Can't use DB indexes with Entity Framework using where clauses

Question

We are using entity framework to work with a table with 50 million entries (at a Microsoft SQL Server).

public async Task<List<TableName>> MyMethod(int fooId, int count)
{
    using (var context = new Context(ConnectionString))
    {
        return
            await context.TableName.AsNoTracking()
                .Where(_ => _.FooId== fooId)
                .OrderByDescending(_ => _.DateCreated)
                .Take(count)
                .ToListAsync();
    }
}

Entity Framework translates this to (beautified):

declare @fooId int = 42
SELECT TOP (100) *
FROM TableName
WHERE FooId = @fooId
ORDER BY DateCreated DESC

The columns FooId and DateCreated both got an index, but SQL Server does a full table scan anyway, which takes very long. This is caused because 42 is assigned outside of the statement (and seems to be related to selectivity). The indexes would be used if you wrote

WHERE FooId = 42

Is there a way to get Entity Framework to optimize the generated query? At the moment, my only way seems to be using raw SQL queries for huge tables in my code :-/ Is there a better workaround?

Edit: More details as requested in the comments: Non beautified query that is generated by entity framework:

SELECT TOP (100) 
    [Project1].[DateCreated] AS [DateCreated], 
    [Project1].[FooId] AS [FooId]
    FROM ( SELECT 
        [Extent1].[DateCreated] AS [DateCreated], 
        [Extent1].[FooId] AS [FooId]
        FROM [dbo].[TableName] AS [Extent1]
        WHERE [Extent1].[FooId] = @p__linq__0
    )  AS [Project1]
    ORDER BY [Project1].[DateCreated] DESC

-- p__linq__0: '42' (Type = Int32, IsNullable = false) 

The create script for the index:

CREATE NONCLUSTERED INDEX [IX_TableName_FooId] ON [dbo].[TableName]
(
    [FooId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 85) ON [SECONDARY]
GO

Create script for my table:

CREATE TABLE [dbo].[TableName](
    [DateCreated] [datetime] NOT NULL,
    [FooId] [int] NULL
) ON [PRIMARY]

Answer 1

It's a problem with parameter sniffing.

Your query will be executed by the stored procedure which gets an input parameter:

EXEC sp_executesql N'SELECT TOP (100) *
FROM TableName
WHERE FooId = = @p__linq__0
ORDER BY DateCreated DESC', ' @p__linq__0 int', @p__linq__0 = 42

You should change the query to change the query parameter to a local variable:

EXEC sp_executesql N'
    DECLARE @pp__linq__0 int = @p__linq__0
    SELECT TOP (100) *
    FROM TableName
    WHERE FooId = = @pp__linq__0
    ORDER BY DateCreated DESC', ' @p__linq__0 int', @p__linq__0 = 42

For example our solution for this issue:

public class ParamPositioningInterceptor : DbCommandInterceptor
{
    public override void ReaderExecuting(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
    {
        if (command.CommandText.StartsWith("SELECT") && command.Parameters.Count > 0)
        {
            StringBuilder sb1 = new StringBuilder(command.CommandText);
            StringBuilder sb2 = new StringBuilder();

            SqlParameter[] array = new SqlParameter[command.Parameters.Count];
            command.Parameters.CopyTo(array, 0);

            foreach (SqlParameter p in array.OrderByDescending(x => x.ParameterName.Length))
            {
                sb1.Replace("@" + p, "@p" + p);

                switch (p.SqlDbType)
                {
                    case SqlDbType.Char:
                    case SqlDbType.VarChar:
                    case SqlDbType.NChar:
                    case SqlDbType.NVarChar:
                        sb2.AppendFormat("DECLARE @p{0} {1}({2}) = @{0}", p, p.SqlDbType, p.Size);
                        break;
                    case SqlDbType.Decimal:
                        sb2.AppendFormat("DECLARE @p{0} {1}({2},{3}) = @{0}", p, p.SqlDbType, p.Precision, p.Scale);
                        break;
                    default:
                        sb2.AppendFormat("DECLARE @p{0} {1} = @{0}", p, p.SqlDbType);
                        break;
                }

                sb2.AppendLine();
            }

            command.CommandText = sb2.Append(sb1).ToString();
        }
        //
        base.ReaderExecuting(command, interceptionContext);
    }
}

Answer 2

If your code is executed exactly like this:

declare @fooId int = 42
SELECT TOP (100) *
FROM TableName
WHERE FooId = @fooId
ORDER BY DateCreated DESC

i.e. has no parameters but local variables only, the estimated number of rows is C^1/2 (C = table cardinality) with non unique value. It will imply full scan.

You can try to pass the real value by using option(recompile).

This will cause your statement to be recompiled when the variable is already assigned, i.e. execution plan will consider a value that you effectivly passed.

To see what is actually executed and what was the estimation and real number of rows you should provide the actual execution plan.