Simultaneous calls

I'm trying to calculate the number of simultaneous calls at the time a particular call is made by looking at the datetime ranges. My query works, but takes ~10 minutes to perform for only 95,000 records, which is too long. Any ideas for optimization?

SELECT r.*,
       rr.ChannelsActive  'ChannelsActive'
FROM #rg r
OUTER APPLY
(
      SELECT SUM(1) AS ChannelsActive
      FROM #rg r_inner 
      WHERE 
      (
             r_inner.CallStart BETWEEN r.CallStart AND r.CallEnd 
            OR r_inner.CallEnd BETWEEN r.CallStart AND r.CallEnd
            OR r.CallStart BETWEEN r_inner.CallStart AND r_inner.CallEnd 
            OR r.CallEnd BETWEEN r_inner.CallStart AND r_inner.CallEnd

      )
 ) rr

Example Data

CREATE TABLE #rg
  (
     CallStart DATETIME,
     CallEnd   DATETIME
  )

CREATE INDEX ix1
  ON #rg(CallStart, CallEnd)

CREATE INDEX ix2
  ON #rg(CallEnd, CallStart);

WITH T(N, R)
     AS (SELECT TOP (95000) ROW_NUMBER() OVER (ORDER BY (SELECT 0)) AS RN,
                            ABS(120 + 30 * SQRT(-2 * LOG(ABS(CAST(CAST(CRYPT_GEN_RANDOM(8) AS BIGINT) AS FLOAT) / 9223372036854775807))) * COS(2 * PI() * ABS(CAST(CAST(CRYPT_GEN_RANDOM(8) AS BIGINT) AS FLOAT) / 9223372036854775807)))
         FROM   sys.all_objects o1,
                sys.all_objects o2)
INSERT INTO #rg
SELECT DATEADD(SECOND, N, GETDATE()),
       DATEADD(SECOND, N + R, GETDATE())
FROM   T 

---

This should do it:

 ;WITH cteCallEvents As
 (
        SELECT *, CallStart As EventTime, 1 As EventType FROM #rg r
    UNION ALL
        SELECT *, CallEnd   As EventTime, 0 As EventType FROM #rg r
 )
 , cteCallCounts As
 (
    SELECT *,
        ROW_NUMBER() OVER(Order By EventTime) as EventCount,
        ROW_NUMBER() OVER(Partition By EventType Order By EventTime) as TypeCount
    FROM cteCallEvents
 )
 SELECT *,
    2*TypeCount - EventCount  As OpenCalls
FROM    cteCallCounts
WHERE   EventType = 1

It should take a couple of seconds at most. Should work on any SQL Server 2005+.

---

Use SQL like this to get a list of start/end events...

Select CallStart, 1 As CallCount From #rg
Union All
Select CallEnd, -1 From #rg
Order By CallStart

...then treat this as a simple running totals problem, which is solved differently depending upon your SQL Server version or can be easily addressed in code if that's an option.

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