XML Data Type Primary Index Columns in VS .NET

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XML Data Type Primary Index Columns
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Column Ordering Information Primary Key
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Description The primary index is clustered by this column. Order is maintained by a special structure called ORDPATH, which keeps the node hierarchy. This column corresponds to the base table primary key. It is duplicated to maintain a reference to the relational data associated with the XML instance. This column maintains the XML node tag name. Instead of keeping the string characters, it is tokenized, so it keeps a reference to the real node tag name in the BLOB structure. This column maintains the node s XSD type, indicating whether the node is an XML element, an XML attribute, XML text, and so on. This column holds the node contents. This column maintains the complete path from the root node to the current node. It is used for path-based lookups.
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The primary index maintains document order and document structure. It is built in reverse order, so lookups can be recursive and work when only the path suffix is known for example, when you use // in XPATH. You can rebuild the XML data by using the information in the primary index. You can create a primary XML index for each XML column in a table. The primary index is built on the relational B+ tree structure of a clustered index on the table s primary key column, so you must create the table s clustered index first.
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SQL Server 2005 indexing features
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Please refer to 4, Creating Indexes, for more information on SQL Server 2005 indexing features.
You create XML indexes by using the same Transact-SQL data definition language (DDL) statements you use to create relational indexes. The syntax for creating a primary XML index is as follows:
CREATE PRIMARY XML INDEX Index_Identifier ON table_name (XML_typed_column_name);
Creating XML Data Type Secondary Indexes
Having a primary index improves performance because instead of having to navigate the BLOB structure, the SQL Server query processor can execute lookups on the primary index. Those lookups are executed sequentially based on the primary index key. However, you can achieve further performance gains by declaring secondary indexes that let SQL Server avoid sequential lookups on the primary index. Secondary indexes are built on top of the primary index, so they provide different lookup schemes depending on the secondary index keys. Table 8-9 lists the three types of secondary indexes you can create.
Table 8-9
Three Types of XML Data Type Secondary Indexes
Secondary Index Type PATH
Description This type of XML secondary index uses the Path and Node Value columns from the primary index. Those two columns allow for more efficient seeks when SQL Server is searching for paths in the XML data. Instead of having to search sequentially for a path in the primary index, SQL Server can fully serve from the secondary index any query that executes pathbased queries.
Lesson 6: Creating XML Indexes
Table 8-9
Three Types of XML Data Type Secondary Indexes
Secondary Index Type VALUE
Description You build this type of XML secondary index by using the Node Value and Path columns from the primary index. Those two columns produce more efficient seeks when searching for specific values in the XML data. Instead of having to search sequentially for a value in the primary index, SQL Server can fully serve from the secondary index a query that executes value-based queries. This type of XML secondary index is based on the Primary Key, Node Value, and Path columns from the primary index. Those three columns give more efficient seeks when SQL Server is searching for specific values that must be associated with their parent row in the base table.
PROPERTY
Here is the syntax to create a secondary PATH XML index:
CREATE XML INDEX Secondary_Index_Identifier ON table_name (XML_typed_column_name); USING XML INDEX Primary_Index_Identifier FOR PATH
The syntax to create a secondary VALUE XML index is as follows:
CREATE XML INDEX Secondary_Index_Identifier ON table_name (XML_typed_column_name); USING XML INDEX Primary_Index_Identifier FOR VALUE
And the syntax to create a secondary PROPERTY XML index is the following:
CREATE XML INDEX Secondary_Index_Identifier ON table_name (XML_typed_column_name); USING XML INDEX Primary_Index_Identifier FOR PROPERTY
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