CS 561 Presentation: Indexing and Querying XML Data for Regular Path Expressions

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CS 561 Presentation Indexing and
Querying XML Data for Regular Path Expressions

• A Paper by Quanzhong Li and Bongki Moon
• Presented by Ming Li

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Our Objective

• Developing a system that will enable us to
  perform XML data queries efficiently.

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XML Queries Languages

• Used for retrieving data from XML files.
• Use a regular path expression syntax.
• e.g. XPath, XQuery.

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Queries Today - Inefficient

• Usually XML tree traversals Inefficient.
• Top-Down Approach
• Bottom-Up Approach
• An example
• the query
• /chapter/_/figure
• (finding all figures in all chapters.)

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Our Objective - Refined

• Developing a system that will enable us to
  perform XML data queries efficiently
• Developing such a system consists of
• Developing a way to efficiently store XML data.
• Developing efficient algorithms for processing
  regular path expressions (e.g. XQuery
  expressions).

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Storing XML Documents - XISS

• XISS - XML Indexing and Storage System.
• Provides us with ways to
• efficiently find all elements or attributes with
  the same name string grouped by document which
  they belong to.
• quickly determine the ancestor-descendant
  relationship between elements and/or attributes
  in the hierarchy of XML data hierarchy.

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Determining Ancestor-Descendent Relationship

• According to Dietzs for two given nodes x and y
  of a tree T, x is an ancestor of y iff x occurs
  before y in the preorder traversal and after y in
  the postorder traversal.
• Example

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Determining Ancestor-Descendent Relationship
cont.

• Advantage the ancestor-descendent relationship
  can be determined in constant time.
• Disadvantage a lack of flexibility.
• e.g. inserting a new node requires recomputation
  of many tree nodes.

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Determining Ancestor-Descendent Relationship
cont.

• A new numbering scheme
• Each node is associated with a ltorder, sizegt
  pair
• For a tree node y and its parent x
• order(y), order(y) size(y) Ì (order(x),
  order(x) size(x)
• For two sibling nodes x and y, if x is the
  predecessor of y in preorder traversal holds
• order(x) size(x) lt order(y).

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Determining Ancestor-Descendent Relationship
cont.

• Fact for two given nodes x and y of a tree T, x
  is an ancestor of y iff
• order(x) lt order(y) order(x) size(x)

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Determining Ancestor-Descendent Relationship
cont.

• Properties
• the ancestor-descendent relationship can be
  determined in constant time.
• flexibility node insertion usually doesnt
  require recomputation of tree nodes.
• an element can be uniquely identified in a
  document by its order value.

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XISS System Overview
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Name Index and Value Table

• Objective minimizing the storage and computation
  overhead by eliminating replicated strings and
  string comparisons.
• Name Index - mapping distinct name strings into
  unique name identifiers (nid).
• Value Table - mapping distinct value strings
  (i.e. attribute value and text value) into unique
  value identifiers (vid).
• Both implemented as a B-tree.

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The Element Index

• Objective quickly finding all elements with the
  same name string.
• Structure

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The Attribute Index

• Objective quickly finding all elements with the
  same name string.
• Structure
• Same structure as the Element Index except that
  the record in attribute index has a value
  identifier vid which is a key used to obtain the
  attribute from the value table.

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The Structure Index

• Objectives
• Finding the parent element and child elements (or
  attributes) for a given element.
• Finding the parent element for a given attribute.
• Structure

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The Structure Index cont.

• Structure
• B-tree using document identifier (did) as a key.
• Leaf nodes linear arrays with records for all
  elements and attributes from an XML document.
• Each record nid, ltorder,sizegt, Parent order,
  Child order, Sibling order, Attribute order.
• Records are ordered by order value.

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Querying Method

• Decomposing path expressions into simple path
  expressions.
• Applying algorithms on simple path expressions
  and their intermediate results.

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Decomposition of Path Expressions

• The main idea
• A complex path expression is decomposed into
  several simple path expressions.
• Each simple path expression produces an
  intermediate result that can be used in the
  subsequent stage of processing.
• The results of the simple path expressions are
  than combined or joined together to obtain the
  final result of the given query.

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Basic Subexpressions - Example
Decomposition of (E1/E2)/ E3 / ((E4_at_aV)
(E5/_/E6))
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Example EA-Join Element and Attribute Join
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EA-Join Element and Attribute Join
Input E1,,Em Ei is a set of elements
having a common document identifier
(did) A1,,An Aj is a set of elements having
a common document identifier (did) Output A
set of (e,a) pairs such that the element e is the
parent of the attribute a.
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EA-Join Element and Attribute Join
The Algorithm // Sort-merge Ei and Aj by
did. (1) foreach Ei and Aj with the same did
do // Sort-merge Ei and Aj by //
PARENT-CHILD relationship (2) foreach e Î Ei and
a Î Aj do (3) if (e is a parent of a) then
output (e,a) end end
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EA-Join Example

• Consider the XML document
• ltEle AttA1gt
• ltEle AttA2gt lt/Elegt
• lt/Elegt
• And the query /Ele_at_AttA1

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EA-Join Querying /Ele_at_AttA1

• ltEle AttA1gt
• ltEle AttA2gt lt/Elegt
• lt/Elegt
• Sort-merging Eles and Atts by parent-child
  relation ship will give us the list
• lt1,3gt, lt2,0gt, lt3,1gt, lt4,0gt
• Finding the elements Eles with a child
  attribute Att with a value A1 from the
  accepted list is easy using the information in
  the Element Record.

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EA-Join Comments

• Only a two-stage sort-merge operation without
  additional cost of sorting
• First merge by did.
• Second merge by examining parent-child
  relationship.
• This merge is based on the order values of the
  element and attribute as defined by the numbering
  scheme.
• Attributes should be placed before their sibling
  elements in the order of the numbering scheme.
• guarantees that elements and attributes with the
  same did can be merged in a single scan.

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Conclusions

• XISS can efficiently process regular path
  expression queries.
• Performance improvement over the conventional
  methods by up to an order of magnitude.
• Future workoptimal page size or the break-even
  point between the two criteria.

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Thank you so much!