1. A computer implemented method implemented with a processor for using cached lexical values for RDF data, comprising: identifying a query to be processed upon RDF data; determining whether a data structure contains an entry associated with lexical mapping information corresponding to the identified query; and if the entry is determined to exist within the data structure, retrieving a lexical value for the query from a cache which stores lexical values using a prefix tree comprising one or more nodes storing common prefix portions of the stored lexical values, wherein retrieving the lexical value from the cache comprises identifying a node of the prefix tree corresponding to the entry using the lexical mapping information, and retrieving the lexical value by traversing the prefix tree from the identified node; or if the entry is determined to not exist within the data structure, retrieving the lexical value corresponding to the identified query from a database.