pureXML overview -- Db2 as an XML database
The pureXML® feature allows you to store well-formed XML documents in database table columns that have the XML data type. By storing XML data in XML columns, the data is kept in its native hierarchical form, rather than stored as text or mapped to a different data model.
Because pureXML data storage is fully integrated, the stored XML data can be accessed and managed by leveraging existing Db2 database server functionality.
The storage of XML data in its native hierarchical form enables efficient search, retrieval, and updates of XML. XQuery, SQL, or a combination of both can be used to query and update XML data. SQL functions that return XML data or take XML arguments (referred to as SQL/XML functions) also enable XML data to be constructed or published from values retrieved from the database.
Querying and updating
XML documents stored in XML columns can be queried and updated using the following methods:
- XQuery is a generalized language for interpreting, retrieving, and modifying XML data. The Db2 database server allows XQuery to be invoked directly or from within SQL. Because the XML data is stored in Db2 tables and views, functions are provided that extract the XML data from specified tables and views by naming the table or view directly, or by specifying an SQL query. XQuery supports various expressions for processing XML data, for updating existing XML objects such as elements and attributes, and for constructing new XML objects. The programming interface to XQuery provides facilities similar to those of SQL to execute queries and retrieve results.
- SQL statements and SQL/XML functions
- Many SQL statements support the XML data type. This enables you to perform many common database
operations with XML data, such as creating tables with XML columns, adding XML columns to existing
tables, creating indexes over XML columns, creating triggers on tables with XML columns, and
inserting, updating, or deleting XML documents. The set of SQL/XML functions, expressions, and
specifications supported by Db2 database server has
been enhanced to take full advantage of the XML data type.
XQuery can be invoked from within an SQL query. In this case, the SQL query can pass data to XQuery in the form of bound variables.
Support for application development is provided by several programming languages, and through SQL and external procedures:
- Programming language support
- Application development support of the new pureXML feature enables applications to combine
XML and relational data access and storage. The following programming
languages support the XML data type:
- C or C++ (embedded SQL or CLI)
- Java™ (JDBC or SQLJ)
- C# and Visual Basic (IBM® Data Server Provider for .NET)
- SQL and external procedures
- XML data can be passed to SQL procedures and external procedures by including parameters of data type XML in CREATE PROCEDURE parameter signatures. Existing procedure features support the implementation of procedural logic flow around SQL statements that produce or make use of XML values as well as the temporary storage of XML data values in variables.
The pureXML feature provides a repository for managing the URI dependencies of XML documents and enables XML data movement for database administration:
- XML schema repository (XSR)
- The XML schema repository (XSR) is a repository for all XML artifacts required to process XML instance documents stored in XML columns. It stores XML schemas, DTDs, and external entities referenced in XML documents.
- Import, export and load utilities
- The import, export and load utilities have been updated to support the native XML data type. These utilities treat XML data like LOB data: both types of data are stored outside the actual table. Application development support for importing, exporting and loading XML data is also provided by updated db2Import, db2Export and db2Load APIs. These updated utilities permit data movement of XML documents stored in XML columns that is similar to the data movement support for relational data.
Several performance oriented features are available to you when working with XML documents stored in XML columns:
- Indexes over XML data
- Indexing support is available for data stored in XML columns. The use of indexes over XML data can improve the efficiency of queries issued against XML documents. Similar to a relational index, an index over XML data indexes a column. They differ, however, in that a relational index indexes an entire column, while an index over XML data indexes part of a column. You indicate which parts of an XML column are indexed by specifying an XML pattern, which is a limited XPath expression.
- The optimizer has been updated to support the evaluation of SQL, XQuery, and SQL/XML functions that embed XQuery, against XML and relational data. The optimizer exploits statistics gathered over XML data, as well as data from indexes over XML data, to produce efficient query execution plans.
- Explain facility
- The Explain facility has been updated to support SQL enhancements for querying XML data and to support XQuery expressions. These updates to the Explain facility allow you to see quickly how Db2 database server evaluates query statements against XML data.
Support for the XML data type is available in tools including the command line processor, IBM Data Studio, and IBM Database Add-Ins for Microsoft Visual Studio.
Annotated XML schema decomposition
The pureXML feature enables you to store and access XML data as XML, in its hierarchical form, there can be cases where accessing XML data as relational data is required. Annotated XML schema decomposition decomposes documents based on annotations specified in an XML schema.