16. The system of claim 14 , wherein the program module for generating a plurality of sub-queries that represent the graph query comprises program modules for: using a dialect of tuple-based algebra to translate the graph query into an algebraic query plan comprising a plurality of algebraic operators; and translating the algebraic query plan into the plurality of sub-queries, said translation comprising program modules for, generating one or more Structured Query Language sub-queries for the algebraic operators that are associated with relational operations in the attributed graph, and generating one or more non-relational sub-queries for the algebraic operators that are associated with traversal operations in the attributed graph.
14. A system for responding to graph queries, comprising: a hardware computing device comprising one or more processing units and memory; and a computer program having program modules executable by the processing units, the processing units being directed by the program modules of the computer program to, receive a graph query that is submitted to a graph database which is modeled by an attributed graph comprising a plurality of nodes each of which is labeled with one or more attributes, and a plurality of edges each of which is labeled with another one or more attributes, each of the nodes representing a different entity instance, each of the edges connecting a different pair of nodes, the attributes of each of the edges specifying the type of relationship that exists between the pair of nodes connected by the edge, the graph query comprising predicates based on one or more of, the topology of the attributed graph, or the attributes of the nodes and edges therein, generate a plurality of sub-queries that represent the graph query, for each of the sub-queries, identify a one of a plurality of independent query execution engines that is available to process the sub-query, send the sub-query to the identified query execution engine for processing, and receive results for the sub-query from the identified query execution engine, and combine the results received to generate a response to the graph query.