Slow Running Function

Hi, when the following function is used within the where statement of a select statement it runs very slowly, anyone know why?



ALTER FUNCTION [dbo].[fn_GetLastDayOfLVPeriod] ( @pInputDate DATETIME )

RETURNS DATETIME

BEGIN



DECLARE @vLVDPeriod varchar(4)

DECLARE @vLVDYear varchar(4)

DECLARE @vOutputDate DATETIME

SELECT @vLVDPeriod = LVDPeriod

FROM dbo.tblMIDates

WHERE CONVERT(varchar(8), Date, 112) = CONVERT(varchar(8), @pInputDate, 112)

SELECT @vLVDyear = LVDYear

FROM dbo.tblMIDates

WHERE CONVERT(varchar(8), Date, 112) = CONVERT(varchar(8), @pInputDate, 112)

SELECT @vOutputDate = MIN(Date)

FROM dbo.tblMIDates

WHERE LVDPeriod = @vLVDPeriod

AND LVDYear = @vLVDyear

RETURN @vOutputDate

END