N+1 Query Problem

Also Known As

TL;DR

Explanation

The N+1 problem occurs when code fetches a list of entities and then executes an additional query for each one — typically to load a related record. For 100 rows this means 101 queries; for 1000 rows, 1001. Database round-trip overhead makes this disproportionately slow compared to a single JOIN or a single IN() query. The fix is to pre-load all related data in one query and look it up from a keyed array in memory.

Diagram

sequenceDiagram
    participant APP as Application
    participant DB as Database
    APP->>DB: SELECT * FROM posts - 1 query
    DB-->>APP: 100 posts returned
    loop For each of 100 posts
        APP->>DB: SELECT * FROM users WHERE id = ?
        DB-->>APP: 1 user
    end
    Note over APP,DB: 1 + 100 = 101 queries total
    Note over APP,DB: Fix: eager load with JOIN<br/>SELECT * FROM users WHERE id IN (1,2,3...)

Common Misconception

Why It Matters

Common Mistakes

• Accessing a relationship inside a loop without eager loading — ORM lazy-loads on every iteration.
• Adding with() to the outer query but forgetting nested relationships (with('orders.items') instead of with('orders')).
• Fixing N+1 in controllers but leaving it in API resources or view composers that loop over collections.
• Not installing a query debugger (Laravel Debugbar, DBAL logger) — N+1 is invisible without query counting.

Code Examples

// N+1: 1 query for posts + N queries for author names
$posts = Post::all();
foreach ($posts as $post) {
    echo $post->author->name; // triggers a new query per post
}

$posts = Post::with('author')->get(); // 2 queries total
foreach ($posts as $post) {
    echo $post->author->name;
}

References

• ↗ https://www.php.net/manual/en/pdo.prepare.php

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