Semi-determinism (extended abstract)
Jan Van den Bussche, Dirk Van Gucht · 1992
We investigate under which conditions a non-deterministic query is semi-deterministic, meaning that two different results of the query to a database are isomorphic. We also consider uniform semi-determinism, meaning that all intermediate results of the computation are isomorphic. Semi-determinism is a concept bridging the new trends of non-determinism and object generation in database query languages. Our results concern decidability, both at compile time and at run time; expressibility of the infamous counting queries; and completeness, which is related to the issue of copy elimination raised by Abiteboul and Kannelakis.