Sql xp 07

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Sql xp 07

  1. 1. Implementing IndexesObjectivesIn this lesson, you will learn to: Create a clustered index Create a nonclustered index Use the Index Tuning Wizard Understand Index Enhancements©NIIT SQL/Lesson 7/Slide 1 of 32
  2. 2. Implementing IndexesGetting Started An index is an internal table structure that SQL Server uses to provide quick access to rows of a table based on the values of one or more columns Advantages of Using Indexes Improve the speed of the execution of queries Enforce uniqueness of data Speed up joins between tables©NIIT SQL/Lesson 7/Slide 2 of 32
  3. 3. Implementing IndexesGetting Started (Contd.) Disadvantages of Using Indexes Takes time to create an index Takes large amount of disk space to store data along with the original data source—the table Gets updated each time the data is modified Types of Indexes Clustered index Nonclustered index©NIIT SQL/Lesson 7/Slide 3 of 32
  4. 4. Implementing IndexesGetting Started (Contd.) Clustered Index In a clustered index: ® The data is physically sorted ® Only one clustered index can be created per table Nonclustered Index In a nonclustered index: ® Thephysical order of the rows is not the same as the index order©NIIT SQL/Lesson 7/Slide 4 of 32
  5. 5. Implementing IndexesGetting Started (Contd.) ® Nonclustered indexes are typically created on columns used in joins and WHERE clauses, and whose values may be modified frequently ® SQLServer creates nonclustered indexes by default when the CREATE INDEX command is given ® There can be as many as 249 nonclustered indexes per table©NIIT SQL/Lesson 7/Slide 5 of 32
  6. 6. Implementing IndexesGetting Started (Contd.) Indexes and Heap Structures SQL Server supports indexes defined on any column in a table, including computed columns ® Ifa table does not have any clustered index, data is not stored in a particular order. This structure is called a heap©NIIT SQL/Lesson 7/Slide 6 of 32
  7. 7. Implementing IndexesGetting Started (Contd.) Features of Indexes Indexes accelerate queries that join tables, and perform sorting and grouping. Indexes can be used to enforce uniqueness of rows. Indexes are useful on columns in which the majority of data is unique. When you modify the data of an indexed column, the associated indexes are updated automatically. You require time and resources to maintain indexes. You should not create an index that is not used frequently.©NIIT SQL/Lesson 7/Slide 7 of 32
  8. 8. Implementing IndexesGetting Started (Contd.) A clustered index should be created before a nonclustered index. A clustered index changes the order of rows. A nonclustered index would need to be rebuilt if it is built before a clustered index Typically, nonclustered indexes are created on foreign keys. Syntax CREATE [UNIQUE] [CLUSTERED|NONCLUSTERED] INDEX index_name ON table_name(column_name[,column_name]…)©NIIT SQL/Lesson 7/Slide 8 of 32
  9. 9. Implementing Indexes7.D.1 Optimizing Query Execution The ExternalCandidate table contains a large amount of data. The first name of each candidate and the name of the recruitment agency are required to create a report. However, it takes a long time to execute the following query. SELECT vFirstName, cName FROM ExternalCandidate JOIN RecruitmentAgencies ON ExternalCandidate.cAgencyCode = RecruitmentAgencies.cAgencyCode Suggest and implement a solution for faster data retrieval.©NIIT SQL/Lesson 7/Slide 9 of 32
  10. 10. Implementing IndexesTask List Identify how to speed up data retrieval Draft the statement to create an index Create the index in the database Verify that the index has been created Verify that the query execution is faster©NIIT SQL/Lesson 7/Slide 10 of 32
  11. 11. Implementing IndexesIdentify how to speed up data retrieval Indexes are used to: Speed up data retrieval Enforce the uniqueness of rows Result: To speed up data retrieval, use indexes©NIIT SQL/Lesson 7/Slide 11 of 32
  12. 12. Implementing IndexesDraft the statement to create an index Action: The tables on which the index would be created are: ExternalCandidate and RecruitmentAgencies The attributes on which the index would be created are: cAgencyCode of ExternalCandidate and cAgencyCode of RecruitmentAgencies The types of indexes to be created are: ExternalCandidate - Nonclustered index; RecruitmentAgencies - Clustered index The names of the indexes to be created are idxRecruitment and idxExternalCandidate©NIIT SQL/Lesson 7/Slide 12 of 32
  13. 13. Implementing IndexesCreate the index in the database Action: In the Query Analyzer window, type: CREATE NONCLUSTERED INDEX idxExternalCandidate ON ExternalCandidate(cAgencyCode) CREATE CLUSTERED INDEX idxRecruitment ON RecruitmentAgencies(cAgencyCode) Press F5 to execute the code©NIIT SQL/Lesson 7/Slide 13 of 32
  14. 14. Implementing IndexesVerify that the index has been created To verify that the index has been created, use the sp_helpindex command Syntax sp_helpindex table_name Action: In the Query Analyzer window, type: sp_helpindex ExternalCandidate Press F5 to execute the command In the Query Analyzer window, type: sp_helpindex RecruitmentAgencies Press F5 to execute the command©NIIT SQL/Lesson 7/Slide 14 of 32
  15. 15. Implementing IndexesVerify that the query execution is faster Action: Execute the query after creating the index. If there is a lot of data, you can note the difference in speed©NIIT SQL/Lesson 7/Slide 15 of 32
  16. 16. Implementing IndexesJust a Minute… How many clustered indexes can be created per table? Which index organizes data logically but does not store data physically?©NIIT SQL/Lesson 7/Slide 16 of 32
  17. 17. Implementing IndexesIndex Tuning Wizard Index Tuning Wizard available in SQL Server is used to select and create the best possible set of indexes and information regarding a database Uses of the Index Tuning Wizard For a given workload, the best possible combination of indexes for a database is recommended The effects of the proposed recommendation about the indexes, distribution of queries among tables, and the query performance in the workload will be analyzed For a small set of problem queries, the way to tune the database will be recommended It will specify the advanced options such as disk space constraints that can be customized©NIIT SQL/Lesson 7/Slide 17 of 32
  18. 18. Implementing Indexes7.D.2 Using the Index Tuning Wizard You have created the following indexes on the Department table: CREATE NONCLUSTERED INDEX idxdepcode ON DEPARTMENT(cDepartmentCode) CREATE NONCLUSTERED INDEX idxdepname ON DEPARTMENT(vDepartmentName) CREATE NONCLUSTERED INDEX idxdephead ON DEPARTMENT(vDepartmentHead) CREATE NONCLUSTERED INDEX idxdeplocation ON DEPARTMENT(vlocation)©NIIT SQL/Lesson 7/Slide 18 of 32
  19. 19. Implementing Indexes7.D.2 Using the Index Tuning Wizard (Contd.) Analyze the use of these indexes on the Department table when the following query is executed: SELECT Requisition.cRequisitionCode, Position.vDescription,vDepartmentName FROM Position JOIN Requisition ON Requisition.cPositionCode= Position.cPositionCode JOIN Department ON Requisition.cDepartmentCode= Department.cDepartmentCode©NIIT SQL/Lesson 7/Slide 19 of 32
  20. 20. Implementing IndexesTask List Identify a method to analyze the use of indexes Perform the analysis Verify that the analysis has been performed©NIIT SQL/Lesson 7/Slide 20 of 32
  21. 21. Implementing IndexesIdentify a method to analyze the use of indexes Result: Use the Index Tuning Wizard to analyze the use of indexes©NIIT SQL/Lesson 7/Slide 21 of 32
  22. 22. Implementing IndexesPerform the analysis Action: Perform the analysis using the Index Tuning Wizard©NIIT SQL/Lesson 7/Slide 22 of 32
  23. 23. Implementing IndexesVerify that the analysis has been performed Action: You can use the following command to view the indexes present on the Department table: sp_helpindex Department Observe that, as recommended by the Index Tuning Wizard, the extra indexes have been removed©NIIT SQL/Lesson 7/Slide 23 of 32
  24. 24. Implementing IndexesIndex Enhancements Fill Factor FILLFACTOR clause improves performance of the system by minimizing the amount of page splitting that occurs each time an index page becomes full Syntax CREATE CLUSTERED INDEX index_name ON table_name (column_name) WITH FILLFACTOR = percentage_fillfactor Pad_Index Specifies the space to leave open on each page (node) in ©NIIT the intermediate levels of the index SQL/Lesson 7/Slide 24 of 32
  25. 25. Implementing IndexesIndex Enhancements (Contd.) Implications of NULL in Unique Indexes: In a table, a unique index cannot be created on a single column if that column contains NULL in more than one row DBCC SHOWCONTIG: The DBCC SHOWCONTIG command is primarily used to find out why the table or the index is heavily fragmented Syntax DBCC SHOWCONTIG [ (table_id [, index_id]) ] ©NIIT SQL/Lesson 7/Slide 25 of 32
  26. 26. Implementing IndexesIndex Enhancements (Contd.) The DBCC INDEXDEFRAG: The DBCC INDEXDEFRAG command is used to defragment clustered and secondary indexes of the specified table or view Syntax DBCC INDEXDEFRAG ( { database_name | database_id | 0 } , { table_name | table_id | view_name | view_id } , { index_name | index_id } )©NIIT SQL/Lesson 7/Slide 26 of 32
  27. 27. Implementing IndexesJust a Minute… Susan wants to minimize the amount of page splitting that occurs each time an index page is full. What should she use?©NIIT SQL/Lesson 7/Slide 27 of 32
  28. 28. Implementing IndexesIndex Enhancements (Contd.) Index Selection: A detailed query analysis is required to determine which index to involve in a query process. This involves: ® Examining the search clauses to identify the columns referenced ® Knowing the importance of the data to determine the usefulness of the index ® Ranking the queries in the order of importance©NIIT SQL/Lesson 7/Slide 28 of 32
  29. 29. Implementing IndexesPerformance Considerations (Contd.) Index Usage Criteria: SQL Server cannot use an index until and unless the query contains a column in a valid search argument or join clause that matches at least the first column of the index ©NIIT SQL/Lesson 7/Slide 29 of 32
  30. 30. Implementing IndexesSummaryIn this lesson, you learned that: Indexes are created to enhance the performance of queries. There are two types of indexes – clustered and nonclustered. Indexes are created using the CREATE INDEX statement. Data is physically sorted in a clustered index. Clustered indexes should be built on an attribute whose values are unique and do not change often. In a nonclustered index, the physical order of rows is not the same as that of the index order.©NIIT SQL/Lesson 7/Slide 30 of 32
  31. 31. Implementing IndexesSummary (Contd.) A nonclustered index should be built on an attribute which is normally used in joins and the WHERE clause. The values of this attribute may often change. A nonclustered index is the default index that is created with the CREATE INDEX command. The Index Tuning Wizard can be used to analyze the optimal use of indexes in the query entered in the Query Analyzer window. SQL Server provides the FILLFACTOR clause to improve performance of the system by minimizing the amount of page splitting that occurs each time an index page becomes full.©NIIT SQL/Lesson 7/Slide 31 of 32
  32. 32. Implementing IndexesSummary (Contd.) The DBCC SHOWCONTIG command is mainly used to find out whether the table or index is heavily fragmented. Table fragmentation normally occurs when a large number of insert and update operations are performed on the table. The DBCC INDEXDEFRAG command is used to defragment clustered and secondary indexes of the specified table or view.©NIIT SQL/Lesson 7/Slide 32 of 32

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