Hippo
Jan Chomicki a System for Computing
University at Buffalo Consistent Query Answers
Jerzy Marcinkowski
to a Class of SQL Queries
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Enforcing data consistency no longer applicable: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 rewriting
For query Q construct Q’ which evaluation returns
consistent answers of Q.
Logic programming
Use disjunctive program to specify repairs and
query result.
Condensed representations of repairs
Conflict Hypergraphs
Vertex – database tuple
(J.S.,BUF) (J.S.,CHO)
Edge – conflicting tuples
Name Town
(D.G.,BUF)
J. Smith Buffalo
J. Smith Chicago
D. Gibs Buffalo (M.A.,BUF)
M. Adams Buffalo
M. Adams Chicago (M.A.,CHO)
M. Adams New York
Repair – Maximal Independent Set (M.A.,NYC)
Hippo – System Description
Conflict hypergraph – stored in RAM
Denial integrity constraints
[ R1 (t1 ) Rn (t n ) ]
Queries: , /,, and pseudo
SQL frontend – RDMBS independent
Platform independent (Java2)
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