A New Inlining Algorithm for Mapping XML DTDs to Relational Schema

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A New Inlining Algorithm for Mapping XML DTDs to
Relational Schema

• Speaker Shiyong Lu
• Email shiyong_at_cs.wayne.edu
• Wayne State University
• Joint work with Yezhou, Mustafa and Farshad

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Introduction

• XML is rapidly emerging on the World Wide Web as
  a standard for representing and exchanging data.
• The amount of XML documents is increasing each
  day.
• It is critical to store and query XML documents
  efficiently and effectively.

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Current approaches of storing and querying XML
documents

• Native XML repositories, e.g., Software AGs
  Tamino 2, eXcelons XIS 1.
• XML support enabled by commercial database
  systems such as SQL Server, Oracle, and DB2 in
  which XMLType is introduced.
• Use RDBMS/ODBMS to store and query XML documents.
  8, 10, 16, 11.

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Issues of the relational approach

• XML data model needs to be mapped into the
  relational model
• XML queries need to be translated into SQL
  queries
• Query results need to be tagged to XML format.

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Our contributions

• We proposed a new inlining algorithm to map DTDs
  to relational schemas.
• Improvements over the shared-inlining 16
• Completeness
• Redundancy elimination for shared nodes
• Optimizations
• Efficiency

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Outline of the talk

• Introduction of XML DTDs
• Mapping DTDS to relational schemas
• Simplifying DTDs
• Creating and inlining DTD graphs
• Generating relational schemas
• An example
• Conclusions and future work

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An overview of DTDs A DTD example

• lt!DOCTYPE memo
• lt!ELEMENT memo (to, from, date, subject?, body)gt
• lt!ATTLIST memo security CDATAgt
• lt!ATTLIST memo lang CDATAgt
• lt!ELEMENT to (PCDATA)gt
• lt!ELEMENT from (PCDATA)gt
• lt!ELEMENT date (PCDATA)gt
• lt!ELEMENT subject (PCDATA)gt
• lt!ELEMENT body (para)gt
• lt!ELEMENT para (PCDATA)gt

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DTD Document Type Defintion

• lt!DOCTYPE root-element doctype-declaration...
• lt!ELEMENT element-name content-modelgt, content
  model , ,, , , ?
• lt!ATTLIST element-name attr-name attr-type
  attr-default ...gt

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DTD Document Type Definition (cont)

• lt!ATTLIST element-name attr-name attr-type
  attr-default ...gtdeclares which attributes are
  allowed or required in which elements attribute
  types
• CDATA any value is allowed (the default)
• (value...) enumeration of allowed values
• ID, IDREF, IDREFS ID attribute values must be
  unique (contain "element identity"), IDREF
  attribute values must match some ID (reference to
  an element)
• ENTITY, ENTITIES, NMTOKEN, NMTOKENS, NOTATION
  just forget these... (consider them deprecated)
• attribute defaults
• REQUIRED the attribute must be explicitly
  provided
• IMPLIED attribute is optional, no default
  provided
• "value" if not explicitly provided, this value
  inserted by default
• FIXED "value" as above, but only this value is
  allowed

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Mapping DTDs to relational schemas

• Simplifying DTDs
• Creating and inlining DTD graphs
• Generating relational schemas

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Simplifying DTDs

• A DTD might be very complex due to nesting, e.g.,
  ltELEMENT a ((b, c, d?)?, (e?, f, (g, h?))?)gt
• A XML query language is concerned about
• The parent-child relationships between XML
  elements
• The relative order relationships between siblings
  (add an ordinal attribute to each relation)

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DTD simplifications rules

• e ? e
• e? ? e
• (e1 en) ? (e1, ,en)
• (a) (e1, ,en) ? (e1, ,en)
• (b) e ? e
• 5. (a) , e, , e, ?,e, ,
• (b) , e, , e, ?,e, ,
• (c) , e, , e, ?,e, ,
• (d) , e, , e, ?,e, ,

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Example of simplifying a DTD

• ltELEMENT a ((b, c, d?)?, (e?, f, (g, h?))?)gt
• simplified to
• ltELEMENT a (b, c, d, e, f, g, h)gt

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Creating and inlining DTD graphs

• We create a DTD graph based on the simplified
  DTD. In the graph, nodes represent XML elements,
  and edges represent operators.
• Idea inline a child c to its parent p if p can
  contain at most one occurrence of c.
• Rationale inlined elements will produce a
  relation.

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Inlining DTD graphs
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Inlining

• Case 1 Element a is connected to b by a ,-edge
  and b has no other incoming edges, inlining b to
  a.
• Case 2 Element a is connected to b by a ,-edge
  but b has other incoming edges, b is a shared
  node, no inlining.
• Case 3 Element a is connected to b by a -edge,
  no inlining.

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Inlinable node

• Definition 2 Given a DTD graph, a node is
  inlinable if and only if it has exactly one
  incoming edge and that edge is a ,-edge.

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Inlinable tree

• Given a DTD graph and a node e in the graph,
• node e and all other inlinable nodes that are
  reachable from e by ,-edge constitute a tree This
  tree is called the inlinable tree for node e
  (rooted at e).

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Complexity of inlining

• Theorem 2 (Complexity)
• Our inlining algorithm can be performed in O(n)
  where n is the number of elements in the input
  DTD.

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The inlining procedure
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The inlining procedure (cont)
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Generating relational schema

• For each node e, a relation e is generated with
  the following attributes
• ID is the primary key, and for each XML
  attribute A of e, a corresponding relational
  attribute A is generated with the same name.
• If e.inlinedSet gt 2, introduce attribute
  nodetype to indicate the type of the XML element
• The names of all the terminal XML elements in
  e.inlinedSet
• If there is a ,-edge from e to node c, then
  introduce c.ID as a foreign key of e referencing
  relation c.

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Generating relational schema (cont)

• If there are at least two relations t_1(ID) and
  t_2(ID) generated by step 1, then we combine all
  the relations of the form t(ID) into one single
  relation table1(ID, nodetype)
• If there are at least two relations t_1(ID, t_1)
  and t_2(ID, t_2) generated by step 1, then we
  combine all the relations of the form t(ID, t)
  into one single relation table2(ID, nodetype,
  pcdata)
• If there is at least one edge in the inlined
  DTD graph, then we introduce relation
  edge(parentID, childID, parentType, childType).

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Improvement over the shared-inlining algorithm

• Completeness
• Redundancy elimination for shared nodes
• Optimizations
• Efficiency
• See the next slide for examples.

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Examples
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A complete example
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DTD graph
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Inlined DTD graph
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Generated relational schema
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Conclusions

• We have developed a new inlining algorithm that
  maps a given input DTD to a relational schema.
• We made several improvements over the
  shared-inlining algorithm. Experimental results
  will be presented in an upcoming paper.

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Future work

• Lossless schema mapping. How to maintain the
  sibling order relationship as well, so that the
  original XML document can be reconstructed!
• Maintain the ID/IDREF/IDREFS in terms of key and
  foreign key constraints.