XMLtoRelational Schema Mapping Algorithm ODTDMap

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XML-to-Relational Schema Mapping Algorithm ODTDMap

• Speaker Artem Chebotko
• Email artem_at_wayne.edu
• Wayne State University
• Joint work with Mustafa Atay, Shiyong Lu and
  Farshad Fotouhi

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Introduction

• XML has emerged as the standard for representing
  and exchanging data on the World Wide Web.
• The increasing amount of XML documents requires
  the need to store and query XML documents
  efficiently.

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Current approaches of storing and querying XML
documents

• Native XML repositories, e.g., Software AGs
  Tamino, eXcelons XIS.
• XML-enabled commercial database systems such as
  SQL Server, Oracle, and DB2
• Using RDBMS/ODBMS to store and query XML
  documents.

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Issues of the relational approach

• Schema Mapping
• XML data model needs to be mapped into the
  relational model
• Data Mapping
• XML documents need to be shredded and composed
  into tuples to be inserted into the relational
  database
• Query Mapping
• XML queries need to be translated into SQL
  queries
• Reverse Data Mapping
• Query results need to be tagged to XML format.

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Our contributions

• We propose a schema mapping algorithm, ODTDMap,
  which generates a relational schema from an XML
  DTD for storing and querying ordered XML
  documents.
• Improvements over the existing algorithms
• Losslessness
• Efficient support for XML queries
• Completeness (recursion, set-valued attributes
  DTD operators)

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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
• An 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.
• Definition 3.2 (DTD graph) The structure of a DTD
  can be represented by a labeled graph, in which
  nodes represent elements and attributes, and
  edges represent their parent-child relationships.
  The edges are labeled by either ' (star edge)
  or , ' (normal edge) where the label ,' is not
  shown for simplicity.
• 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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Inlinable node and subtree, shared node

• Definition 3.3 (Inlinable node) Given a DTD
  graph, a node is inlinable if and only if it has
  exactly one incoming edge and that edge is a
  normal edge.
• Definition 3.4 (Inlinable subtree) Given a DTD
  graph and a node e in the graph, e and all other
  inlinable nodes that are reachable from e by
  normal edges constitute a subtree. This subtree
  is called the inlinable subtree for the node e
  (it is rooted at e).
• Definition 3.5 (Shared node) Given a DTD graph, a
  node is called a shared node if it has more than
  one incoming edge.

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Inlining

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

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Inlining (cont)
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Inlining DTD graphs
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Complexity of inlining

• Theorem 3.7 (Time Complexity)
• The time complexity of our inlining algorithm is
  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)INCORRECT
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The inlining procedure (cont)CORRECT
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Generating relational schema
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Generating schema mapping info.

• Definition 3.8 (s Mapping) s is a mapping from X
  to R, where X is the set of XML element and
  attribute types in the input XML DTD, and R is
  the set of relations in the relational database.
  Given an XML element type e, s(e) will return the
  corresponding relation that is used to store e.
  Similarly, given an XML attribute type a of
  element type e, s(e.a) will return the
  corresponding relation that is used to store a of
  e.

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A complete example
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DTD graphInlined DTD graph
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Generated relational schema
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Conclusions

• We defined the schema mapping algorithm ODTDMap,
  which has several improvements over the existing
  ones.
• It is lossless in the sense that one can
  reconstruct original XML document in the given
  document order, based on the target relational
  schema generated by ODTDMap.
• It has efficient support for recursive queries
  and schemas.
• It defines how to map set-valued XML attributes.
• Experimental results showed good performance and
  scalability of the algorithm.

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Future work

• Extending our work to XML Schema to support data
  types other than string type.
• Maintain the ID/IDREF/IDREFS in terms of key and
  foreign key constraints.