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06.19.2007 at 06:20PM PDT, ID: 22644899
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8.8

Nested Select statements with poor performance

Asked by mpdillon in MS SQL Server

Tags: , ,

I am experiencing extremely poor performace from the Select statement below:

Select sum(Amount) as Amt From TableA
where IDno in (Select BaseIDNo From TableB
                        where Zip in (Select ZIp From TableC
                                             where SlsPsnNo = '501' Group By Zip)
                        Group By BaseIDNo)  

TableA = 16 records
TableB = 35469 records
TableC = 26334 records

This query returns one record in 5 munutes.
If I break out the inner selects:

(Select BaseIDNo From TableB
                          where Zip in (Select ZIp From TableC
                                                where SlsPsnNo = '501' Group By Zip)
                          Group By BaseIDNo)  
This returns a result in less than 1 second. It returns a single record whos value is 29.

Therefore, the full query could be reduced to something like:

Select sum(Amount) as Amt From TableA
where IDno in (29)
Why is this taking 5 minutes? I do not understand the order the in which the Select statements are processed or what is causing the delay.

How can I optimize this query?

The only solution I know is to build a temp table from the two Selects which return the value 29 and to then use an inner join to use it as part of the main Select.

Thanks in advance for you assistance.

pat


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[+][-]06.19.2007 at 06:45PM PDT, ID: 19321492

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Zone: MS SQL Server
Tags: select, nested, performance
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Solution Provided By: dqmq
Participating Experts: 4
Solution Grade: A
 
 
[+][-]06.19.2007 at 07:59PM PDT, ID: 19321707

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[+][-]06.19.2007 at 08:09PM PDT, ID: 19321740

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[+][-]06.19.2007 at 09:32PM PDT, ID: 19322094

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[+][-]06.20.2007 at 05:27AM PDT, ID: 19323910

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[+][-]06.20.2007 at 09:09AM PDT, ID: 19325860

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[+][-]06.20.2007 at 09:32AM PDT, ID: 19326061

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