JuniorJunior (1–3 yrs)CodingSQLAmazonShopify

N+1 Query Problem

What is the N+1 query problem in ORMs, and how do you fix it?

Answers use simple, clear English.

Quick interview answer

Loading a list of N parents then querying children per parent yields 1+N queries. Fix with eager loading/joins, batch IN queries, or DataLoader-style batching. Measure with query logs.

Detailed answer

Loading a list of N parents then querying children per parent yields 1+N queries. Fix with eager loading/joins, batch IN queries, or DataLoader-style batching. Measure with query logs.

Real example & use case

Blog API listing posts with comments drops from 101 queries to 1–2 after join/include.

Pros & cons

Pros of eager load: fewer round-trips. Cons: over-fetching wide joins — select only needed columns.

Code example

-- Bad: 1 + N
SELECT * FROM posts;
-- then per post: SELECT * FROM comments WHERE post_id = ?;

-- Better:
SELECT p.*, c.*
FROM posts p
LEFT JOIN comments c ON c.post_id = p.id
WHERE p.author_id = @author;

Practice code · sql (view only · no execution)

-- Bad: 1 + N
SELECT * FROM posts;
-- then per post: SELECT * FROM comments WHERE post_id = ?;

-- Better:
SELECT p.*, c.*
FROM posts p
LEFT JOIN comments c ON c.post_id = p.id
WHERE p.author_id = @author;

#orm#performance#sql