Query in loops

Rule

Placing expensive operations like SQL queries or system calls inside loops leads to significant performance issues due to repeated and unnecessary executions. This can cause:

• Excessive load on the database or external systems

• Increased latency and slower performance

• Hard-to-detect scalability bottlenecks

Examples

❌ Bad Practice:

foreach ($users as $user) {
    $orders = $db->query("SELECT * FROM orders WHERE user_id = {$user->id}"); // BAD
    process($orders);
}

✅ Good Practice:

$userIds = array_map(fn($u) => $u->id, $users);
$allOrders = $db->query("SELECT * FROM orders WHERE user_id IN (" . implode(',', $userIds) . ")");

foreach ($users as $user) {
    $orders = filterOrdersForUser($allOrders, $user->id);
    process($orders);
}