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Enforcing data consistency no longer applicable: Data Integration – Consistent data sources, but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