CTE with INSERT, UPDATE, DELETE (writable CTEs) in PostgreSQL - Time & Space Complexity

Time Complexity: CTE with INSERT, UPDATE, DELETE (writable CTEs)

O(n)

Understanding Time Complexity

When using writable CTEs in PostgreSQL, we want to know how the time to run the query changes as the data grows.

We ask: How does the number of rows affect the work done by INSERT, UPDATE, or DELETE inside a CTE?

Scenario Under Consideration

Analyze the time complexity of this writable CTE example.

WITH updated_rows AS (
  UPDATE employees
  SET salary = salary * 1.1
  WHERE department = 'Sales'
  RETURNING *
)
DELETE FROM employees
WHERE id IN (SELECT id FROM updated_rows WHERE salary > 100000);

This code first increases salaries for Sales employees, then deletes those whose salary exceeds 100,000.

Identify Repeating Operations

Look for repeated work inside the query.

• Primary operation: Scanning and updating rows in the employees table matching the department condition.
• How many times: Each matching row is processed once during UPDATE, then some rows are checked again for DELETE.

How Execution Grows With Input

As the number of employees grows, the work grows roughly in proportion.

Input Size (n)	Approx. Operations
10	About 10 rows updated and checked
100	About 100 rows updated and checked
1000	About 1000 rows updated and checked

Pattern observation: The operations grow linearly with the number of rows affected.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the query grows roughly in direct proportion to the number of rows involved.

Common Mistake

[X] Wrong: "The DELETE inside the CTE runs separately and doubles the work for all rows."

[OK] Correct: The DELETE only processes rows returned by the UPDATE, so it works on a smaller subset, not the whole table again.

Interview Connect

Understanding how writable CTEs scale helps you explain query performance clearly and shows you can reason about real database operations.

Self-Check

"What if the DELETE condition was changed to remove rows based on a different column not related to the UPDATE? How would that affect time complexity?"

Practice

1. What is the main purpose of using writable CTEs in PostgreSQL?

easy

A. To speed up SELECT queries by caching results

B. To create temporary tables that last beyond the current transaction

C. To perform INSERT, UPDATE, or DELETE operations inside a WITH clause

D. To define functions that return multiple rows

Solution

1. Step 1: Understand writable CTEs

   Writable CTEs allow data modification commands like INSERT, UPDATE, or DELETE inside a WITH clause.

2. Step 2: Compare options

   Options B, C, and D describe other features unrelated to writable CTEs.

3. Final Answer:

   To perform INSERT, UPDATE, or DELETE operations inside a WITH clause -> Option C

4. Quick Check:

   Writable CTEs = Data modification in WITH [OK]

Hint: Writable CTEs = data changes inside WITH clause [OK]

Common Mistakes:

• Confusing writable CTEs with temporary tables
• Thinking writable CTEs speed up SELECT queries
• Mixing writable CTEs with function definitions

2. Which of the following is the correct syntax to update rows using a writable CTE in PostgreSQL?

easy

A. UPDATE employees WITH (salary = salary * 1.1) RETURNING *;

B. WITH updated AS (UPDATE employees SET salary = salary * 1.1) RETURNING *;

C. WITH updated AS (SELECT * FROM employees WHERE salary > 5000) UPDATE updated SET salary = salary * 1.1;

D. WITH updated AS (UPDATE employees SET salary = salary * 1.1 RETURNING *) SELECT * FROM updated;

Solution

1. Step 1: Review writable CTE update syntax

   The correct syntax uses WITH updated AS (UPDATE ... RETURNING *) followed by a SELECT from updated.

2. Step 2: Analyze options

   WITH updated AS (UPDATE employees SET salary = salary * 1.1 RETURNING *) SELECT * FROM updated; matches the correct syntax. Options A, C, and D have syntax errors or misuse RETURNING.

3. Final Answer:

   WITH updated AS (UPDATE employees SET salary = salary * 1.1 RETURNING *) SELECT * FROM updated; -> Option D

4. Quick Check:

   Writable CTE update = WITH + UPDATE + RETURNING + SELECT [OK]

Hint: Use WITH (UPDATE ... RETURNING *) then SELECT [OK]

Common Mistakes:

• Omitting RETURNING clause in writable CTE
• Trying to UPDATE inside WITH without SELECT
• Misplacing RETURNING outside the CTE

3. Given the table products(id, name, stock) with rows: (1, 'Pen', 10), (2, 'Pencil', 5), what will be the result of this query?

WITH updated AS (UPDATE products SET stock = stock + 10 WHERE name = 'Pen' RETURNING *) SELECT * FROM updated;

medium

A. [{id:1, name:'Pen', stock:20}]

B. [{id:1, name:'Pen', stock:10}]

C. [{id:2, name:'Pencil', stock:15}]

D. Empty result set

Solution

1. Step 1: Identify updated rows

   The UPDATE increases stock by 10 only for product with name 'Pen' (id=1), changing stock from 10 to 20.

2. Step 2: Understand RETURNING and SELECT

   The RETURNING * returns the updated row, and the SELECT outputs it.

3. Final Answer:

   [{id:1, name:'Pen', stock:20}] -> Option A

4. Quick Check:

   Updated Pen stock = 10 + 10 = 20 [OK]

Hint: RETURNING shows updated rows immediately [OK]

Common Mistakes:

• Assuming all rows are updated
• Ignoring RETURNING clause output
• Confusing stock before and after update

4. Identify why this writable CTE query returns an empty result set even if matching rows exist:

WITH deleted AS (DELETE FROM orders WHERE status = 'cancelled') SELECT * FROM deleted;

medium

A. The WHERE clause syntax is incorrect

B. Missing RETURNING clause in DELETE statement

C. SELECT * FROM deleted should be SELECT COUNT(*)

D. DELETE cannot be used inside WITH clause

Solution

1. Step 1: Check DELETE in writable CTE

   DELETE inside WITH must include RETURNING to produce rows for the outer SELECT.

2. Step 2: Analyze the query

   The query lacks RETURNING, so 'deleted' has no rows to select from, resulting in an empty result set.

3. Final Answer:

   Missing RETURNING clause in DELETE statement -> Option B

4. Quick Check:

   Writable CTE DELETE needs RETURNING [OK]

Hint: Always add RETURNING with DELETE in writable CTEs [OK]

Common Mistakes:

• Omitting RETURNING after DELETE in CTE
• Thinking DELETE can't be in WITH clause
• Misunderstanding SELECT after writable CTE

5. You want to insert new users and immediately update their status to 'active' using writable CTEs. Which query correctly does this in one statement?

hard

A. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob') RETURNING id, name) UPDATE users SET status = 'active' FROM ins WHERE users.id = ins.id RETURNING users.*;

B. INSERT INTO users(name) VALUES ('Alice'), ('Bob'); WITH upd AS (UPDATE users SET status = 'active' WHERE name IN ('Alice', 'Bob') RETURNING *) SELECT * FROM upd;

C. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob')) UPDATE users SET status = 'active' WHERE id IN (SELECT id FROM ins);

D. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob')) UPDATE ins SET status = 'active' WHERE name IN ('Alice', 'Bob');

Solution

1. Step 1: Insert and capture new rows

   Use a writable CTE to insert and RETURNING id, name to identify new users.

2. Step 2: Update using CTE results

   Update users joining with inserted rows on id, setting status to 'active', and RETURNING updated rows.

3. Step 3: Analyze options

   WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob') RETURNING id, name) UPDATE users SET status = 'active' FROM ins WHERE users.id = ins.id RETURNING users.*; correctly chains insert and update in one statement with proper RETURNING and join. INSERT INTO users(name) VALUES ('Alice'), ('Bob'); WITH upd AS (UPDATE users SET status = 'active' WHERE name IN ('Alice', 'Bob') RETURNING *) SELECT * FROM upd; separates insert and update into multiple statements. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob')) UPDATE users SET status = 'active' WHERE id IN (SELECT id FROM ins); lacks RETURNING so CTE returns no rows and subquery updates nothing. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob')) UPDATE ins SET status = 'active' WHERE name IN ('Alice', 'Bob'); incorrectly attempts to UPDATE the CTE which is invalid.

4. Final Answer:

   WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob') RETURNING id, name) UPDATE users SET status = 'active' FROM ins WHERE users.id = ins.id RETURNING users.*; -> Option A

5. Quick Check:

   Writable CTE insert + update with join = WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob') RETURNING id, name) UPDATE users SET status = 'active' FROM ins WHERE users.id = ins.id RETURNING users.*; [OK]

Hint: Chain INSERT and UPDATE with RETURNING and FROM join [OK]

Common Mistakes:

• Omitting RETURNING clause before using CTE in subquery
• Splitting insert and update into separate statements
• Omitting RETURNING to get updated rows