here is my code: var mainQuery = (IQueryable<dynamic>)SortedAndPaged().Select(selector); var aggregateQuery = Aggregate();

FutureScalar<dynamic> futureAggregate;
dynamic[] futureMain;

try
{
    futureAggregate = Query.ExecuteFutureScalar(key, () => aggregateQuery.FirstOrDefault());
    futureMain = mainQuery.ToArray();
}
catch (StorageException e)
{
    if (e.InnerException != null && typeof(SqlException) == e.InnerException.GetType() && e.InnerException.Message.Contains("Arithmetic overflow error"))
    {
        success = false;
        message = "Ошибка аггрегирования";
        Session.Current.Persist();
        AggregateInfo.Clear();
        futureAggregate = Query.ExecuteFutureScalar(key, () => aggregateQuery.FirstOrDefault());
        futureMain = mainQuery.ToArray();
    }
    else
    {
        throw;
    }
}

I try to execute two queries in one batch, one for tabular data, another - for Summary (aggregates) and if I catch overflow exception - exclude this aggregates, and try to execute it one more time, but:

There is already an open DataReader associated with this Command which must be closed first.

[InvalidOperationException: There is already an open DataReader associated with this Command which must be closed first.]
System.Data.SqlClient.SqlInternalConnectionTds.ValidateConnectionForExecute(SqlCommand command) +5008639
System.Data.SqlClient.SqlConnection.ValidateConnectionForExecute(String method, SqlCommand command) +23
System.Data.SqlClient.SqlCommand.ValidateCommand(String method, Boolean async) +144
System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result) +87
System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method) +32
System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method) +141
System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior) +12
System.Data.Common.DbCommand.ExecuteReader() +12
Xtensive.Storage.Providers.Sql.Driver.ExecuteReader(Session session, DbCommand command) +46

[StorageException: Error 'Unknown' while executing query 'SELECT TOP 1 [a].[column1], ...'. Original message: There is already an open DataReader associated with this Command which must be closed first.]
Xtensive.Storage.Providers.Sql.Driver.ExecuteReader(Session session, DbCommand command) +81
Xtensive.Storage.Providers.Sql.BatchingCommandProcessor.ExecuteBatch(Int32 numberOfTasks, QueryRequest lastRequest) +282
Xtensive.Storage.Providers.Sql.BatchingCommandProcessor.ExecuteRequestsWithReader(QueryRequest request) +50
Xtensive.Storage.Providers.Sql.ExecuteTupleReader>d__1.MoveNext() +121
Xtensive.Storage.Providers.Sql.<OnEnumerate>d__0.MoveNext() +151
Xtensive.Storage.Rse.Providers.<GetEnumerator>d__0.MoveNext() +206
System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +327
System.Linq.Enumerable.ToList(IEnumerable`1 source) +58
Xtensive.Storage.Rse.<GetEnumerator>d__4.MoveNext() +219
System.Linq.WhereSelectEnumerableIterator`2.MoveNext() +87
Xtensive.Core.<Batch>d__20`1.MoveNext() +194
Xtensive.Core.<ApplyBeforeAndAfter>d__28`1.MoveNext() +154
Xtensive.Storage.<ToTransactional>d__0`1.MoveNext() +193
System.Linq.<SelectManyIterator>d__14`2.MoveNext() +399
System.Linq.Buffer`1..ctor(IEnumerable`1 source) +217
System.Linq.Enumerable.ToArray(IEnumerable`1 source) +78
Werp.Controller.Services.GridReadService`1.GetObject() in D:\work\platform\Werp.Controller.Services\GridReadService.cs:94
Werp.View.Ext.Handlers.ReadHandler.ProcessRequest(HttpContext context) in D:\work\platform\Werp.View.Ext\Handlers\ReadHandler.ashx.cs:166
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +100
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

simple example to reproduce bug: entity:

[Serializable]
[HierarchyRoot]
public class MyEntity : Entity
{
    [Field, Key]
    public int Id { get; private set; }

    [Field]
    public int Number { get; set; }
}

and code:

using (Session.Open(domain))
{
    using (var transactionScope = Transaction.Open())
    {
        var helloWorld = new MyEntity { Number = int.MaxValue };
        var helloWorld2 = new MyEntity { Number = int.MaxValue };
        transactionScope.Complete();
    }

    using (var transactionScope = Transaction.Open())
    {
        FutureScalar<int> futureAggregate;
        MyEntity[] query;
        try
        {
            futureAggregate = Query.ExecuteFutureScalar(() => Query.All<MyEntity>().Sum(a => a.Number));
            query = Query.All<MyEntity>().ToArray();
        }
        catch (StorageException e)
        {
            futureAggregate = Query.ExecuteFutureScalar(() => Query.All<MyEntity>().Count());
            query = Query.All<MyEntity>().ToArray();
        }

        foreach (var myEntity in query)
            Console.WriteLine(myEntity.Number);
        transactionScope.Complete();
    }
}

checked on sql 2008 and DO 4.3.5