att_abstract={{In many applications including loosely coupled cloud databases,
collaborative editing and network monitoring, data from multiple
sources or authors is regularly used for query answering. For reasons
such as system failures, unspecified subjects or network issues,
data may be temporarily unavailable or generally nonexistent.
Hence, not all data needed for query answering may be available.

In this paper, we propose a natural class of completeness patterns,
expressed by selections on database tables, to specify complete
parts of database tables. We then show how to modify the operators
of relational algebra so that they manipulate these completeness
patterns to compute completeness patterns pertaining to query
answers. Our proposed algebra is computationally sound and complete
with respect to the information that the patterns provide. We
show that stronger completeness patterns can be obtained by considering
not only the schema but also the database instance and we
extend the algebra to take into account this additional information.
We develop novel techniques to efficiently implement the computation
of completeness patterns on query answers and demonstrate
their scalability on real data.}},
	att_categories={C_BB.1, C_NSS.2, C_IIS.6},
	att_copyright_notice={{(c) ACM, 2015. This is the author's version of the work. It is posted here by permission of ACM for your personal use. Not for redistribution. The definitive version was published in 2015 {{, 2015-05-31}}.
	author={Divesh Srivastava and Simon Razniewski and Werner Nutt and Flip Korn},
	institution={{ACM SIGMOD International Conference on Management of Data}},
	title={{Identifying the Extent of Completeness of Query Answers over Partially Complete Databases}},