问分组时的SQL性能问题-超时

EN

ContactID | A_DTStart        | A_DTEnd
      123 | 2014-03-04 23:43 | 2014-03-05 02:01
      121 | 2014-02-03 12:34 | 2014-02-03 14:44
      ...

With
  AT1 As (Select
      EventLogUnpvt.ContactID,
      DateAdd(minute, -1 * IsNull(MasterLesson.DTstart, 0),
      Convert(date,EventLog.EventDate) +
      Convert(datetime,Convert(time,LandingLog.OffBlocks))) As A_DTstart,
      DateAdd(day, Case
        When
        (Convert(float,Convert(datetime,Convert(time,LandingLog.OffBlocks))) >
        Convert(float,Convert(datetime,Convert(time,LandingLog.OnBlocks)))) Then
        1 Else 0
      End, DateAdd(minute, IsNull(MasterLesson.DTend, 0),
      Convert(date,EventLog.EventDate) +
      Convert(datetime,Convert(time,LandingLog.OnBlocks)))) As A_DTend
    From
      LandingLog Inner Join
      EventLog On LandingLog.EventLogID = EventLog.ID Inner Join
      StudentLesson On EventLog.StudentLessonID = StudentLesson.StudentLessonID
      Inner Join
      MasterLesson On MasterLesson.ID = StudentLesson.LessonID Inner Join
      EventLogUnpvt On EventLogUnpvt.ID = EventLog.ID
    Where
      EventLog.EventDate > GetDate() - 2),
  a As (Select
      a.*,
      Row_Number() Over (Partition By a.ContactID Order By a.A_DTstart) As seqnum
    From
      AT1 a),
  ate As (Select
      a.ContactID,
      a.A_DTstart,
      a.A_DTend,
      a.seqnum,
      1 As grp,
      a.A_DTstart As grp_start
    From
      a
    Where
      a.seqnum = 1
    Union All
    Select
      a.ContactID,
      a.A_DTstart,
      a.A_DTend,
      a.seqnum,
      (Case When a.A_DTstart - ate.grp_start <= 14.0 / 24 Then ate.grp
        Else ate.grp + 1 End),
      (Case When a.A_DTstart - ate.grp_start <= 14.0 / 24 Then ate.grp_start
        Else a.A_DTstart End)
    From
      ate Join
      a On ate.ContactID = a.ContactID And ate.seqnum = a.seqnum - 1)
Select
  ate.*
From
  ate