ASP.NET Core MVC Interview Question #146

What is the N+1 query problem and why can it hurt an MVC page?

Dependency Injection, Services, Error Handling, Logging & Performance Senior Advanced

Quick Interview Answer

The N+1 problem occurs when an application executes one query to load a collection and then performs additional queries for individual items or related data. A page containing many rows can therefore trigger a large number of database round trips.

Detailed Explanation

Prevent it by designing the data query for the page, projecting only required fields, loading necessary relationships appropriately, and inspecting generated SQL and query counts when diagnosing performance.

Code Example

var products = await db.Products
    .AsNoTracking()
    .Select(p => new ProductListItem
    {
        Id = p.Id,
        Name = p.Name,
        CategoryName = p.Category.Name
    })
    .ToListAsync(token);