How to Prove that using subselect queries in SQL is killing performance of server

Posted by adopilot on Stack Overflow See other posts from Stack Overflow or by adopilot
Published on 2010-12-28T12:28:33Z Indexed on 2010/12/28 12:54 UTC
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Filed under:

sql

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database-design

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performance-tuning

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sql-subselect

One of my jobs it to maintain our database, usually we have troubles with lack of performance while getting reports and working whit that base.
When I start looking at queries which our ERP sending to database I see a lot of totally needlessly subselect queries inside main queries.
As I am not member of developers which is creator of program we using, they do not like much when I criticize they code and job. Let say they do not taking my review as serious statements. So I asking you few questions about subselect in SQL

Does subselect is taking a lot of more time then left outer joins?
Does exists any blog, article or anything where I subselect is recommended not to use ?
How I can prove that if we avoid subselesct in query that query is going to be faster ?

Our database server is MSSQL2005

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