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  SQL Server Tips by Burleson

Indexes

The concept behind indexes is to change the order of the data (clustered index) or to add metadata (non-clustered index) for improving the performance of queries.

Clustered indexes

  • Physically stored in order (ascending or descending)
  • Only one per table
  • When a primary key is created a clustered index is automatically created as well.
  • If the table is under heavy data modifications or the primary key is used for searches, a clustered index on the primary key is recommended.
  • Columns with values that will not change at all or very seldom, are the best choices.

Non-clustered indexes

  • Up to 249 nonclustered indexes are possible for each table or indexed view.
  • The clustered index keys are used for searching therefore clustered index keys should be chosen with a minimal length.
  • Covered queries (all the columns used for joining, sorting or filtering are indexed) should be non-clustered.
  • Foreign keys should be non-clustered.
    If the table is under heavy data retrieval from fields other than the primary key, one clustered index and/or one or more non-clustered indexes should be created for the column(s) used to retrieve the data.

The above book excerpt is from:

Super SQL Server Systems
Turbocharge Database Performance with C++ External Procedures

ISBN: 0-9761573-2-2
Joseph Gama, P. J. Naughter

 http://www.rampant-books.com/book_2005_2_sql_server_external_procedures.htm  

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