Worktable showing high logical reads

Hello all,

I have a store procedure where i have one sub query to return no . of attendance between period selection 

basically like this,

SELECT EmployeeCode,COUNT(DISTINCT ATTENDANCE_DATE)AS Mkt_Attendance 
FROM [dbo].[View_Marketing_Person_Attendance] 
WHERE CONVERT(DATE,ATTENDANCE_DATE) BETWEEN @from AND @to
GROUP BY EmployeeCode

@from and @to parameters passed to procedure and used in sub query to filter attendance of period of employees 

and i have join that query to main table in procedure using a EmployeeCode and displaying 

,SUM(DISTINCT Mkt_Attendance) AS Mkt_Attendance in Main select area

(using sum because having grouping summarize query )

Now , above query is run very fast as it use index seek on Attendance_date based in view defination of a log table and returns only 265 rows for period 2015-04-01 and 2015-06-30 in 0 sec. approx.

now when i am running a procedure for 2015-04-01 and 2015-06-30 it executes in 11 sec. after commenting a sub query 

and when i removed a comment and executes it takes 32 sec. but a strange is in statistics it shows 

Table 'Worktable'. Scan count 93, logical reads 6576643, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

worktable only shows when i use above sub query as it is because of the sub query i know.

please , help why the work table is showing high reads and is slowing query this much .

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Dilip Patil..