previous value.

B The <code>last_modified</code> column will be set to NULL.

C The update will fail because the trigger function does not return OLD.

D The <code>last_modified</code> column will be set to the current timestamp at update time.

💡 Hint

Think about what the trigger function does before returning the new row.

The trigger function sets the last_modified column to the current timestamp using NOW() before the row is updated. It returns the modified NEW row, so the update applies this new timestamp.

📝 Syntax

intermediate

Identify the syntax error in this trigger function definition

Which option contains the correct syntax for creating a trigger function in PostgreSQL?

PostgreSQL

CREATE OR REPLACE FUNCTION trg_func()
RETURNS TRIGGER AS $$
BEGIN
  -- some logic
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

A CREATE FUNCTION trg_func() RETURNS TRIGGER AS $$ BEGIN RETURN NEW; END; $$ LANGUAGE plpgsql;

B CREATE OR REPLACE FUNCTION trg_func RETURNS TRIGGER AS $$ BEGIN RETURN NEW; END; $$ LANGUAGE plpgsql;

C CREATE OR REPLACE FUNCTION trg_func() RETURNS TRIGGER AS $$ BEGIN RETURN NEW; END; $$ LANGUAGE plpgsql;

D CREATE OR REPLACE FUNCTION trg_func() RETURNS TRIGGER AS $$ BEGIN RETURN NEW; END $$ LANGUAGE plpgsql;

💡 Hint

Check for missing parentheses, semicolons, and correct keywords.

Option C correctly includes parentheses after the function name, the RETURNS clause, the semicolon after END, and the language declaration.

❓ optimization

advanced

Optimize this trigger function to avoid unnecessary updates

Given this trigger function that updates a last_modified timestamp on every update:

CREATE OR REPLACE FUNCTION update_last_modified()
RETURNS TRIGGER AS $$
BEGIN
  NEW.last_modified := NOW();
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

How can you optimize it to only update last_modified if some column other than last_modified actually changed?

A Add an IF statement comparing OLD and NEW rows for changes excluding last_modified, then update last_modified only if changed.

B Always update last_modified regardless of changes to ensure consistency.

C Remove the trigger function and update last_modified manually in application code.

D Use a BEFORE INSERT trigger instead of BEFORE UPDATE to update last_modified.

💡 Hint

Think about how to detect changes between OLD and NEW rows.

By comparing OLD and NEW rows for changes excluding last_modified, the trigger can avoid unnecessary updates and reduce write load.

🔧 Debug

advanced

Why does this trigger function cause an infinite loop?

Consider this trigger function and trigger on table orders:

CREATE OR REPLACE FUNCTION trg_update_total()
RETURNS TRIGGER AS $$
BEGIN
  NEW.total := NEW.quantity * NEW.price;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_update_total
BEFORE UPDATE ON orders
FOR EACH ROW
EXECUTE FUNCTION trg_update_total();

Updating any row in orders causes an infinite loop. Why?

A The table <code>orders</code> does not have columns <code>quantity</code> or <code>price</code>.

B The trigger updates the row, which fires the trigger again, causing recursion without a stop condition.

C The trigger is defined as AFTER UPDATE instead of BEFORE UPDATE.

D The trigger function is missing a RETURN statement, causing an error.

💡 Hint

Think about what happens when a trigger modifies the row it is triggered on.

The trigger modifies the row during an update, which causes the update to fire the trigger again, creating an infinite loop of updates.

🧠 Conceptual

expert

Which statement about trigger functions in PostgreSQL is true?

Choose the correct statement about trigger functions in PostgreSQL.

A Trigger functions can be written in any language supported by PostgreSQL, but must return type TRIGGER.

B Trigger functions must always return the NEW row for BEFORE triggers and the OLD row for AFTER triggers.

C Trigger functions can only be used with BEFORE INSERT triggers, not UPDATE or DELETE.

D Trigger functions cannot access the OLD row data in BEFORE INSERT triggers.

💡 Hint

Consider the flexibility of languages and return types for trigger functions.

Trigger functions in PostgreSQL can be written in any supported language (like plpgsql, plpython, etc.) but must always return type TRIGGER.

Practice

1. What is the correct return type for a trigger function in PostgreSQL?

easy

A. void

B. trigger

C. integer

D. boolean

Solution

1. Step 1: Understand trigger function requirements

   Trigger functions must return a special type that PostgreSQL recognizes for triggers.

2. Step 2: Identify the correct return type

   The return type for trigger functions is always trigger, not standard types like void or integer.

3. Final Answer:

   trigger -> Option B

4. Quick Check:

   Trigger functions return type = trigger [OK]

Hint: Trigger functions always return type 'trigger' [OK]

Common Mistakes:

• Using void or integer as return type
• Forgetting to specify return type
• Confusing trigger function with normal function

2. Which of the following is the correct way to start a trigger function in PostgreSQL?

easy

A. CREATE FUNCTION my_trigger() RETURNS trigger AS $$ BEGIN RETURN NEW; END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION my_trigger() RETURNS boolean AS $$ BEGIN RETURN TRUE; END; $$ LANGUAGE plpgsql;

C. CREATE TRIGGER my_trigger() RETURNS trigger AS $$ BEGIN RETURN OLD; END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION my_trigger() RETURNS void AS $$ BEGIN END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Check syntax for trigger function creation

   Trigger functions must be created with CREATE FUNCTION, return type trigger, and use plpgsql language.

2. Step 2: Identify correct function body and return statement

   Trigger functions must return NEW or OLD row, so RETURN NEW; is correct here.

3. Final Answer:

   CREATE FUNCTION my_trigger() RETURNS trigger AS $$ BEGIN RETURN NEW; END; $$ LANGUAGE plpgsql; -> Option A

4. Quick Check:

   Trigger function syntax = CREATE FUNCTION my_trigger() RETURNS trigger AS $$ BEGIN RETURN NEW; END; $$ LANGUAGE plpgsql; [OK]

Hint: Trigger functions use RETURNS trigger and RETURN NEW or OLD [OK]

Common Mistakes:

• Using RETURNS void instead of trigger
• Using CREATE TRIGGER instead of CREATE FUNCTION
• Returning boolean or void instead of NEW or OLD

3. Given this trigger function, what will be the result when a new row is inserted?

CREATE FUNCTION check_age() RETURNS trigger AS $$
BEGIN
  IF NEW.age < 18 THEN
    RAISE EXCEPTION 'Age must be 18 or older';
  END IF;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

medium

A. The row is inserted regardless of age

B. The function returns NULL causing an error

C. The row is always rejected

D. The row is inserted only if age is 18 or older

Solution

1. Step 1: Analyze the condition in the trigger function

   The function checks if NEW.age < 18 and raises an exception if true.

2. Step 2: Understand effect of exception and return

   If age is less than 18, insertion stops with error. Otherwise, RETURN NEW; allows insertion.

3. Final Answer:

   The row is inserted only if age is 18 or older -> Option D

4. Quick Check:

   Exception blocks underage inserts = The row is inserted only if age is 18 or older [OK]

Hint: Exception stops insert; RETURN NEW allows it [OK]

Common Mistakes:

• Thinking all rows insert regardless
• Assuming RETURN NEW inserts row without checks
• Confusing RAISE EXCEPTION with warnings

4. Identify the error in this trigger function code:

CREATE FUNCTION update_timestamp() RETURNS trigger AS $$
BEGIN
  NEW.updated_at = NOW();
  RETURN NEW
END;
$$ LANGUAGE plpgsql;

medium

A. Cannot assign to NEW in trigger functions

B. Incorrect return type, should be void

C. Missing semicolon after RETURN NEW

D. Missing BEGIN keyword

Solution

1. Step 1: Check syntax for statements inside function

   Each statement must end with a semicolon in PL/pgSQL.

2. Step 2: Identify missing semicolon

   The line RETURN NEW lacks a semicolon at the end, causing syntax error.

3. Final Answer:

   Missing semicolon after RETURN NEW -> Option C

4. Quick Check:

   PL/pgSQL statements end with semicolon [OK]

Hint: Check semicolons after each statement [OK]

Common Mistakes:

• Forgetting semicolon after RETURN NEW
• Thinking NEW cannot be assigned
• Confusing return type with void

5. You want to create a trigger function that automatically sets a column last_modified to the current timestamp whenever a row is updated. Which of the following trigger function definitions correctly achieves this?

hard

A. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified := NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN OLD.last_modified := NOW(); RETURN OLD; END; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION set_last_modified() RETURNS void AS $$ BEGIN NEW.last_modified := NOW(); END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified = NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Identify correct assignment operator in PL/pgSQL

   PL/pgSQL uses := for assignment, not =.

2. Step 2: Check return type and returned row

   Trigger functions must return type trigger and return NEW for BEFORE UPDATE triggers.

3. Step 3: Evaluate each option

   CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified := NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql; uses correct assignment :=, returns NEW, and has correct return type. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified = NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql; uses = which is invalid for assignment. CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN OLD.last_modified := NOW(); RETURN OLD; END; $$ LANGUAGE plpgsql; assigns to OLD which is read-only. CREATE FUNCTION set_last_modified() RETURNS void AS $$ BEGIN NEW.last_modified := NOW(); END; $$ LANGUAGE plpgsql; returns void which is invalid.

4. Final Answer:

   CREATE FUNCTION set_last_modified() RETURNS trigger AS $$ BEGIN NEW.last_modified := NOW(); RETURN NEW; END; $$ LANGUAGE plpgsql; -> Option A

5. Quick Check:

   Use := for assignment and return NEW [OK]

Hint: Use := for assignment and return NEW in trigger [OK]

Common Mistakes:

• Using = instead of := for assignment
• Returning OLD instead of NEW
• Using void return type for trigger function