Lesson 1: Managing Index Fragmentation in .NET framework

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Lesson 1: Managing Index Fragmentation
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Practice 2: Use ALTER INDEX REORGANIZE to Reorganize an Index
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In this practice, you will reorganize an index by using the ALTER INDEX REORGANIZE statement. 1. If necessary, start SSMS and connect to the instance containing the AdventureWorks sample database. Open the Query Editor pane. 2. In the Query Editor pane, type in the following Transact-SQL statement to view current fragmentation levels, reorganize the indexes on the HumanResources.Employee table, and view the fragmentation levels after the reorganization:
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USE AdventureWorks; --View the current fragmentation levels. SELECT index_id, avg_fragmentation_in_percent, avg_page_space_used_in_percent FROM sys.dm_db_index_physical_stats (DB_ID( AdventureWorks ), OBJECT_ID( HumanResources.Employee ),NULL, NULL, DETAILED ) WHERE index_id <> 0; --does not return information about heaps --Reorganize all indexes on the table. ALTER INDEX ALL ON HumanResources.Employee REORGANIZE; --View the fragmentation levels after the index reorganization. SELECT index_id, avg_fragmentation_in_percent, avg_page_space_used_in_percent FROM sys.dm_db_index_physical_stats (DB_ID( AdventureWorks ), OBJECT_ID( HumanResources.Employee ),NULL, NULL, DETAILED ) WHERE index_id <> 0; --does not return information about heaps
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Lesson Summary
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Indexes become fragmented during INSERT, DELETE, and UPDATE operations, and this fragmentation can degrade query performance. Internal index fragmentation occurs when the index pages are not filled to the maximum amount allowed under the current fill factor setting. External index fragmentation occurs when the physical ordering of index pages does not match the logical ordering of the pages. You can check index fragmentation levels by running a SELECT statement against the sys.dm_db_index_physical_stats dynamic management function. You can correct index fragmentation by executing either the ALTER INDEX REORGANIZE or the ALTER INDEX REBUILD Transact-SQL statement.
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Using Transact-SQL to Manage Databases
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Lesson Review
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The following questions are intended to reinforce key information presented in this lesson. The questions are also available on the companion CD if you prefer to review them in electronic form.
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NOTE
Answers
Answers to these questions and explanations of why each answer choice is right or wrong are located in the Answers section at the end of the book.
1. You are a DBA tasked with maintaining an installation of SQL Server 2005.One of your jobs is to determine the index fragmentation levels for all user tables in your database. Which dynamic management view or function can you use to review index fragmentation levels A. sys.dm_db_index_operational_stats B. sys.dm_db_index_usage_stats C. sys.dm_db_missing_index_details D. sys.dm_db_index_physical_stats 2. You are a DBA tasked with maintaining an installation of SQL Server 2005.You need to determine whether your tables contain external fragmentation. Which column would you use to find whether your indexes are externally fragmented A. avg_fragment_size_in_pages B. avg_page_space_used_in_percent C. avg_fragmentation_in_percent D. avg_record_size_in_bytes 3. You are a DBA tasked with maintaining an installation of SQL Server 2005.One of your jobs is to correct the index fragmentation levels for all user tables in your database. During your fragmentation investigation, you determine that an index has external fragmentation levels greater than 30 percent. Which statement would you use to correct this amount of external fragmentation A. ALTER INDEX REBUILD B. ALTER INDEX...REORGANIZE C. ALTER INDEX DISABLE D. ALTER INDEX SET STATISTICS_NORECOMPUTE = ON
Lesson 2: Managing Statistics
Lesson 2: Managing Statistics
Another important aspect of achieving top query performance is the statistical information that SQL Server creates about the distribution of values in a column. During its evaluation of a query, the query optimizer uses these statistics to estimate the cost of using an index to satisfy the query. To ensure optimum query performance, you need to understand the importance of statistics and decide when to let SQL Server automatically generate and update them, and when to manually generate and update them.
After this lesson, you will be able to:
Explain the purpose of statistics. Manage index and column statistics.
Estimated lesson time: 25 minutes
Understanding Statistics
When SQL Server creates column and index statistics, the database engine sorts the values of the columns on which the statistics are being built and creates a histogram. Histograms are based on up to 200 values contained in the column, separated by intervals. The histogram specifies how many rows exactly match each interval value, how many rows fall within an interval, and the density of values contained within an interval. These statistics on column values help the query optimizer determine whether using an index improves query performance. SQL Server 2005 introduces additional information that is collected by statistics created on char, varchar, varchar(max), nchar, nvarchar, nvarchar(max), text, and ntext columns. This additional information, called a string summary, helps the query optimizer estimate the selectivity of query predicates on string patterns, which leads to better estimates of result set sizes when a query uses LIKE conditions.
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