Issue with DateCreated - sql-server

Question

I have a table dbo.participation:
ID INT NOT NULL IDENTITY(1,1) PRIMARY KEY CLUSTERED,
User VARCHAR(MAX) NOT NULL,
ParticipationLevel TINYINT NOT NULL,
Selector VARCHAR(MAX) NOT NULL,
DateCreated DATETIME NOT NULL
I created the code below but unfortunately it shows bad performance for #DateStart and #DateStop
SELECT
dateadd(month, datediff(month, 0, DateCreated), 0) AS MDate
,COUNT(CASE WHEN ParticipationLevel >= 10 THEN Selector ELSE NULL END) AS ParticipationLevel1
,COUNT(CASE WHEN ParticipationLevel >= 30 THEN Selector ELSE NULL END) AS ParticipationLevel2
FROM
Participation
WHERE
(#DateStart IS NULL OR (#DateStart IS NOT NULL
AND DateCreated >= #DateStart))
AND (#DateEnd IS NULL OR (#DateEnd IS NOT NULL
AND DateCreate < #DateEnd))
GROUP BY
Dateadd(month, datediff(month, 0, DateCreate), 0)
Do you happen to have any ideas how to improve my code or alternatively how to modify the table to improve performance?

Answer 1

You need an index along the following lines
CREATE INDEX ix
ON dbo.Participation(DateCreated)
INCLUDE (ParticipationLevel);
And you should rewrite the query to get rid of the OR and to avoid the unnecessary reference to a column defined as NOT NULL.
(Note a simple COUNT(Selector) would not look up the value as SQL Server recognizes it can't be NULL but wrapping in an expression defeats this logic)
SELECT DATEADD(month, DATEDIFF(month, 0, DateCreated), 0) AS MDate,
COUNT(CASE
WHEN ParticipationLevel >= 10 THEN 1
END) AS ParticipationLevel1,
COUNT(CASE
WHEN ParticipationLevel >= 30 THEN 1
END) AS ParticipationLevel2
FROM Participation
WHERE DateCreated >= ISNULL(#DateStart, '17530101')
AND DateCreated <= ISNULL(#DateEnd, '99991231')
GROUP BY DATEDIFF(month, 0, DateCreated)
This can give a plan with a seek as below
Note that it would be possible to get rid of the sort by processing chunks of the index a month at the time (possibly in a recursive CTE) but this may be overkill.
Code for that could look something like
/*Cheap to find out from the index*/
IF #DateStart IS NULL
SELECT #DateStart = MIN(DateCreated)
FROM dbo.Participation
IF #DateStart IS NULL
SELECT #DateEnd = MAX(DateCreated)
FROM dbo.Participation
/*Adjust to start of month*/
SELECT #DateStart = DATEADD(month, DATEDIFF(month, 0, #DateStart), 0),
#DateEnd = DATEADD(month, 1 + DATEDIFF(month, 0, #DateEnd), 0);
WITH Dates
AS (SELECT #DateStart AS MDate
UNION ALL
SELECT dateadd(MONTH, 1, MDate) AS MDate
FROM Dates
WHERE dateadd (MONTH, 1, MDate) <= #DateEnd)
SELECT D.MDate,
CA.ParticipationLevel1,
CA.ParticipationLevel2
FROM Dates D
CROSS APPLY (SELECT COUNT(CASE
WHEN ParticipationLevel >= 10
THEN 1
END) AS ParticipationLevel1,
COUNT(CASE
WHEN ParticipationLevel >= 30
THEN 1
END) AS ParticipationLevel2
FROM Participation P WITH (INDEX = ix)
WHERE P.DateCreated >= D.MDate
AND P.DateCreated < DATEADD(MONTH, 1, D.MDate)
GROUP BY () /* So no grouping row returned for empty months */
) CA(ParticipationLevel1, ParticipationLevel2)
OPTION (MAXRECURSION 0);
Which gives a plan with repeated seeks and no sorts

Answer 2

Below two checks not needed in your WHERE clause
#DateStart IS NOT NULL AND
#DateEnd IS NOT NULL AND
SELECT dateadd(month, datediff(month, 0, DateCreated), 0) AS MDate
,COUNT(CASE WHEN ParticipationLevel >= 10 THEN Tracking ELSE NULL END) AS ParticipationLevel1
,COUNT(CASE WHEN ParticipationLevel >= 30 THEN Tracking ELSE NULL END) AS ParticipationLevel2
FROM Participation
WHERE (#DateStart IS NULL OR DateCreated >= #DateStart) AND (#DateEnd IS NULL OR DateCreate < #DateEnd)
GROUP BY Dateadd(month, datediff(month, 0, DateCreate), 0)