Transforming XPath Queries for BottomUp Query Processing

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Transforming XPath Queries for Bottom-Up Query
Processing

• Yoshiharu Ishikawa
• Takaaki Nagai
• Hiroyuki Kitagawa
• University of Tsukuba
• ishikawa,kitagawa_at_is.tsukuba.ac.jp

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Presentation Overview

• Background
• Motivation and Our Approach
• The Proximal Nodes Model
• Query Translation
• Translation Example
• Related Work
• Conclusions and Future Work

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Background

• XML content-description language on the Web
• XPath
• pattern-based query language for XML
• extracts XML nodes based on the specified pattern
• has navigational semantics
• XSLT uses XPath for the node specification
• XQuery also uses XPath

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XML Example
ltitemlistgt ltitem category"audio equipment"gt
ltcatalog-infogt lttypegtCD playerlt/typegt
ltmanufacturergtStar Electronicslt/manufacturergt
ltcatalog-nogtCDP-R55Nlt/catalog-nogt
lt/catalog-infogt ltsales-infogt
ltprod-yeargt2001lt/prod-yeargt
ltpricegt125.00lt/pricegt lt/sales-infogt
lt/itemgt ... lt/itemlistgt
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XPath Query

• Sample query Q retrieve prices of CD players
• XPath sentence
• contains location steps separated by "/"
• a location step has the format axisnode_testpre
  dicate...predicate
• location steps can be abbreviated
• e.g., /descendantfoo ? //foo, /attributebar ?
  _at_bar

/itemlist/item_at_category "audio equipment"
catalog-info/type "CD player"/sales-info/price
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Presentation Overview

• Background
• Motivation and Our Approach
• The Proximal Nodes Model
• Query Translation
• Translation Example
• Related Work
• Conclusions and Future Work

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XPath Semantics

• XPath assumes top-down query processing
• Not efficient for large XML databases
• Bottom-up processing is better in some cases

query /article/authorsauthor "Miller"
article
article
top-down
bottom-up
authors
authors
author
author
author
author
"Smith"
"White"
"Chen"
"Miller"
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Bottom-Up Query Processing

• We can process the example query when
• we can determine the specified leaf elements
  (i.e., "Miller") with the help of an index, and
• we can select the parent for a specific author
  node.
• We do not need to access all the authors/author
  elements

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Our Objective and Approach

• Our Objective
• Efficient bottom-up processing of XPath queries
  with the help of index structures
• Our Approach
• Use of the proximal nodes model as the underlying
  retrieval model
• The model enables bottom-up query evaluation
• Development of transformation rules from XPath
  queries to proximal nodes expressions

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Presentation Overview

• Background
• Motivation and Our Approach
• The Proximal Nodes Model
• Query Translation
• Translation Example
• Related Work
• Conclusions and Future Work

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The Proximal Nodes Model (1)

• Proposed by Navarro and Baeza-Yates 7 as a
  structured document retrieval model
• Uses bottom-up query processing approach
• XML data can be treated as nested nodes
• a node corresponds to an element or attribute in
  XML
• each node has an associated text region (called
  the segment) segments can take nested structure
• Expressive power and efficiency are well-balanced
• evaluation cost is almost O(n) n is the no. of
  nodes

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The Proximal Nodes Model (2)

• The model consists of three components
• Text pattern matching language
• specifies pattern matching conditions
• implementation dependent
• returns a set of the matched nodes
• example "ABC Corporation"
• Retrieval operators based on document structures
• returns a set of nodes for a given element or
  attribute name
• example chapter, price
• Operators to integrate partial retrieval results
• calculates the result node set from the given
  node sets
• efficient computation based on segment
  relationships

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Proximal Nodes Operators
P and Q are nodes with associated segments
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Example of Proximal Nodes Expression

• Example expression of proximal nodes model
• Query processing steps
• 1. determine the node sets that corresponds to
  the elements "item" and "type" using indexes
• 2. determine the node set that corresponds to the
  pattern "CD player" using an index
• 3. compute the result of "same" operator
• 4. compute the result of "with" operator

item with (type same "CD player")
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Presentation Overview

• Background
• Motivation and Our Approach
• The Proximal Nodes Model
• Query Translation
• Translation Example
• Related Work
• Conclusions and Future Work

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Translation Rules (1)

• Supports major XPath patterns
• Based on the XPath semantic description by Wadler
  10
• Use of denotational semantics

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Translation Rules (2)
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Translation Rules (3)
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Auxiliary Functions
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Simplification Using the Knowledge of Document
Structure

• If we know the DTD of the target XML, we can
  derive more simplified translation results

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Presentation Overview

• Background
• Motivation and Our Approach
• The Proximal Nodes Model
• Query Translation
• Translation Example
• Related Work
• Conclusions and Future Work

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Translation Example

• Original query Q
• Translation result
• t1 item with (item with (category same "audio
  equipment"))
• t2 catalog-info child t1
• t3 t1 with (t1 with (((type child t2) child t2)
  same "CD player"))
• t4 sales-info child t3
• ans (((price child t4) child t4) child t3)
  child itemlist

/itemlist/item_at_category "audio equipment"
catalog-info/type "CD player"/sales-info/price
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Simplification of Query Plan (1)

• The translated result contains multiple
  application of an operator
• We can delete redundant operators considering the
  operator semantics
• Example
• t1 item with (item with (category same "audio
  equipment")) ? item with (category same "audio
  equipment")

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Simplification of Query Plan (2)

• If we can use the DTD information, we can further
  simplify the expressions
• Example
• t3 t1 with ((type child (catalog-info child
  t1)) same "CD player") ? t1 with ((type in t1)
  same "CD player")
• Simplified query plan for query Q
• t1 item with (category name "audio equipment")
• ans price in (t1 with ((type in t1) same "CD
  player"))

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Presentation Overview

• Background
• Motivation and Our Approach
• The Proximal Nodes Model
• Query Translation
• Translation Example
• Related Work
• Conclusions and Future Work

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Related Work

• Translation of XQL queries into proximal nodes
  expressions (Baeza-YatesNavarro 2)
• Rewriting techniques for XQL queries (Wood 13)
• Use of document structure for the query
  optimization 3,11,12,13
• Optimization of regular path expressions in the
  context of semistructured DBs 4,8

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Presentation Overview

• Background
• Motivation and Our Approach
• The Proximal Nodes Model
• Query Translation
• Translation Example
• Related Work
• Conclusions and Future Work

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Conclusions and Future Work

• Conclusions
• Bottom-up processing approach for XPath queries
• Support of major XPath query patterns
• Translation to proximal nodes expressions
• Simplification and optimization techniques
• Future work
• Support of more complete XPath semantics
• Application of hybrid approach (top-down and
  bottom-up)