EF Core generates non-optimized SQL for .Include().First() queries

Pomelo is not correctly generating SQL code. The following code produces the following SQL:

SELECT 't'.'UserId', 't'.'Username', 'o'.'OrderId', 'o'.'Total', 'o'.'UserId'
FROM (
    SELECT 'u'.'UserId', 'u'.'Username'
    FROM 'Users' AS 'u'
    WHERE 'u'.'UserId' = 1
    LIMIT 1
) AS 't'
LEFT JOIN 'Orders' AS 'o' ON 't'.'UserId' = 'o'.'UserId'
ORDER BY 't'.'UserId'

There is absolutely no reason that a second select statement is being generated. It is redundant and as far as I am aware it is a performance problem. It should be generating the following SQL code:

SELECT u.userid, u.username, o.orderid, o.total, o.userid 
FROM users AS u 
JOIN orders AS o ON o.userid = u.userid
WHERE u.userid = 1

C# Dummy code

namespace LearningCSharp1
{
    class Program
    {
        public static async Task Main(string[] args)
        {
            var context = new Context();

            var user = await context.Users
                .Where(u => u.UserId == 1)
                .Include(u => u.Order)
                .FirstAsync();
        }
    }

    class User
    {
        public int UserId { get; set; }
        public string Username { get; set; } = null!;
        public IEnumerable<Order> Order { get; set; } = new List<Order>();

        public override string ToString()
        {
            return $"Id: {UserId} | Username: {Username}";
        }
    }

    class Order
    {
        public int OrderId { get; set; }
        public decimal Total { get; set; }
        public User User { get; set; } = null!;
        public int UserId { get; set; }
    }

    class Context : DbContext
    {
        public DbSet<User> Users => Set<User>();
        public DbSet<Order> Orders => Set<Order>();

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder
                .UseMySql(
                    "Server=localhost;Database=efcore;User=root;Password=password.",
                    ServerVersion.AutoDetect("Server=localhost;Database=efcore;User=root;Password=password.")
                )
                .UseQueryTrackingBehavior(QueryTrackingBehavior.NoTracking)
                .LogTo(Console.WriteLine, LogLevel.Information)
                .EnableSensitiveDataLogging();
        }
    }
}

Further technical details

Pomelo.EntityFrameworkCore.MySql version: 6.0.1 Microsoft.AspNetCore.App version: 6.0.6

[lauxjpn]

Now I realized why perf numbers are that way @lauxjpn . Both are running different SQL which generates different results. MultiSelect version takes first User and associated orders which can be one or more so result set could have 5 rows too. SingleSelect is doing Limit 1 on top level so it will only take first User and first matching order and discard other orders if they are matching and should be included in the result when populating Orders collection via EF. Hence it is only selecting 1 row and not 5. Hence results makes sense, why SingleSelect is faster.

@smitpatel Good catch, that’s it! And that is why the statement needs to be generated by EF Core the way it already currently is, since otherwise only the first Order entity would be returned, which would be an incorrect translation of the LINQ query.

So looks good and works as expected.

[smitpatel]

Why do both single and first produce the same SQL?

They don’t produce same SQL either. The LIMIT in both queries is different number.

Now I realized why perf numbers are that way @lauxjpn . Both are running different SQL which generates different results. MultiSelect version takes first User and associated orders which can be one or more so result set could have 5 rows too. SingleSelect is doing Limit 1 on top level so it will only take first User and first matching order and discard other orders if they are matching and should be included in the result when populating Orders collection via EF. Hence it is only selecting 1 row and not 5. Hence results makes sense, why SingleSelect is faster.