213Algebraic Optimization of RDF Graph Pattern Queries on MapReduce
• Perfect Triplegroup: It is an unambiguous triplegroup, which contains no
properties except those in specic star patterns. Therefore, it can be con-
sidered as a valid answer for a query. The triplegroups tg
1
and tg
2
in Figure
6.13b are examples of perfect triplegroups.
Since the current TG _ GroupFilter semantics assume perfect triplegroups as
input, we need to extend its denition to deal with ambiguous triplegroups. Specically,
the TG _ GroupFilter needs to split any ambiguous triplegroup ta into a set of sub-
groups (not necessarily disjoint) representing all perfect triplegroups that are derivable
ta with respect to the query. However, care must be taken not to introduce ...