UPDATE with RETURNING clause in PostgreSQL - Time & Space Complexity

Time Complexity: UPDATE with RETURNING clause

O(n)

Understanding Time Complexity

When we update rows in a database, it takes time depending on how many rows we change.

We want to know how the time grows when we update more rows and use the RETURNING clause to get results back.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

UPDATE employees
SET salary = salary * 1.1
WHERE department = 'Sales'
RETURNING id, salary;
    

This code updates salaries for all employees in the Sales department and returns their id and new salary.

Identify Repeating Operations

• Primary operation: Scanning and updating each matching row in the employees table.
• How many times: Once for each employee in the Sales department.

How Execution Grows With Input

As the number of employees in Sales grows, the update and returning work grows too.

Input Size (n)	Approx. Operations
10	About 10 updates and 10 rows returned
100	About 100 updates and 100 rows returned
1000	About 1000 updates and 1000 rows returned

Pattern observation: The work grows directly with the number of rows updated and returned.

Final Time Complexity

Time Complexity: O(n)

This means the time grows in a straight line with the number of rows updated and returned.

Common Mistake

[X] Wrong: "The RETURNING clause does not affect the time because it just shows results."

[OK] Correct: Returning rows means the database must collect and send data for each updated row, adding to the work done.

Interview Connect

Understanding how updates and returning results scale helps you explain database performance clearly and confidently.

Self-Check

"What if we removed the WHERE clause and updated all rows? How would the time complexity change?"

Practice

1. What does the RETURNING clause do in a PostgreSQL UPDATE statement?

easy

A. It returns the updated rows immediately after the update.

B. It deletes the rows after updating them.

C. It creates a backup of the updated rows.

D. It prevents the update from happening.

Solution

1. Step 1: Understand the purpose of UPDATE

   The UPDATE statement changes data in existing rows of a table.

2. Step 2: Role of RETURNING clause

   The RETURNING clause immediately outputs the rows that were updated, showing their new values.

3. Final Answer:

   It returns the updated rows immediately after the update. -> Option A

4. Quick Check:

   RETURNING shows updated rows [OK]

Hint: RETURNING shows updated rows right after update [OK]

Common Mistakes:

• Thinking RETURNING deletes rows
• Assuming RETURNING creates backups
• Believing RETURNING stops the update

2. Which of the following is the correct syntax to update a table and return the updated rows in PostgreSQL?

easy

A. UPDATE table_name SET column1 = value1 WHERE condition RETURNING *;

B. UPDATE table_name SET column1 = value1 RETURN * WHERE condition;

C. UPDATE table_name RETURNING * SET column1 = value1 WHERE condition;

D. UPDATE table_name SET column1 = value1 WHERE condition SHOW RETURNING *;

Solution

1. Step 1: Identify correct UPDATE syntax

   The UPDATE statement must have SET clause before WHERE clause.

2. Step 2: Position of RETURNING clause

   RETURNING clause comes after WHERE clause to output updated rows.

3. Final Answer:

   UPDATE table_name SET column1 = value1 WHERE condition RETURNING *; -> Option A

4. Quick Check:

   RETURNING follows WHERE in UPDATE [OK]

Hint: RETURNING always comes after WHERE clause [OK]

Common Mistakes:

• Placing RETURNING before SET clause
• Using SHOW RETURNING which is invalid
• Putting RETURNING between SET and WHERE

3. Given the table employees with columns id, salary, and department, what will this query return?

UPDATE employees SET salary = salary + 500 WHERE department = 'Sales' RETURNING id, salary;

medium

A. Only the number of rows updated, no data returned.

B. All employees' ids and salaries regardless of department.

C. All employees' ids and new salaries in the Sales department after the raise.

D. An error because RETURNING cannot specify columns.

Solution

1. Step 1: Understand the UPDATE condition

   The query updates salary by adding 500 only for employees in the Sales department.

2. Step 2: Effect of RETURNING clause

   RETURNING id, salary outputs the id and updated salary of only those employees updated.

3. Final Answer:

   All employees' ids and new salaries in the Sales department after the raise. -> Option C

4. Quick Check:

   RETURNING outputs updated rows only [OK]

Hint: RETURNING shows only updated rows, not all rows [OK]

Common Mistakes:

• Thinking all rows are returned
• Expecting only count of updated rows
• Believing RETURNING cannot specify columns

4. What is wrong with this query?

UPDATE products RETURNING * SET price = price * 1.1 WHERE category = 'Books';

medium

A. The price calculation is invalid syntax.

B. The RETURNING clause is placed before SET, causing a syntax error.

C. The WHERE clause is missing.

D. RETURNING cannot be used with UPDATE.

Solution

1. Step 1: Check clause order in UPDATE

   UPDATE syntax requires SET before WHERE and RETURNING last.

2. Step 2: Identify error in query

   RETURNING appears before SET, which is invalid and causes syntax error.

3. Final Answer:

   The RETURNING clause is placed before SET, causing a syntax error. -> Option B

4. Quick Check:

   RETURNING must come after WHERE and SET [OK]

Hint: RETURNING always comes last after WHERE [OK]

Common Mistakes:

• Placing RETURNING before SET
• Omitting WHERE clause when needed
• Misordering clauses in UPDATE

5. You want to increase the salary by 10% for employees in the 'Engineering' department and immediately get their id, old_salary, and new_salary. Which query correctly does this using UPDATE with RETURNING?

hard

A. UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary, salary / 1.1 AS old_salary;

B. UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary, salary * 1.1 AS old_salary;

C. UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary AS old_salary, salary * 1.1 AS new_salary;

D. UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary AS new_salary, salary / 1.1 AS old_salary;

Solution

1. Step 1: Understand the salary update

   Salary is multiplied by 1.1 to increase by 10%, so new salary is salary * 1.1.

2. Step 2: Calculate old salary in RETURNING

   After update, salary column shows new salary. To get old salary, divide new salary by 1.1.

3. Step 3: Check aliasing in RETURNING

   RETURNING must show id, new_salary (salary), and old_salary (salary / 1.1) with correct aliases.

4. Final Answer:

   UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary AS new_salary, salary / 1.1 AS old_salary; -> Option D

5. Quick Check:

   Old salary = new salary / 1.1 [OK]

Hint: Use RETURNING with expressions and aliases for old and new values [OK]

Common Mistakes:

• Confusing old and new salary calculations
• Wrong aliasing in RETURNING clause
• Using expressions incorrectly in RETURNING