SQL Server takes forever to process when I use a local variable instead of specifying a date directly?

I apologize in advance for my newbness! Using SQL Server 2005 (9.00.4053.00).

It takes 2:22 to run this query:

DECLARE @Start_date CHAR(10);
SET @Start_date = '2010-07-01';

SELECT 
    DATEPART(wk, rp.dtScanDate) AS 'Week Number',
    COUNT(rp.qwImageID) AS 'Documents',
    SUM(CAST(rp.iCompressedSize AS BIGINT))/1024/1024 AS 'Compressed',
    SUM(CAST(rp.iUncompressedSize AS BIGINT))/1024/1024 AS 'Uncompressed'
INTO [#TempRVOSPolicy]
FROM
    Policy rp
WHERE
    dtScanDate >= @Start_date
GROUP BY 
    DATEPART(wk, rp.dtScanDate)
ORDER BY
    DATEPART(wk, rp.dtScanDate);

But it takes 0:00 to run this one:

DECLARE @Start_date CHAR(10);
SELECT 
    DATEPART(wk, rp.开发者_Python百科dtScanDate) AS 'Week Number',
    COUNT(rp.qwImageID) AS 'Documents',
    SUM(CAST(rp.iCompressedSize AS BIGINT))/1024/1024 AS 'Compressed',
    SUM(CAST(rp.iUncompressedSize AS BIGINT))/1024/1024 AS 'Uncompressed'
INTO [#TempRVOSPolicy]
FROM
    Policy rp
WHERE
    dtScanDate >= '2010-07-01'
GROUP BY 
    DATEPART(wk, rp.dtScanDate)
ORDER BY
    DATEPART(wk, rp.dtScanDate);

Why is this?

Take a look at the Query Plan that SQL Server generates.

My guess is that the query optimizer optimizes away the cast from char(10) to datetime (does it a single time) in the second query whereas it needs to cast the value once for each row in the first.

what happens when you do this?

DECLARE @Start_date datetime;
SET @Start_date = '20100701';

If you always run one query after another, it may return you cashed result (for second query)...