24 XML Databases

Dr R. Baskaran

IDs & IDREFs

An element can have at most one attribute of type ID.
The ID attribute value of each element in an XML document must be distinct.
Thus, the ID attribute value is an object identifier.
An attribute of type IDREF must contain the ID value of an element in the same document.
An attribute of type IDREFS contains a set of (0 or more) ID values. Each ID value must contain the ID value of an element in the same document.

Example

</department>

</course>

….

<course_incharge staff_id=“54783” dept_name=“CSE”> <name> Arvind </name>

</course_incharge>

….

</university>

XML Schema

XML Schema is a more sophisticated schema language which addresses the drawbacks of DTDs. Supports
Typing of values
integer, string, etc
Also, constraints on min/max values
User-defined, complex types
Many more features, including
uniqueness and foreign key constraints, inheritance
XML Schema is itself specified in XML syntax, unlike DTDs
More-standard representation, but verbose
XML Scheme is integrated with namespaces
BUT: XML Schema is significantly more complicated than DTDs.

Rationale for XML in Databases

There are a number of reasons to directly specify data in XML or other document formats such as JSON.
For XML in particular, they include:
An enterprise may have a lot of XML in an existing standard format.
Data may need to be exposed or ingested as XML, so using another format such as relational forces ouble-modeling of the data.
XML is very well suited to sparse data, deeply nested data and mixed content (such as text with embedded markup tags).
XML is human readable whereas relational tables require expertise to access.
Metadata is often available as XML.
Semantic web data is available as RDF/XML.

XML in Databases

Steve O’Connell gives one reason for the use of XML in databases: the increasingly common use of XML for data transport hich has meant that “data is extracted from databases and put into XML documents and vice-versa”.
In content-based applications, the ability of the native XML database also minimizes the need for extraction or entry of metadata to support searching and navigation.

Querying and Transforming XML Data

Translation of information from one XML schema to another
Querying on XML data
Above two are closely related, and handled by the same tools
Standard XML querying/translation languages
XPath
Simple language consisting of path expressions
XSLT
Simple language designed for translation from XML to XML and XML to HTML
XQuery
An XML query language with a rich set of features

Tree Model of XML Data

Query and transformation languages are based on a tree model of XML data.
An XML document is modeled as a tree, with nodes corresponding to elements and attributes.
Element nodes have child nodes, which can be attributes or subelements.
Text in an element is modeled as a text node child of the element
Children of a node are ordered according to their order in the XML
Element and attribute nodes (except for the root node) have a single parent, which is an element node.
The root node has a single child, which is the root element of the document.

XPath

XPath is used to address (select) parts of documents using path expressions
A path expression is a sequence of steps separated by “/”
Think of file names in a directory hierarchy
Result of path expression: set of values that along with their containing elements/attributes match the specified path
E.g. /university/course_incharge/name evaluated on the university data we saw earlier returns

<name>Arvind</name>

g./university/course_incharge/name/text( ) returns the same names, but without the enclosing tags Arvind, Balu

XQuery

XQuery is a general purpose query language for XML data
XQuery is derived from the Quilt query language, which itself borrows from SQL, XQL and XML-QL
XQuery uses a

for … let … where … order by …result …

syntax

for ó SQL from

where ó SQL where

order by ó SQL order by

result ó SQL select

let allows temporary variables, and has no equivalent in SQL

XSLT

A stylesheet stores formatting options for a document, usually separately from document.
g. an HTML style sheet may specify font colors and sizes for headings, etc.
The XML Stylesheet Language (XSL) was originally designed for generating HTML from XML.
XSLT is a general-purpose transformation language.
Can translate XML to XML, and XML to HTML.
XSLT transformations are expressed using rules called
Templates combine selection using XPath with construction of results.

Application Program Interface

There are two standard application program interfaces to XML data:
SAX (Simple API for XML)
Based on parser model, user provides event handlers for parsing events
g. start of element, end of element
DOM (Document Object Model)
XML data is parsed into a tree representation
Variety of functions provided for traversing the DOM tree
g.: Java DOM API provides Node class with methods getParentNode( ), getFirstChild( ), getNextSibling( ) getElementsByTagName( ), …
Also provides functions for updating DOM tree

Storage of XML Data

XML data can be stored in
Non-relational data stores
Flat files
Natural for storing XML
XML database
Database built specifically for storing XML data, supporting DOM model and declarative querying
Currently no commercial-grade systems
Relational databases
Data must be translated into relational form
Advantage: mature database systems
Disadvantages: overhead of translating data and queries

Alternative Representations :
String Representation
Tree Representation
Map to relations

String Representation

Store each top level element as a string field of a tuple in a relational database.

Use a single relation to store all elements, or
Use a separate relation for each top-level element type.
E.g. account, customer, depositor relations.
each with a string-valued attribute to store the element.

Indexing:

Store values of subelements/attributes to be indexed as extra fields of the relation, and build indices on these fields.
E.g. customer_name or account_number.
Some database systems support function indices, which use the result of a function as the key value.
The function should return the value of the required subelement/attribute.

String Representation

Benefits:

Can store any XML data even without DTD
As long as there are many top-level elements in a document, strings are small compared to full document
Allows fast access to individual elements.

Drawback:

Need to parse strings to access values inside the elements
Parsing is slow.

Tree Representation

Tree representation: model XML data as tree and store using relations nodes(id, parent_id, type, label, value)

Each element/attribute is given a unique identifier
Type indicates element/attribute
Label specifies the tag name of the element/name of attribute
Value is the text value of the element/attribute
Can add an extra attribute position to record ordering of children

Tree Representation

Benefit:

Can store any XML data, even without DTD.

Drawbacks:

Data is broken up into too many pieces, increasing space overheads.
Even simple queries require a large number of joins, which can be slow.

Mapping XML data to Relations

Relation created for each element type whose schema is known:
An id attribute to store a unique id for each element.
A relation attribute corresponding to each element attribute.
A parent_id attribute to keep track of parent element.
As in the tree representation.
Position information (ith child) can be store too.
All sub-elements that occur only once can become relation attributes.
For text-valued sub-elements, store the text as attribute value.
For complex sub-elements, can store the id of the sub-element.
Sub-elements that can occur multiple times represented in a separate table.
Similar to handling of multivalued attributes when converting ER diagrams to tables.

Storing XML Data in Relational Systems

Applying above ideas to department elements in university-1 schema, with nested course elements, we get

department(id, dept_name, faculty, students)

course(parent id, course_code, dept_name, title, credits)

Publishing: process of converting relational data to an XML format
Shredding: process of converting an XML document into a set of tuples to be inserted into one or more relations
XML-enabled database systems support automated publishing and shredding
Many systems offer native storage of XML data using the xml data type. Special internal data structures and indices are used for efficiency

XML Enabled databases

XML enabled databases typically offer one or more of the following approaches to storing XML within the traditional relational structure:

XML is stored into a CLOB (Character Large OBject).
XML is `shredded` into a series of Tables based on a Schema.
XML is stored into a native XML Type as defined by ISO Standard 9075-14.

RDBMS that support the ISO XML Type are:

IBM DB2 (pureXML)
Microsoft SQL Server.
Oracle Database.
PostgreSQL

Native XML databases

These databases are typically better when much of the data is in XML or other non-relational formats. BaseX, Berkeley DB XML edition, eXist, MarkLogic Server, Qizx, Sedna
All the above databases uses XML as an interface to specify documents as tree structured data that may contain unstructured text, but on disk the data is stored as “optimized binary files.”
This makes query and retrieval faster.
For MarkLogic it also allows XML and JSON to co-exist in one binary format.

Key Features

Has an XML document as at least one fundamental unit of (logical) storage, just as a relational database has a row in a table as a fundamental unit of (logical) storage.
Need not have any particular underlying physical storage model.
For example, NXDs can use optimized, proprietary storage formats.
This is a key aspect of XML databases.
Managing XML as large strings is inefficient due to the extra markup in XML.
Compressing and indexing XML allows the illusion of directly accessing, querying and transforming XML while gaining the performance advantages of working with optimized binary tree structures.

Summary

Introduction to XML
XML Syntax
Querying and Transforming XML data
Different representations of XML in relational data
Storing XML data in Relational Systems
XML Database Applications