13. XML databases Are XML documents just sequential files? What - - PowerPoint PPT Presentation

XML-13 J. Teuhola 2013 247

• 13. XML databases
• Are XML documents just sequential files?
• What about typical database features:

– Queries and indexing, based on tags/keywords – Updating of document structure/content – Piecewise processing – Transactions and concurrency in multi-user environments – Recovery from failures during transactions

• Important design decisions:

– Implementation data model – Query language

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Implementation alternatives for XML databases

• 1. Relational database; alternatives for storage:

–

Document as a field, seq. processing & extensions

–

Non-typed DOM implementation; DOM-tree pre- sented using parent-child relations

–

Typed DOM impl.; a relation for each node type

• 2. Object-oriented database

–

More direct mapping of DOM model to OO concepts

–

Both typed and non-typed implementations possible; non-typed more flexible

• 3. Native XML database

–

Database schema based on DTD or XML schema

–

Support for hierarchical structures

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Example document collection: 2 courses

<?xml version=“1.0”?> <course> <cname>Adv DB</cname> <teacher>Timo</teacher> <audience> <student>Pasi</student> <student>Pirjo</student> </audience> </course> <?xml version=“1.0”?> <course> <cname>C++</cname> <teacher>Esa</teacher> <audience> <student>Pasi</student> <student>Pia</student> </audience> </course>

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Relational alternative 1: XML data type for a column

Courses-relation cid course document c1 <?xml…?><course><cname>AdvDB</cname><teacher> Timo</teacher><audience><student>Pasi</student> <student>Pirjo</student></audience></course> c2 <?xml version=“1.0”?><course><cname>C++</cname> <teacher>Esa</teacher><audience> <student>Pasi </student><student>Pia</student></audience></course>

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Relational alternative 2: Non-typed nodes

Nodes-relation

node-id element parent text-value n1 course

• n2

cname n1 Adv DB n3 teacher n1 Timo n4 audience n1

• n5

student n4 Pasi n6 student n4 Pirjo n7 course

• n8

cname n7 C++ … … … …

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Relational alternative 3: Typed nodes

Courses cid cname teacher c1 Adv DB Timo c2 C++ Esa Audience student cid Pasi c1 Pirjo c1 Pasi c2 Pia c2 Note 1: In each solution, the DTD or XML schema must be stored separately. Note 2: The solution uses the information that cname and teacher are 1-valued.

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Query languages for XML

• Query goals:

– Retrieve documents satisfying a selection condition. – Retrieve subparts of a document on the basis of a selection condition.

• Conditions can be structural (based on node

relationships) or content-based (based on text).

• Several propositions have been made for XML query

languages.

• XPath is a popular choice, and also a basis for more

advanced languages.

• XQuery 1.0:

– W3C recommendation (Jan 2007) – Returns the answer in XML form

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XQuery design goals

• Declarative syntax, with two alternatives

– Human-readable syntax (cf. SQL) – XML syntax (called XQueryX)

• Ability to create and transform XML trees

(cf. XSLT) for output

• Combining information from multiple documents
• Support for namespaces
• Support for simple and complex data types
• Utilization of XML Schema information

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XQuery vs. XPath

• XQuery 1.0 is a superset of XPath 2.0;

every XPath expression is a legitimate XQuery expression (exception: only axes child, des- cendant, parent, attribute, self and descendant-

• r-self are required to be implemented)
• Extensions over XPath:

– Ability to join information from different sources – Ability to generate new XML structures – User-defined functions – Arbitrary computations

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XQuery vs. XSLT

• Both can be used for extracting, combining and

transforming XML data.

• Same processing power
• Different design principles and origins:

XQuery inspired by SQL; XSLT by CSS.

• Strengths of XSLT:

– Recursive traversals and arbitrary-depth processing – Efficient implementations

• Strengths of XQuery:

– Simpler for simple tasks – Less verbose

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‘Prolog’ definitions in XQuery queries

• Version declaration (“xquery version 1.0”)
• Handling of boundary whitespace

(e.g. “declare boundary-space preserve”)

• Namespace definition

(e.g. “declare namespace ns = …”)

• Importing a schema from a URI

(“import schema at …”)

• Declaring variables with initial values

(declare variable $name = …;)

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XQuery expressions

• Atomic expressions:

– primitive types integer, boolean, string, etc. – simple constructor functions for types, imported from schemas, e.g. xs:string(“Adv DB”), xs:date(“2006-10-12”)

• XML expressions for new elements, attributes,

character data, … can be constructed

• Enclosed expressions, syntax: { expression }

The expression result will be positioned into the context where it occurs.

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XQuery expressions: Example

• FLWOR expressions

(for – let – where – order by – return), e.g. <large-courses> { for $c in fn:doc(“courses.xml”)//course let $s := $c/audience/student where fn:count($s) gt 4 return <large> { $c/name/text() } </large> } </large-courses>

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XQuery expressions: Example (cont.)

Source document: <?xml version="1.0" encoding="UTF-8"?> <courses> <course> <name>Adv DB</name> <audience> <student>Pekka</student> <student>Paula</student> </audience> </course> <course> <name>C++<name> <audience> <student>Paavo</student> <student>Pirjo</student> <student>Pekka</student> <student>Pirkko</student> <student>Pauli</student> </audience> </course> </courses Result of the previous query: <large-courses> <large>C++</large> </large-courses>

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XML support in some commercial DBMSs

• IBM DB2 9 ‘Viper’:

– Hybrid relational/XML database management system – Storage alternatives:

• XML collection (decomposed storage, composed output)
• CLOB (Character Large OBject) columns
• XML columns & indexes

– SQL and XQuery support for both normal and XML columns

• Oracle 11g XML DB:

– Storage alternatives: Object-relational, CLOB, binary – Support for XQuery

• Microsoft SQL Server 2005:

– Storage alternatives:

• XML-type columns
• Decomposed storage (‘shredding’); composing for output

– Support for (subset of) XQuery and XML-DML

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Some ’native’ XML databases

• dbXML

– Open-source (dbXML Group; SourceForge) – XPath is the main query language

• eXist

– Open-source (project led by Wolfgang Meier) – Support for XPath, XQuery, XUpdate

• xDB

– Commercial (EMC corp.) – Support for XPath, XQuery, XUpdate – See: https://community.emc.com/community/edn/xmltech