EntityFramework LINQ query count fails but query returns result. How to optimize LINQ query?

Question

I have the below LINQ query that performs a self-left-outer-join. The querys looks a little complex but is simply doing a self join on itself(purpose if to join each record with

Answer 1

Not quite sure that's the problem, but at least let try to eliminate the potential effect of the so called Parameter Sniffing Problem by eliminating the dateStart / dateEnd parameters by manually building expression with constant values.

First, a little helper method:

using System;
using System.Linq;
using System.Linq.Expressions;

public static class QueryableUtils
{
    public static IQueryable<T> WhereBetween<T>(this IQueryable<T> source, Expression<Func<T, DateTime>> dateSelector, DateTime? startDate, DateTime? endDate)
    {
        if (startDate == null && endDate == null) return source;
        var startCond = startDate != null ? Expression.GreaterThanOrEqual(dateSelector.Body, Expression.Constant(startDate.Value)) : null;
        var endCond = endDate != null ? Expression.LessThanOrEqual(dateSelector.Body, Expression.Constant(endDate.Value)) : null;
        var predicate = Expression.Lambda<Func<T, bool>>(
            startCond == null ? endCond : endCond == null ? startCond : Expression.AndAlso(startCond, endCond),
            dateSelector.Parameters[0]);
        return source.Where(predicate);
    }
}

Then try the following and see if it helps:

//if (dateStart.HasValue)
//    query = query.Where(e => e.outer.ValueDate >= dateStart.Value);
//if (dateEnd.HasValue)
//    query = query.Where(e => e.outer.ValueDate <= dateEnd.Value);
query = query.WhereBetween(e => e.outer.ValueDate, dateStart, dateEnd);

Answer 2

You can simply use AsParallel() to enable multithreading execution of the linq query. You should then check your tables' indexes to improve performances.