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  SQL Server Tips by Robin Schumacher

What is Bad SQL?

Before the DBA can identify problem SQL in the database, the question of ďWhat is bad SQL?Ē needs to be asked. What criteria should be used when the hunt for problem SQL in critical systems is performed?

Even seasoned experts disagree on what constitutes efficient and inefficient SQL, so there is no way to sufficiently answer this question to every Microsoft professionalís satisfaction. The following list contains general criteria that can be used when evaluating the output from various database monitors or personal diagnosticScripts:

  • Overall Response (Elapsed) Time: This is how much time the query took to parse, execute, and fetch the data needed to satisfy the query. It should not include the network time needed to make the round trip from the requesting client workstation to the database server.

  • CPU Time: This is how much CPU time the query took to parse, execute, and fetch the data needed to satisfy the query.

  • Physical I/O: Often used as the major statistic in terms of identifying good vs. bad SQL, this is a measure of how many disk reads were caused by the query in order to satisfy the userís request. While controlling disk I/O is desired, it is important the DBA not focus solely on physical I/O as the single benchmark of inefficient SQL. Make no mistake, disk access is slower than memory access and also consumes processing time making the physical to logical transition, but the entire I/O picture of a SQL statement should be considered. This includes looking at a statementís logical I/O as well.

  • Logical I/O: This is a measure of how many memory reads the query took to satisfy the userís request. The goal of tuning I/O for a query should be to examine both logical and physical I/O and use appropriate mechanisms to keep both to a minimum.

  • Repetition: This is a measure of how often the query has been executed. A problem in this area is not as easy to spot as the others unless the DBA is very familiar with the application. A query that takes a fraction of a second to execute may still be causing a headache on the system if it is executed erroneously. One example would be a query that executes in a runaway T-SQL loop over and over again.

There are other criteria that can be examined. Examples include criteria such as sort activity, temp table usage, or access plan statistics that show items such as Cartesian joins and the like; however, more often than not, these measures are reflected in the criteria listed above. Fortunately, SQL Server records most of the above measures. This makes tracking the SQL that has been submitted against Microsoft databases much easier than other database engines.

The above book excerpt is from:

High-Performance SQL Server DBA
Tuning & Optimization Secrets

ISBN: 0-9761573-6-5
Robin Schumacher  


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