Query execution time in Entity Framework vs in SQL Server

Question

I have a query written in Linq To Entities:

db.Table<Operation>()
    .Where(x => x.Date >= dateStart)
    .Where(x => x.Date < dateEnd)
    .GroupBy(x => new
    {
        x.EntityId,
        x.EntityName,
        x.EntityToken
    })
    .Select(x => new EntityBrief
    {
        EntityId = x.Key.EntityId,
        EntityName = x.Key.EntityName,
        EntityToken = x.Key.EntityToken,
        Quantity = x.Count()
    })
    .OrderByDescending(x => x.Quantity)
    .Take(5)
    .ToList();

The problem is that it takes 4 seconds when executing in the application using EF. But when I take the created pure SQL Query from that query object (using Log) and fire it directly on SQL Server, then it takes 0 seconds. Is it a known problem?

Answer 1

Firstly, try improving your query:

var entityBriefs = 
  Table<Operation>().Where(x => x.Date >= dateStart && x.Date < dateEnd)
                    .GroupBy(x => x.EntityId)
                    .OrderByDescending(x => x.Count())
                    .Take(5)
                    .Select(x => new EntityBrief
                    {
                        EntityId = x.Key.EntityId, 
                        Quantity = x.Count()
                    });

var c = entityBriefs.ToDictionary(e => e.EntityId, e => e);

var entityInfo = Table<Operation>().Where(o => mapping.Keys.Contains(o.EntityId).ToList();

foreach(var entity in entityInfo)
{
   mapping[entity.EntityId].EntityName = entity.EntityName;
   mapping[entity.EntityId].EntityToken = entity.EntityToken;
}

You may also compile queries with the help of CompiledQuery.Compile , and use it further with improved performance.

http://msdn.microsoft.com/en-us/library/bb399335%28v=vs.110%29.aspx

Answer 2

The problem was with the database locks. I used wrong isolation level, so my queries were blocked under some circumstances. Now I use read-commited-snapshot and the execution time looks good.