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  SQL Server Tips by Gama and Naughter

 

Pages and Extents

 

SQL Server stores data in blocks of 8 Kb called Pages and eight contiguous extents are the basic unit of storage for tables and indexes, called Extents. There are two types of extents:  

Uniform extents - the entire extent contains data from one table.

 

Mixed extents - the extent contains data from two to eight different tables. This is the case of either tables with less than eight pages or the last pages from a table with a total number of pages multiple of eight.

Data Pages are pages that contain data from tables, there are two ways to organize such pages: 

Clustered tables - tables with one clustered index, the pages are linked in a doubly-linked list using the index as a key and the index is stored as a B-tree structure. Indexed views have an identical structure as Clustered tables.

 

Heaps - tables with no clustered index, the data pages are stored in no particular order and not linked.

 

Non-clustered indexes have a B-tree index structure but the data pages are stored in no particular order.
 


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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