I have a select statement which is infact a subquery within a larger select statement built up programmatically. The problem is if I elect to include this subquery it acts as a bottle neck and the whole query becomes painfully slow.
An example of the data is as follows:
Payment .Receipt_no|.Person |.Payment_date|.Type|.Reversed| 2|John |01/02/2001 |PA | | 1|John |01/02/2001 |GX | | 3|David |15/04/2003 |PA | | 6|Mike |26/07/2002 |PA |R | 5|John |01/01/2001 |PA | | 4|Mike |13/05/2000 |GX | | 8|Mike |27/11/2004 |PA | | 7|David |05/12/2003 |PA |R | 9|David |15/04/2003 |PA | |
The subquery is as follows :
select Payment.Person, Payment.amount from Payment inner join (Select min([min_Receipt].Person) 'Person', min([min_Receipt].Receipt_no) 'Receipt_no' from Payment [min_Receipt] inner join (select min(Person) 'Person', min(Payment_date) 'Payment_date' from Payment where Payment.reversed != 'R' and Payment.Type != 'GX' group by Payment.Person) [min_date] on [min_date].Person= [min_Receipt].Person and [min_date].Payment_date = [min_Receipt].Payment_date where [min_Receipt].reversed != 'R' and [min_Receipt].Type != 'GX' group by [min_Receipt].Person) [1stPayment] on [1stPayment].Receipt_no = Payment.Receipt_no
This retrieves the first payment of each person by .Payment_date (ascending), .Receipt_no (ascending) where .type is not 'GX' and .Reversed is not 'R'. As Follows:
Payment .Receipt_No|.Person|.Payment_date 5|John |01/01/2001 3|David |15/04/2003 8|Mike |27/11/2004
I am unable to move the subquery out to a temporary table as temporary tables are simply not supported within the programming language used by my application.
Edit : Incorrect statement. Temporary tables are supported and therefore this is a valid option.
The Query was rewritten as the following.
select min(Payment.Person) 'Person', min(Payment.receipt_no) 'receipt_no' from Payment a where a.type<>'GX' and (a.reversed not in ('R') or a.reversed is null) and a.payment_date = (select min(payment_date) from Payment i where i.Person=a.Person and i.type <> 'GX' and (i.reversed not in ('R') or i.reversed is null)) group by a.Person
I added this as a subquery within my much larger query, however it still ran very slowly. So I tried rewriting the query whilst trying to avoid the use of aggregate functions and came up with the following.
SELECT receipt_no, person, payment_date, amount FROM payment a WHERE receipt_no IN (SELECT top 1 i.receipt_no FROM payment i WHERE (i.reversed NOT IN ('R') OR i.reversed IS NULL) AND i.type<>'GX' AND i.person = a.person ORDER BY i.payment_date DESC, i.receipt_no ASC)
Which I wouldn't necessarily think of as being more efficient. In fact if I run the two queries side by side on my larger data set Query 1. completes in a matter of milliseconds where as Query 2. takes several seconds.
However if I then add them as subqueries within a much larger query, the larger query completes in hours using Query 1. and completes in 40 seconds using Query 2.
I can only attribute this to the use of aggregate functions in one and not the other.