Consistent Query Answering for Primary Keys in Logspace
Paraschos Koutris, Jef Wijsen

TL;DR
This paper classifies the complexity of consistent query answering with primary key violations, showing a dichotomy between logspace expressibility and coNP-completeness for self-join-free conjunctive queries.
Contribution
It refines previous complexity classifications by establishing a dichotomy between symmetric stratified Datalog expressibility and coNP-completeness, especially highlighting the practical case of foreign key joins.
Findings
CERTAINTY(q) is either in L or coNP-complete.
CERTAINTY(q) is in L for foreign key join queries.
Provides a practical complexity classification for common database queries.
Abstract
We study the complexity of consistent query answering on databases that may violate primary key constraints. A repair of such a database is any consistent database that can be obtained by deleting a minimal set of tuples. For every Boolean query q, CERTAINTY(q) is the problem that takes a database as input and asks whether q evaluates to true on every repair. In [KW17], the authors show that for every self-join-free Boolean conjunctive query q, the problem CERTAINTY(q) is either in P or coNP-complete, and it is decidable which of the two cases applies. In this paper, we sharpen this result by showing that for every self-join-free Boolean conjunctive query q, the problem CERTAINTY(q) is either expressible in symmetric stratified Datalog or coNP-complete. Since symmetric stratified Datalog is in L, we thus obtain a complexity-theoretic dichotomy between L and coNP-complete. Another new…
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