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rust.n_plus_one

Performance High

Detects N+1 query patterns where database queries are executed inside loops.

Why It Matters

N+1 queries:

Kill performance — 100 items = 101 queries
Overwhelm databases — Connection pool exhaustion
Increase latency — Linear time complexity

Example

// ❌ Before (N+1 query pattern)
async fn get_users_with_orders(db: &Pool) -> Vec<UserWithOrders> {
    let users = sqlx::query_as!(User, "SELECT * FROM users")
        .fetch_all(db).await?;

    let mut result = Vec::new();
    for user in users {
        let orders = sqlx::query_as!(Order,
            "SELECT * FROM orders WHERE user_id = ?", user.id)
            .fetch_all(db).await?; // Query per user!
        result.push(UserWithOrders { user, orders });
    }
    result
}

// ✅ After (batch query with JOIN)
async fn get_users_with_orders(db: &Pool) -> Vec<UserWithOrders> {
    let rows = sqlx::query!(
        "SELECT u.*, o.* FROM users u
         LEFT JOIN orders o ON o.user_id = u.id")
        .fetch_all(db).await?;

    // Group by user
    rows.into_iter()
        .group_by(|r| r.user_id)
        .map(|(_, group)| /* build UserWithOrders */)
        .collect()
}

What Unfault Detects

SQLx queries inside loops
Diesel queries per iteration
Missing eager loading patterns

Auto-Fix

Unfault suggests JOIN queries or batch fetching strategies.