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  SQL Server Tips by Burleson

Access Path Effectiveness

The second determinant of access efficiency is the route through which the information must travel to be accessed. Index lookups versus table scans must be taken into account to determine how well code has been written or how well the DBMS optimizer chooses the best path. Design considerations also come into play.

Small table scans should be overlooked, as it is often more efficient for a database to cache and scan a small table than to use any available indexing mechanisms. Another important factor that contributes to access path effectiveness is the consistent application of object demographics to the data dictionary.

Most DBAs have witnessed an access path, which worked well for objects with a certain volume of data, begin to fail when the demographics of those objects changed and the database’s optimizer was not informed of the change.

The final huge factor is I/O consumption, which is comprised of logical and physical consumption. While physical I/O may take longer to accomplish a database request, a heavy load of logical I/O is not conducive to quick response time either. Mitigating both should be a goal of the performance model. Even if the database is servicing requests well in memory, it might not matter if the code is not accessing the data properly or utilizing the right techniques to get only the data that is necessary.

This is the point at which some DBAs who rely only on singular measures, such as the cache hit ratio, fail to properly measure performance. Yes, a user thread may sport a 98% cache hit ratio, but if that same thread used 500,000 logical reads to produce a result that SQL could have achieved in 5,000 logical reads, wouldn’t overall speed be increased with the use of SQL?

Measuring code efficiency is not simple since individual SQL statements can be difficult to track and trace; however, there are a few components that will indicate positive or negative trends in this territory.

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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