NEW and OLD record access in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of trigger function using OLD and NEW

Consider a PostgreSQL trigger function that logs changes on a table. What will be the output of the following function when an UPDATE occurs changing column price from 100 to 120?

CREATE OR REPLACE FUNCTION log_price_change() RETURNS trigger AS $$
BEGIN
  RAISE NOTICE 'Price changed from % to %', OLD.price, NEW.price;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

A Price changed from 100 to 120

B Price changed from 100 to NULL

C Price changed from NULL to 120

D Price changed from 120 to 100

💡 Hint

OLD contains the row before the update, NEW contains the row after.

In an UPDATE trigger, OLD holds the original row values, and NEW holds the new values. So the price changed from 100 (OLD.price) to 120 (NEW.price).

🧠 Conceptual

intermediate

Understanding OLD and NEW in DELETE triggers

In a DELETE trigger in PostgreSQL, which of the following statements is true about the availability of OLD and NEW records?

A Both OLD and NEW contain the row being deleted

B NEW contains the row being deleted; OLD is NULL

C OLD contains the row being deleted; NEW is NULL

D Both OLD and NEW are NULL

💡 Hint

Think about what data exists before and after a DELETE.

When a row is deleted, the OLD record holds the row before deletion, but there is no NEW record because the row no longer exists after deletion.

📝 Syntax

advanced

Identify the syntax error in trigger function using OLD and NEW

Which option contains a syntax error in this PostgreSQL trigger function that tries to compare OLD and NEW values?

CREATE OR REPLACE FUNCTION check_update() RETURNS trigger AS $$
BEGIN
  IF NEW.value <> OLD.value THEN
    RAISE NOTICE 'Value changed';
  END IF;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

A IF (NEW.value <> OLD.value) THEN

B IF NEW.value <> OLD.value

C IF NEW.value <> OLD.value THEN

D IF NEW.value != OLD.value THEN

💡 Hint

Check the syntax of IF statements in PL/pgSQL.

In PL/pgSQL, IF statements require THEN keyword after the condition. Option B misses THEN, causing a syntax error.

❓ optimization

advanced

Optimizing trigger function to avoid unnecessary updates

You have a BEFORE UPDATE trigger that updates a timestamp column only if certain columns changed. Which approach is best to avoid unnecessary updates?

Options show different ways to compare OLD and NEW records.

A

IF NEW.col1 <> OLD.col1 OR NEW.col2 <> OLD.col2 THEN
  NEW.updated_at = now();
END IF;

B NEW.updated_at = now();

C

IF OLD IS DISTINCT FROM NEW THEN
  NEW.updated_at = now();
END IF;

D

IF NEW IS DISTINCT FROM OLD THEN
  NEW.updated_at = now();
END IF;

💡 Hint

PostgreSQL supports IS DISTINCT FROM for row comparisons.

Using 'NEW IS DISTINCT FROM OLD' compares entire rows safely including NULLs, avoiding multiple OR conditions and unnecessary updates.

🔧 Debug

expert

Debugging a trigger function that causes infinite recursion

A BEFORE UPDATE trigger function updates the same table's row by setting NEW.updated_at = now(). However, this causes infinite recursion and stack overflow. What is the best fix?

A Add a condition to update NEW.updated_at only if OLD.updated_at is different

B Disable triggers on the table temporarily during update

C Change the trigger to AFTER UPDATE instead of BEFORE UPDATE

D Use a separate function to update updated_at outside the trigger

💡 Hint

Think about what causes the trigger to fire repeatedly.

Updating NEW.updated_at unconditionally causes the trigger to fire again. Adding a condition to update only if the timestamp changed prevents recursion.

Practice

1. In a PostgreSQL trigger function, which record variable would you use to access the new row data after an INSERT operation?

easy

A. PREVIOUS

B. NEW

C. CURRENT

D. OLD

Solution

1. Step 1: Understand trigger timing for INSERT

   For an INSERT operation, the new row is being added, so the trigger can access the new data using the NEW record.

2. Step 2: Identify correct record variable

   The OLD record is not available for INSERT because there is no previous row. Therefore, NEW is used to access the inserted row.

3. Final Answer:

   NEW -> Option B

4. Quick Check:

   INSERT uses NEW = D [OK]

Hint: Use NEW for inserted or updated rows, OLD for deleted or old rows [OK]

Common Mistakes:

• Using OLD in INSERT triggers
• Confusing NEW and OLD for UPDATE
• Assuming CURRENT or PREVIOUS exist

2. Which of the following is the correct syntax to access the old value of a column named price inside a DELETE trigger in PostgreSQL?

easy

A. OLD.price

B. NEW.price

C. OLD->price

D. NEW->price

Solution

1. Step 1: Identify record variable for DELETE

   In a DELETE trigger, the row is being removed, so the old data is accessible via OLD.

2. Step 2: Use correct syntax for column access

   PostgreSQL uses dot notation to access columns in record variables, so OLD.price is correct.

3. Final Answer:

   OLD.price -> Option A

4. Quick Check:

   DELETE uses OLD.column = A [OK]

Hint: Use dot notation with OLD for deleted row columns [OK]

Common Mistakes:

• Using NEW in DELETE triggers
• Using arrow (->) instead of dot for record access
• Confusing syntax for JSON operators

3. Consider this trigger function snippet for an UPDATE operation:

IF NEW.quantity < OLD.quantity THEN
  RAISE NOTICE 'Quantity decreased from % to %', OLD.quantity, NEW.quantity;
END IF;

What will be the output if the old quantity was 10 and the new quantity is 7?

medium

A. Quantity decreased from 10 to 7

B. Quantity decreased from 7 to 10

C. No output

D. Syntax error

Solution

1. Step 1: Understand the condition in the IF statement

   The condition checks if the new quantity is less than the old quantity. Here, 7 < 10 is true.

2. Step 2: Analyze the RAISE NOTICE output

   The message prints the old quantity first, then the new quantity, so it will output: 'Quantity decreased from 10 to 7'.

3. Final Answer:

   Quantity decreased from 10 to 7 -> Option A

4. Quick Check:

   NEW < OLD triggers notice = A [OK]

Hint: Compare NEW and OLD values carefully in UPDATE triggers [OK]

Common Mistakes:

• Mixing up NEW and OLD values in output
• Assuming no output when condition is true
• Confusing syntax of RAISE NOTICE

4. You wrote this trigger function for DELETE:

CREATE FUNCTION trg_delete_check() RETURNS trigger AS $$
BEGIN
  IF NEW.id IS NULL THEN
    RAISE EXCEPTION 'ID cannot be null';
  END IF;
  RETURN OLD;
END;
$$ LANGUAGE plpgsql;

What is the error in this function?

medium

A. RAISE EXCEPTION syntax is wrong

B. RETURN OLD is invalid in DELETE triggers

C. Using NEW in a DELETE trigger where only OLD is available

D. Function must return VOID, not trigger

Solution

1. Step 1: Check record variables in DELETE triggers

   In DELETE triggers, the NEW record is not available because no new row is inserted or updated.

2. Step 2: Identify incorrect usage of NEW

   The function incorrectly uses NEW.id, which causes an error. It should use OLD.id instead.

3. Final Answer:

   Using NEW in a DELETE trigger where only OLD is available -> Option C

4. Quick Check:

   DELETE triggers have OLD, not NEW = C [OK]

Hint: Use OLD in DELETE triggers; NEW is unavailable [OK]

Common Mistakes:

• Using NEW in DELETE triggers
• Returning OLD incorrectly
• Misunderstanding trigger return types

5. You want to create a trigger that logs changes to a salary column only when the salary is updated to a higher value. Which trigger condition and record access correctly implements this in PostgreSQL?

hard

A. IF OLD.salary > NEW.salary THEN INSERT INTO log_table VALUES (OLD.id, OLD.salary); END IF;

B. IF NEW.salary < OLD.salary THEN INSERT INTO log_table VALUES (NEW.id, NEW.salary); END IF;

C. IF NEW.salary = OLD.salary THEN INSERT INTO log_table VALUES (NEW.id, NEW.salary); END IF;

D. IF NEW.salary > OLD.salary THEN INSERT INTO log_table VALUES (NEW.id, NEW.salary); END IF;

Solution

1. Step 1: Understand the condition for logging

   The trigger should log only when the new salary is greater than the old salary, so the condition is NEW.salary > OLD.salary.

2. Step 2: Use correct record variables for UPDATE

   The new salary and id come from NEW because the row is updated with new values.

3. Final Answer:

   IF NEW.salary > OLD.salary THEN INSERT INTO log_table VALUES (NEW.id, NEW.salary); END IF; -> Option D

4. Quick Check:

   Log when NEW > OLD salary = B [OK]

Hint: Compare NEW and OLD to detect increases, then log NEW data [OK]

Common Mistakes:

• Reversing NEW and OLD in condition
• Logging when salary decreases
• Using equality instead of greater than