Schema Mapping as Query Discovery

1
Schema Mapping as Query Discovery

• Renee J. Miller
• Laura M. Haas
• Mauricio A. Hernandez
• Presented by Helen Chen

2
Introduction

• Modern applications need schema mappings
• Current schema mapping process is done manually
• In Clio, schema mapping query discovery
• Modern DBMS manage not only data but also queries

3
Introduction (cont)

• Schema mappings cannot be fully automated
• Outside sources are needed
• Clio is a prototype tool for semi-automated
  schema mapping/query discovering

4
Characteristics of Clio

• Clio is VC driven
• VCs are an appropriate abstraction for eliciting
  information from the user or DBA
• Using reasoning about queries and query
  containment can help the user derive correct
  schema mappings

5
Principle in Mapping Construction

• All possible values in source ? target
• Use union rather than join
• A value from the source ? target
• Use join rather than cross product
• Override the principles is permitted

once
6
Search Space

• Vertical compositions (join)
• Requires to consider mappings between schemas
  with constraints and dependencies
• Horizontal compositions (set operators)
• Source and target schemas do not represent the
  same information

7
Query Discovery Notation

• Let S1, Sn represent the n source relation
• Let T1, Tm represent the m target relation
• Use symbol A to denote source attributes
• The domain of an attribute A is denoted dom(A)
• The meta-data associated with A is denoted ?(A)
• Use symbol B to denote target attributes

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Query Discovery Notation (cont)

• Value correspondence ? i ltfi, pigt
• A function (fi)
• q gt1
• fi dom(A1) x dom(Aq) x m(A1) x m(Aq) ?
  dom(B)
• A filter (pi)
• pi dom(A1) x dom(Ar) x m(A1) x m(Ar) ?
  boolean

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Core Query Discovery Algorithm
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Example

• Consider the following value correspondences
• f1 S1.A ? T.C
• f2 S2.A ? T.D
• f3 S2.B ? T.C
• All three filters are True

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Example (cont)

• P ?1, ?2,?2, ?3,?1,?2,?3
• G ?1, ?2,?2, ?3,?1,?2,?3
• Cover
• ?1 ?1, ?2,?2, ?3
• ?2 ?1,?2, ?3
• SQL Query

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Another Example
f1 PayRate(HrRate)WorkdOn(Hrs) ? Personnel(Sal)
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Another Example (cont)
f2 Professor(Sal) ? Personnel(Sal) p2 True
f1 PayRate(HrRate)WorkdOn(Hrs) ?
Personnel(Sal) p1 True
q3 SELECT P.HrRateW.Hrs FROM
PayRate P, WorksOn W, Student S
WHERE P.Rank W.ProjRank AND
S.Yr P.Rank UNION ALL SELECT
Sal FROM Professor
? ?1, ?2
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Incremental Query Discovery Algorithm
?u

SQL Query
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Conclusion

• Schema mapping construction process is searching
  for the most reasonable mapping
• Clio uses VCs to help users create schema
  mappings
• Clio can produce both flat and nested relational
  targets
• VC framework can be extended to both GAV and LAV

16
Limitation

• VCs are entered by user of linguistic techniques
  semi-automated