Choosing Secondary XML Indexes in .NET framework

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Choosing Secondary XML Indexes
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The PATH secondary index is best used for queries that filter based on the XML structure or for queries for which the complete XML path is unknown. It can also be used for queries that combine path-based queries and value filtering. For example, the query /Employees/Employee[@Bonus] retrieves all Employee elements that have a
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Managing XML Data
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Bonus attribute. And /Departments/Department[@ID = 10] retrieves all nodes under the Departments element that have an ID attribute with the value 10. The VALUE secondary index works best for queries that filter based on values and if the path is not fully specified or if it includes a wildcard. For example, // Employee[@YearsInRole = 8] retrieves all Employee elements (no matter where they appear in the XML structure) that have a YearsInRole attribute with a value of 8. And //Employees/Employee[@* = "Smith"] retrieves all Employee elements that have any attribute with the value Smith. The PROPERTY secondary index is best used for queries that use the value method of the XML data type and that filter based on the table s primary key, as this example shows:
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SELECT EmployeeData.value('(/Employee/FirstName)[1]', 'nvarchar(100)') FROM EmployeesTable WHERE EmployeeID = 101
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Which type of secondary index works best for queries that filter based on values and if the path is not fully specified or if it includes a wildcard The VALUE secondary index works best for those types of queries.
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Create Appropriate Indexes for XML Data
In this practice, you will create the appropriate indexes on the LogRecord XML column in the UniversalLog table. 1. In the TK4318 database, modify the UniversalLog table and add a clustered primary key constraint on the ID column:
ALTER TABLE UniversalLog ADD CONSTRAINT ULogPK PRIMARY KEY CLUSTERED (ID)
2. Execute a CREATE INDEX statement to create the XML primary index:
CREATE PRIMARY XML INDEX LogRecordPrimaryIdx ON UniversalLog (LogRecord);
3. Execute a CREATE INDEX statement to create an XML PATH secondary index:
CREATE XML INDEX LogRecordSecondaryIdxPath ON UniversalLog (LogRecord) USING XML INDEX LogRecordPrimaryIdx FOR PATH;
Lesson 6: Creating XML Indexes
4. Execute a CREATE INDEX statement to create the XML VALUE secondary index:
CREATE XML INDEX LogRecordSecondaryIdxValue ON UniversalLog (LogRecord) USING XML INDEX LogRecordPrimaryIdx FOR VALUE;
5. Execute a CREATE INDEX statement to create the XML PROPERTY secondary index:
CREATE XML INDEX LogRecordSecondaryIdxProperty ON UniversalLog (LogRecord) USING XML INDEX LogRecordPrimaryIdx FOR PROPERTY;
Lesson Summary
Indexes help the SQL Server query engine optimize the query execution plan. The XML data type primary index requires a clustered index on the base table s primary key column. XML data type columns accept one primary index and three types of secondary indexes. Create secondary indexes based on the type of queries that will be executed: PATH, VALUE, or PROPERTY.
Lesson Review
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.
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 this book.
1. Users of the sales application have been complaining about the time it takes to generate the TotalSalesPerDay report. The report is created from the SalesByDate XML data type column in the Sales tables. The TotalSalesPerDay report is fed by the TotalSalesPerDaySP stored procedure, which executes the following query:
SELECT SalesByDate.query('/Sales[//@reportDate = sql:variable("@today")]') FROM Sales
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Managing XML Data
How can you improve the performance on this query A. Create a PATH secondary XML index. B. Create a PROPERTY secondary XML index. C. Create a VALUE secondary XML index. D. Create a clustered index on the XML column. 2. The end of the fiscal year is coming up, and users of the accounting application are inserting 200 new records per minute. Each record is made up of four XML documents representing different tax forms that need to be filled in. The users have been complaining because the rate of inserted records per minute was three times higher last year at this time. Which action would provide the best performance in this application A. Create a PROPERTY secondary XML index. B. Drop the secondary indexes on the XML columns. C. Create a PATH secondary XML index. D. Drop all indexes on the XML columns.
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