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  SQL Server Tips by Robin Schumacher

Object Fragmentation

Plainly stated, the subject of SQL Server fragmentation is not that simple to handle. There are a variety of factors to consider when diagnosing fragmentation and contemplating the reorganization of database objects.

SQL Server fragmentation can be broken down into two basic types:

  • External Fragmentation: This situation exists when indexes have a logical order, based on their key value, which does not match the actual physical order inside the database file that houses the index. When an ordered scan is performed on an index, I/O performance is enhanced when the physical order of the database pages matches the logical order of the index. This is because the disk head can scan in one direction instead of moving back and forth to obtain the needed information.

  • Internal Fragmentation: This situation exists when tables and indexes are not utilizing space as efficiently as they should. The amount of wasted space in the database pages artificially inflates the size of the table or index and causes more I/O to be performed than would otherwise be needed if the object were compacted.

Fragmentation is not a performance factor once data reaches the SQL Server memory caches.


The above book excerpt is from:

High-Performance SQL Server DBA
Tuning & Optimization Secrets

ISBN: 0-9761573-6-5
Robin Schumacher

 http://www.rampant-books.com/book_2005_2_sql_server_dba.htm  

 

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