317Experimental Cloud Query Processing System for Social Networks
the query involves a bound variable, (e.g., [?Y ub:subOrganizationOf <http://www.U0.edu>]), then,
before making the key–value pairs, a bound component selectivity estimation can be applied. The
resulting estimate for the TPs will account for the cost of the Map Output phase. The selected triples
are written to a local disk.
Reduce Input (RI) phase: In this phase, the triples from the Map Output phase are read via HTTP
and then sorted based on their key values. After sorting, the triples with identical keys are grouped
together. Therefore, the cost estimation for the RI phase is equal to the MO phase. The number of key–
value pairs that are sorted in RI is equal to the number o ...