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N+1 problem

Learn the N+1 query trap.

N+1 problem

The N+1 problem is one query returning N rows, then a separate query per row. It is slow.

-- YANLIŞ: her sipariş için ayrı müşteri sorgusu
-- DOĞRU: JOIN ile tek sorgu
SELECT o.id, c.name FROM orders o JOIN customers c ON o.customer_id = c.id;

JOIN solves N+1.

o.id, c.name is selected from orders o. customers c is joined on o.customer_id = c.id, keeping only matching rows.

N+1 problem — details

The N+1 problem is common in ORMs. It is solved with JOIN or eager loading.

For example, listing 100 orders and fetching each customer with a separate query means 101 queries; a single JOIN returns the same data in one query.

Practice N+1 problem

Solve the N+1 problem with JOIN: combine orders and customers in one query.

Starter code

query.sql
-- solve N+1
SELECT o.id, c.name FROM orders o

Sample solution

query.sql
SELECT o.id, c.name FROM orders o JOIN customers c ON o.customer_id = c.id;

Expected output

id  name
1   Ayşe
2   Mehmet

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Add JOIN

Question 
SELECT o.id, c.name FROM orders o ____ customers c ON o.customer_id = c.id
Answer 
SELECT o.id, c.name FROM orders o JOIN customers c ON o.customer_id = c.id

Explanation: JOIN combines orders and customers on customer_id; an indexed column speeds up the plan.

2 N+1 problem: match the terms

Term Meaning
N+1 many queries
JOIN single query
slow problem
ORM common

3 What solves the N+1 problem?

  • JOIN Correct answer
  • DISTINCT
  • INDEX
  • VIEW

Explanation: JOIN solves N+1.

All lessons in Advanced

1 CTE

  1. What is a CTE?
  2. Using WITH
  3. Multiple CTEs
  4. CTE vs subquery
  5. CTE with JOIN
  6. Summary - CTE

2 Recursive CTE

  1. What is a recursive CTE?
  2. Anchor and recursive parts
  3. Walk a hierarchy
  4. Tracking depth
  5. Stopping recursion
  6. Summary - Recursive CTE

3 Window functions

  1. What is a window function?
  2. ROW_NUMBER
  3. RANK and DENSE_RANK
  4. LAG and LEAD
  5. PARTITION BY
  6. Summary - Window functions

4 Project: Analytics report

  1. Analytics design
  2. CTE summary
  3. Top users with ROW_NUMBER
  4. Growth with LAG
  5. Final report

5 Pivot and unpivot

  1. What is pivot?
  2. Pivot with CASE
  3. FILTER clause
  4. Unpivot
  5. Dynamic pivot
  6. Summary - Pivot

6 Materialized view

  1. What is a materialized view?
  2. CREATE
  3. REFRESH
  4. View vs materialized view
  5. Index on materialized views
  6. Summary - Materialized view

7 Stored procedure

  1. What is a procedure?
  2. CREATE PROCEDURE
  3. Parameters
  4. CALL
  5. Control flow
  6. Summary - Stored procedure

8 Function

  1. What is a function?
  2. CREATE FUNCTION
  3. RETURN
  4. Calling a function
  5. Function vs procedure
  6. Summary - Function

9 Trigger

  1. What is a trigger?
  2. BEFORE and AFTER
  3. INSERT trigger
  4. UPDATE trigger
  5. Write an audit trigger
  6. Summary - Trigger

10 Transaction isolation

  1. What is isolation?
  2. READ UNCOMMITTED
  3. READ COMMITTED
  4. REPEATABLE READ
  5. SERIALIZABLE
  6. Summary - Isolation

11 Locking

  1. What is a lock?
  2. Shared and exclusive locks
  3. Deadlock
  4. SELECT FOR UPDATE
  5. Lock timeout
  6. Summary - Locking

12 Query optimization

  1. What is EXPLAIN?
  2. Seq scan vs index scan
  3. Cost
  4. Sargable
  5. N+1 problem
  6. Summary - Optimization

13 Partitioning

  1. What is partitioning?
  2. Range partition
  3. List partition
  4. Hash partition
  5. Partition pruning
  6. Summary - Partitioning

14 Advanced final

  1. Hierarchy report
  2. Window analysis
  3. Procedure and trigger
  4. Optimization