Query Processing with XML

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Query Processing with XML

• CSE 350 Advanced Database Topics
• Jeffrey R. Ellis

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Query Processing Topics

• Why?
• Java and Other Programming Languages
• XPath/XSLT
• XQuery (W3C-sponsored Query Language)
• Current Research
• Other Query Languages
• XISS (XML Indexing and Storage System)

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FIRST Distinction between XML and HTML/Web
Technologies

• XML spotlight is analogous to Java
• Immediate benefits applied to World Wide Web
• Long-range, more exciting benefits in
  applications
• XML IS NOT AN HTML REPLACEMENT
• HTML marks pages up for presentation on the web
• XML marks text for semantic information purposes
• XML can encode HTML pages, but HTML works well on
  the Web

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XML Data Storage

• XML Documents
• Data is delineated semantically
• Schemas/DTDs control contents of elements
• Semi-structured attitude allows flexibility
• Text is human-readable and machine-parsable
• Open standards work with common tools
• File data storage allows for easy sharing
• Can queries control access to data?

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Traditional Database Storage

• Databases
• Data is delineated semantically
• Schemas control contents of rows
• No flexibility from semi-structured storage
• Data is not human-readable, but only
  machine-parsable
• Proprietary standards prevent interoperability
• Proprietary storage prevents data sharing
• Queries control access to data

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XML for Query Processing

• If we can get efficient query processing, XML
  document storage provides many benefits over
  traditional database storage.
• Sample application
• Employee database document
• XML Schema assumed to exist
• Employee information queried as per standard HR
  processing

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• lt?xml version"1.0"?gt
• lt!DOCTYPE employees SYSTEM "employee.xsd"gt
• ltemployeesgt
• ltemp gender'm'gt
• ltnamegt
• ltlastgtBisselllt/lastgt
• ltfirstgtBrianlt/firstgt
• lt/namegt
• ltpositiongtIT Specialistlt/positiongt
• ltsalarygt35,000lt/salarygt
• ltlocationgtCTlt/locationgt
• lt/empgt
• ltemp gender'm'gt
• ltnamegt
• ltlastgtPhamlt/lastgt
• ltfirstgtHunglt/firstgt
• ltmigtQlt/migt
• lt/namegt
• ltpositiongtSenior IT Specialistlt/positiongt

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Tree Structure of XML Document

• Remember that XML documents are trees

emp
gender
name
position
salary
location
last
first
mi
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Query Processing Programming Languages

• XML Documents are flat files
• Any language with file I/O can read XML document
• Any language with string parsing capabilities can
  use XML data
• Query processing done through language syntax
• Obvious result different from traditional
  databases

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Query Processing Programming Languages

• Strategy
• Basic File I/O through language
• Basic String matching to identify elements
• Processing possible, but not necessarily
  efficient
• Languages have gathered XML processing tools in
  libraries
• xerces Apache library for Java and C
• Two methods for parsing XML data
• DOM
• SAX

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DOM

• Document Object Model
• Defined by W3C for XML, HTML, and stylesheets
• Provides an hierarchical, object-view of the
  document
• DOMParser parses through file, then provides
  access to nodes
• Key Every item in XML document is a node

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DOM Example
Node (Element) nameemp attribute1 child1
Node (Attr) namegender valuem parent
Node (Element) namename parent child1
Node (Element) namelast parent child1
Node (Text) valueBissell parent
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SAX

• Simple API for XML
• Defined by XML-DEV mailing list
• Provides an event-driven processing of the
  document
• XMLReader parses through file and activates
  different methods and functions based on the
  elements retrieved
• Key Methods are defined in interface,
  implemented in user code

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DOM versus SAX

• SAX is primarily Java-based DOM defined for most
  languages
• DOM requires storage of entire document in
  memory SAX processes as it reads
• DOM mirrors a document that can be revisited
  suited for document processing
• SAX mirrors object lifecycles suited for data
  processing

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Query Processing - XPath/XSLT

• Standard XML technologies XPath and XSLT provide
  a ready-made querying infrastructure
• XPath identifies the location of various document
  elements
• XSL Stylesheets provide methods for tranforming
  data from one format to another
• Combining XPath and XSLT provides easy generation
  of result sets based on queries

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XPath

• Provides element, value, and attribute
  identification
• employees/emp/name/first Brian, Hung,
  Sara, Brian
• //salary 35,000, 40,000, 35,000, 60,000
• count(/employees/emp) 4
• //mi Q

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XSLT

• Stylesheet transforms data from one form into
  another
• ltxsltemplate matchnamegt
• ltxslvalue-of selectfirst/gt
• ltxslvalue-of selectlast/gt
• lt/xsltemplategt
• Brian Bissell, Hung Pham, Sara Menillo, Brian
  Chicos

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Combine XPath and XSLT for Queries

• Query Find the last name and position of each
  employee named Brian
• ltxsltemplate match'employees'gt
• ltxslfor-each select'emp'gt
• ltxslif test'name/first"Brian"'gt
• ltxslvalue-of select'name/last'/gt
• ltxsltextgtlt/xsltextgt
• ltxslvalue-of select'position'/gt
• ltxsltextgt lt/xsltextgt
• lt/xslifgt
• lt/xslfor-eachgt
• lt/xsltemplategt

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Combine XPath and XSLT for Queries

• Query Find the average salary of all
  non-managers
• ltxsltemplate match'employees'gt
• ltxslvariable name'running_sum'gt
• ltxslvalue-of select'sum(emp/salary../p
  osition!"Manager")'/gt
• lt/xslvariablegt
• ltxslvariable name'running_count'gt
• ltxslvalue-of select'count(empposition!
  "Manager")'/gt
• lt/xslvariablegt
• ltxslvalue-of select'running_sum div
  running_count'/gt
• lt/xsltemplategt

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Results XSLT/XPath

• Many SQL queries can be accomplished
• XPath provides element (data) access
• XPath provides basic functions (e.g., sum() )
• XPath provides WHERE functionality
• XSLT provides SELECT functionality
• XSLT provides ORDER BY functionality (sort)
• XSLT provides result set formatting
• UNION functionality provided ..?

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Querying with XPath and XSLT

• Important questions
• Is it sufficient?
• Is it efficient?
• Is there a better way?
• XML community has need to design a full query
  language
• XQuery Working draft published 7 June 2001

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Query Processing - XQuery

• XML provides flexibility in representing many
  kinds of information
• Good query language must be likewise flexible
• Pre-XQuery languages are good for specific types
  of data
• Goal Small, easily implementable language in
  which queries are concise and easily understood.

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XQuery Forms

1. Path expressions
2. Element constructors
3. FLWR expressions
4. Operator/Function expressions
5. Conditional expressions
6. Quantified expressions
7. Data Type expressions

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XQuery Path Expressions

• Contribution of XPath
• XQuery 1.0 and XPath 2.0 Data Model
• document(sample1.xml)//emp/salary
• /employees/emp/name../_at_genderf
• //emp1 TO 3/name/first

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XQuery Element Constructors

• Queries can generate new elements
• Similar to XSLT abilities
• ltworkergt
• name/last
• position
• lt/workergt

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XQuery FLWR Expressions

• For clause/Let clause/Where clause/Return
• Similar to SQL
• FOR e IN document(sample1.xml)//emp
• WHERE e/salary gt 38000
• AND e/_at_gender f
• RETURN e/name

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XQuery Operator/Function Expressions

• Pre-defined and user-defined operators and
  functions
• Still under development Union, Intersect, Except
• FOR e IN //employees/emp
• WHERE not(empty(e//mi))
• RETURN e/name

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XQuery Conditional Expressions

• If-then-else expressions are not yet limited to
  boolean (ongoing discussion)
• FOR e IN /employees/emp
• RETURN
• ltworkergt
• name
• IF (e/positionManager)
• THEN ltmanager /gt
• lt/workergt

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Quanitifed Expressions

• Some/Every conditions
• Some/Every evaluates to True or False
• FOR e IN //employees
• WHERE SOME p IN e//emp/position Manager
• RETURN e

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Data Types

• Data Types based on those available from XML
  Schema
• Data types can be literal (Brian), from
  constructor functions (date(2001-10-11) ), or
  from casting ( CAST AS xsdinteger(24) )
• User-defined data types are also allowable and
  parsable

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XQuery

• More choices than XSLT/XPath combination
• Work in progress
• Current W3C efforts into query language
• Influencing the future design of the core XML
  technologies (XPath)
• Hopes to be fully flexible for all future XML
  applications

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Query Processing Research

• XQuery specification continues to undergo review
  and change
• 6 of 7 specification documents released since
  June
• All specifications released in 2001
• Other avenues of research
• Other Query languages
• Indexing strategies
• Implementation

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Query Processing Other Query Languages

• Many query languages exist
• Quilt (basis for XQuery)
• W3C early languages (XML-QL, XQL)
• Adopted traditional languages (OQL, XSQL)
• Research papers (XML-GL, YATL, Lorel)
• Other query languages often optimized for a
  particular subset of XML documents
• Query language field MAY be standardizing to
  XQuery

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Query Processing Indexing Strategy

• Query language less important better indexing
  techniques lead to efficiency
• XISS (XML Indexing and Storage System)
• September 19, 2001 publishing
• Builds sets of indexes on XML data elements and
  attributes on initial parse of XML document
• Lookup becomes constant-time through the various
  built indexes
• Demonstrated successes in test runs

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Query Processing - Implementation

• XML is currently in state of flux
• Standards are still being revised
• Industry cautious before embracing a new
  technology
• Economic slowdown may prevent new research and
  development efforts
• XML still waiting for its Killer App,
  application that forces immediate acceptance

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XML Query Processing

• XML is a functional database storage language
• Efficient query language needed to turn XML into
  a viable database
• Query language solutions are being developed
• Java/C hooks first developed OK
• XSLT/XPath implemented GOOD
• XQuery being designed GREAT?
• Future additions ????