Computing possible and certain answers over order-incomplete data

Antoine Amarilli, Mouhamadou Lamine Ba, Daniel Deutch, Pierre Senellart

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


This paper studies the complexity of query evaluation for databases whose relations are partially ordered; the problem commonly arises when combining or transforming ordered data from multiple sources. We focus on queries in a useful fragment of SQL, namely positive relational algebra with aggregates, whose bag semantics we extend to the partially ordered setting. Our semantics leads to the study of two main computational problems: the possibility and certainty of query answers. We show that these problems are respectively NP-complete and coNP-complete, but identify tractable cases depending on the query operators or input partial orders. We further introduce a duplicate elimination operator and study its effect on the complexity results.

Original languageEnglish
Pages (from-to)42-76
Number of pages35
JournalTheoretical Computer Science
StatePublished - 10 Dec 2019
Externally publishedYes


  • Certain answer
  • Partial order
  • Possible answer
  • Uncertain data

ASJC Scopus subject areas

  • Theoretical Computer Science
  • Computer Science (all)


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