What is the N+1 query problem and how do you solve it in Eloquent?

// N+1 PROBLEM: 1 + N queries (disaster at scale)
$users = User::all(); // 1 query
foreach ($users as $user) {
    echo $user->posts->count(); // N queries! One per user
}
      

// SOLUTION: 2 queries total (scales perfectly)
$users = User::with('posts')->get(); // 1 query users + 1 query all posts
foreach ($users as $user) {
    echo $user->posts->count(); // 0 queries - pre-loaded!
}
      

// Multiple relationships
$users = User::with(['posts', 'profile', 'roles'])->get(); // 4 queries total

// Nested (posts + each post's comments)
$users = User::with('posts.comments')->get(); // Users + Posts + Comments (3 queries)

// Constrained eager loading
$users = User::with(['posts' => function ($query) {
    $query->where('published', true)->latest();
}])->get();
      

// Already have $users collection? Load later:
$users = User::all(); // 1 query
$users->load('posts'); // 1 more query - now eager loaded!
foreach ($users as $user) {
    echo $user->posts->count(); // 0 queries
}
      

// Just need COUNT? Avoid loading full relation:
$users = User::withCount('posts')->get();
foreach ($users as $user) {
    echo $user->posts_count; // 0 queries - just integer!
}