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POOptimize SQL subquery containing multiple inner joins and aggregate functions
 

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    <p>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.</p> <p>An example of the data is as follows:</p> <pre><code>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 | | </code></pre> <p>The subquery is as follows :</p> <pre><code>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 </code></pre> <p>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:</p> <pre><code>Payment .Receipt_No|.Person|.Payment_date 5|John |01/01/2001 3|David |15/04/2003 8|Mike |27/11/2004 </code></pre> <h2>Following Ahmads post -</h2> <p>From the following results</p> <pre><code>(3|David |15/04/2003) and (9|David |15/04/2003) </code></pre> <p>I would only want the record with the lowest receipt_no. So</p> <pre><code>(3|David |15/04/2003) </code></pre> <p>So I added the aggregate function 'min(Payment.receipt_no)' grouping by person.</p> <p>Query 1.</p> <pre><code>select min(Payment.Person) 'Person', min(Payment.receipt_no) 'receipt_no' from Payment a where a.type&lt;&gt;'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 &lt;&gt; 'GX' and (i.reversed not in ('R') or i.reversed is null)) group by a.Person </code></pre> <p>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.</p> <p>Query 2.</p> <pre><code>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&lt;&gt;'GX' AND i.person = a.person ORDER BY i.payment_date DESC, i.receipt_no ASC) </code></pre> <p>Which I wouldn't necessarily think as 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.</p> <p>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.</p> <p>I can only attribute this to the use of aggregate functions in one and not the other.</p>
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    1. COwhat is the database you are working with?
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    2. COThe Database I'm working with is Visual dataflex 14.0 with a Sql server 2008 R2 back end. However any Sql commands I use would have to be backwardly compatible to atleast Sql server 2005. Preferably sql server 2000 if possible.
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    3. COTry http://codereview.stackexchange.com
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