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Hippoa System for Computing Consistent Query Answers to a Class of SQL Queries
Jan Chomicki
University at Buffalo
Jerzy Marcinkowski
Wroclaw University
Slawomir Staworko
University at Buffalo
Motivation - Inconsistent data
Enforcing data consistency no longer applicable: Data Integration – Consistent data sources, but
inconsistent global view. Long-running transactions. Efficiency reasons.
Consistent Query Answers
Repair Instance satisfying the constraint. The set of changes is minimal.
There can be an exponential number of repairs.
Tuple t is a consistent answer to Q if t is an answer to Q in every repair.
Computing CQA
Query rewritingFor query Q construct Q’ which evaluation returns
consistent answers of Q.
Logic programmingUse disjunctive program to specify repairs and
query result.
Condensed representations of repairs
Conflict Hypergraphs
Vertex – database tuple Edge – conflicting tuples
Name Town
J. Smith Buffalo
J. Smith Chicago
D. Gibs Buffalo
M. Adams Buffalo
M. Adams Chicago
M. Adams New York
(J.S.,BUF)
(D.G.,BUF)
(M.A.,BUF)
(M.A.,CHO)
(M.A.,NYC)
(J.S.,CHO)
Repair – Maximal Independent Set
Hippo – System Description
Conflict hypergraph – stored in RAM Denial integrity constraints
Queries: SQL frontend – RDMBS independent Platform independent (Java2)
])()([ 11 nn tRtR pseudo and ,/,,
Hippo is fast
Selection and Join – as fast as underlying database system.
(QR takes approx. twice the time) Union and Difference – takes approx. Twice
the time of simple query evaluation.
(QR the same for difference).
Future Work
Projection– In general problem is co-NP-data-complete.– Find an efficient heuristic.– Characterize hypergraphs where projection is
easy
Preferences– User provides preferences on resolving conflicts.– Computing Preferred CQA still easy.
References
1. M. Arenas, L. Bertossi, J. Chomicki. Consistent Query Answers in Inconsistent Databases. PODS’99
2. J. Chomicki, J. Marcinkowski. Minimal Change Integrity Maintenance using Tuple Deletions. Under revision for Information and Computation.
3. J. Chomicki, J. Marcinkowski, S. Staworko. Computing Consistent Query Answers using Conflict Hypergraphs. Under conference submission.
4. http://www.cse.buffalo.edu/~chomicki