---
title: "N+1 query"
slug: "n-plus-one-query"
category: "glossary"
tags: ["glossary", "backend", "database", "orm", "performance", "sql"]
status: "stable"
last_updated: 2026-10-01
summary: "An N+1 query problem is one query for a list plus one extra query per row for related data, so cost grows with list size."
related: ["[[glossary/foreign-key]]", "[[glossary/query-plan]]", "[[glossary/secondary-index]]", "[[backend/prisma-client]]", "[[backend/postgres]]"]
---

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## Overview

An N+1 query runs one query for a list of N rows and then one query per row for related data: a page showing 100 orders issues 101 queries. It hides behind ORM lazy loading and GraphQL resolvers. Detect it by counting queries per request in development, using query logs or `pg_stat_statements` call counts, and fix it before production data sizes expose it.

## Rules

- Load related rows in bulk: a join, the ORM's eager loading (Django `select_related` and `prefetch_related`, SQLAlchemy `selectinload`, Prisma `include`), or one batched `WHERE id = ANY(...)` query.
- In GraphQL, batch per request with a DataLoader.
- A join across a one-to-many relation multiplies rows and can cost more than a second batched query. Compare plans with `EXPLAIN` ([[glossary/query-plan]]).
- Index the foreign-key column used for the lookup ([[glossary/foreign-key]]).

```sql
SELECT id FROM orders LIMIT 100;
SELECT * FROM order_items WHERE order_id = $1;              -- N+1: repeated 100 times
SELECT * FROM order_items WHERE order_id = ANY($1::bigint[]); -- batched: one query for all 100
```

## Related

- [[glossary/foreign-key]]: the relationship being loaded.
- [[glossary/query-plan]]: compare join and batched strategies.
- [[glossary/secondary-index]]: index the lookup column.
- [[backend/prisma-client]]: relation loading in Prisma.
- [[backend/postgres]]: query tuning.