CHECKSUM_AGG And BINARY_CHECKSUM Performance Problems

Gentlemen,

I am using the following query to get a list of grouped checksum data.

SELECT CAST(Field0_datetime AS INT),
CHECKSUM_AGG(BINARY_CHECKSUM(Field1_bigint, Field2_datetime,
Field3_datetime, Field4_bigint, Field5_bigint, CAST(Field6_float
Decimal(38,6)), Field7_datetime))
FROM Table1
WHERE Field0_datetime BETWEEN '2003-01-01' AND '2003-01-20'
GROUP BY CAST(Field0_datetime AS INT)

Please notice the used filter: from January 1 to January 20.
That query takes about 6 minutes do return the data. The result is 18
records.

However, when I execute the same query filtering BETWEEN '2003-01-01' and
'2003-01-10', this time it takes only 1 second to return data.
When I execute the query filtering BETWEEN '2003-01-10' and '2003-01-20' the
query takes another 1 second to return data.

So why 6 minutes to process them together??

The table have an index by Field0_datetime.

It contains about 1.5 millions records total, using around 1.7Gb of
diskspace, indexes included.

From 2003-01-01 and 2003-01-20, there are 11401 records selected. Don't look
like that much.

The situation is repeatable, I mean, if I execute the queries back and
again, they takes the about the same ammount of time to execute, so I don't
think this problem is related to cache or something like that.

I would appreciate any advice about what might be wrong with my situation.

Thanks a lot and kind regards,

Orly Junior
IT Professional