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Creating XML Indexes

This topic describes how to create primary and secondary XML indexes.

Creating a Primary XML Index

To create a primary XML index, use the CREATE INDEX (Transact-SQL)Transact-SQL DDL statement. Not all options available for non-XML indexes are supported on XML indexes.

Note the following when you are creating an XML index:

  • To create a primary XML index, the table that contains the XML column being indexed, called the base table, must have a clustered index on the primary key. This makes sure that if the base table is partitioned, the primary XML index can be partitioned by using the same partitioning scheme and partitioning function.

  • If an XML index exists, the clustered, primary key of the table cannot be modified. You will have to drop all XML indexes on the table before modifying the primary key.

  • A primary XML index can be created on a single xml type column. You cannot create any other type of index with the XML type column as a key column. However, you can include the xml L type column in a non-XML index. Each xml type column in a table can have its own primary XML index. However, only one primary XML index per xml type column is permitted.

  • XML indexes exist in the same namespace as non-XML indexes. Therefore, you cannot have an XML index and a non-XML index on the same table with the same name.

  • IGNORE_DUP_KEY and ONLINE options of are always set to OFF for XML indexes. You can specify these options with a value of OFF.

  • The filegroup or partitioning information of the user table is applied to the XML index. Users cannot specify these separately on an XML index.

  • The DROP_EXISTING index option can drop a primary XML index and create a new primary XML index, or drop a secondary XML index and create a new secondary XML index. However, this option cannot drop a secondary XML index to create a new primary XML index or vice versa.

  • Primary XML index names have the same restrictions as view names.

You cannot create an XML index on an xml type column in a view, on a table valued variable with xml type columns, or xml type variables.

  • To change an xml type column from untyped to typed XML, or vice versa, by using the ALTER TABLE ALTER COLUMN option, no XML index on the column should exist. If one does exist, it must be dropped before the column type change is tried.

  • The option ARITHABORT must be set to ON when an XML index is created. To query, insert, delete, or update values in the XML column using XML data type methods, the same option must be set on the connection. If it is not, the XML data type methods will fail.

    Note

    Information about an XML index can be found in catalog views. However, sp_helpindex is not supported. Examples provided later in this topic show how to query the catalog views to find XML index information.

When creating or recreating a primary XML index on an XML data type column that contains values of the XML Schema types xs:date or xs:dateTime (or any subtypes of these types) that have a year of less than 1, the index creation will fail in SQL Server 2008. SQL Server 2005 allowed these values, so this problem can occur when creating indexes in a database generated in SQL Server 2005. For more information, see Typed XML Compared to Untyped XML.

Example: Creating a Primary XML Index

Table T (pk INT PRIMARY KEY, xCol XML) with an untyped XML column is used in most of the examples. These can be extended to typed XML in a straightforward way. For more information about how to use typed XML, see Implementing XML in SQL Server.) For simplicity, queries are described for XML data instances as shown in the following:

<book genre="security" publicationdate="2002" ISBN="0-7356-1588-2">
   <title>Writing Secure Code</title>
   <author>
      <first-name>Michael</first-name>
      <last-name>Howard</last-name>
   </author>
   <author>
      <first-name>David</first-name>
      <last-name>LeBlanc</last-name>
   </author>
   <price>39.99</price>
</book>

The following statement creates an XML index, called idx_xCol, on the XML column xCol of table T:

CREATE PRIMARY XML INDEX idx_xCol on T (xCol)

Creating a Secondary XML Index

Use the CREATE INDEX (Transact-SQL)Transact-SQL DDL statement to create secondary XML indexes and specify the type of the secondary XML index that you want.

Note the following when you are creating secondary XML indexes:

  • All indexing options that apply to a nonclustered index, except IGNORE_DUP_KEY and ONLINE, are permitted on secondary XML indexes. The two options must always be set to OFF for secondary XML indexes.

  • The secondary indexes are partitioned just like the primary XML index.

  • DROP_EXISTING can drop a secondary index on the user table and create another secondary index on the user table.

You can query the sys.xml_indexes catalog view to retrieve XML index information. Note that the secondary_type_desc column in the sys.xml_indexes catalog view provides the type of secondary index:

SELECT  * 
FROM    sys.xml_indexes

The values returned in the secondary_type_desc column can be NULL, PATH, VALUE, or PROPERTY. For the primary XML index, the value returned is NULL.

Example: Creating Secondary XML Indexes

The following example illustrates how secondary XML indexes are created. The example also shows information about the XML indexes that you created.

CREATE TABLE T (Col1 INT PRIMARY KEY, XmlCol XML)
GO
-- Create primary index.
CREATE PRIMARY XML INDEX PIdx_T_XmlCol 
ON T(XmlCol)
GO
-- Create secondary indexes (PATH, VALUE, PROPERTY).
CREATE XML INDEX PIdx_T_XmlCol_PATH ON T(XmlCol)
USING XML INDEX PIdx_T_XmlCol
FOR PATH
GO
CREATE XML INDEX PIdx_T_XmlCol_VALUE ON T(XmlCol)
USING XML INDEX PIdx_T_XmlCol
FOR VALUE
GO
CREATE XML INDEX PIdx_T_XmlCol_PROPERTY ON T(XmlCol)
USING XML INDEX PIdx_T_XmlCol
FOR PROPERTY
GO

You can query the sys.xml_indexes to retrieve XML indexes information. The secondary_type_desc column provides the secondary index type.

SELECT  * 
FROM    sys.xml_indexes

You can also query the catalog view for index information.

SELECT *
FROM sys.xml_indexes
WHERE object_id = object_id('T')

You can add sample data and then review the XML index information.

INSERT INTO T VALUES (1,
'<doc id="123">
<sections>
<section num="2">
<heading>Background</heading>
</section>
<section num="3">
<heading>Sort</heading>
</section>
<section num="4">
<heading>Search</heading>
</section>
</sections>
</doc>')
GO
-- Check XML index information.
SELECT *
FROM   sys.dm_db_index_physical_stats (db_id(), object_id('T'), NULL, NULL, 'DETAILED')
GO
-- Space usage of primary XML index
DECLARE @index_id int
SELECT  @index_id = i.index_id
FROM    sys.xml_indexes i 
WHERE   i.name = 'PIdx_T_XmlCol' and object_name(i.object_id) = 'T'
 
SELECT *
FROM sys.dm_db_index_physical_stats (db_id(), object_id('T') , @index_id, DEFAULT, 'DETAILED')
go
--- Space usage of secondary XML index (for example PATH secondary index)  PIdx_T_XmlCol_PATH
DECLARE @index_id int
SELECT  @index_id = i.index_id 
FROM    sys.xml_indexes i 
WHERE  i.name = 'PIdx_T_XmlCol_PATH' and object_name(i.object_id) = 'T'
 
SELECT *
FROM sys.dm_db_index_physical_stats (db_id(), object_id('T') , @index_id, DEFAULT, 'DETAILED')
go
 
-- Space usage of all secondary XML indexes for a particular table
SELECT i.name, object_name(i.object_id), stats.* 
FROM   sys.dm_db_index_physical_stats (db_id(), object_id('T'), NULL, DEFAULT, 'DETAILED') stats
JOIN sys.xml_indexes i ON (stats.object_id = i.object_id and stats.index_id = i.index_id)
WHERE secondary_type is not null
-- Drop secondary indexes.
DROP INDEX PIdx_T_XmlCol_PATH ON T
GO
DROP INDEX PIdx_T_XmlCol_VALUE ON T
GO
DROP INDEX PIdx_T_XmlCol_PROPERTY ON T
GO
-- Drop primary index.
DROP INDEX PIdx_T_XmlCol ON T
-- Drop table T.
DROP TABLE T
Go