8. The apparatus according to claim 5 , wherein the structured document processing apparatus further stores respective relevant lexical identifiers of the lexical items, assigns structure model tree identifiers to a plurality of nodes of a structure model tree, and assigns a structure model tree identifier of a node where one of the relevant lexical identifiers occurs most frequently as a relevant structure model tree identifier, the structured document search apparatus further comprises a lexical score value assigning unit configured to assign a relative value of one of the lexical items to each of the lexical items as a lexical score value based on the relevant lexical identifiers and the relevant structure model tree identifiers, the value increasing with an increase of number of the lexical items having an identical relevant lexical identifier which matches with the lexical items having an identical relevant structure model tree identifier, and wherein when the query language conducts search processing including a plurality of keywords, and when combining processing of index information is executed in the search processing, the priority calculation unit calculates the priority processing levels of the processes for making the plan based on the lexical score values, the lowest processing cost, and the structure score values.
5. A structured document search apparatus comprising: a connection unit connectable to a structured document processing apparatus that stores a structure model tree indicating a typical structure of a structured document, and frequency-of-occurrence information indicating locations of each of lexical items included in the structured document; a reception unit configured to receive query data which is described in a query language and is required to generate a new composite document formed of partial structures of a plurality of structured documents; a parsing unit configured to parse the query data including elements; a query graph generation unit configured to generate a query graph indicating a relationship among the elements based on the parsed query data; a selection unit configured to select, based on the frequency-of-occurrence information and the query graph, a similar structure similar to the structure model tree from the structured document processing apparatus; a storage unit configured to store similarities indicating similar levels between lexical items; a calculation unit configured to calculate a structure score value indicating a similar degree of the selected similar structure based on the similarities; a cost calculation unit configured to calculate a plurality of processing costs for selecting plans indicating processing orders of elements based on the frequency-of-occurrence information; a selection unit configured to select, from the plans, a plan which has a lowest processing cost of the calculated processing costs; a priority calculation unit configured to calculate processing priority levels of a plurality of processes for making the plan based on the lowest processing cost and the structure score values; a processing execution unit configured to execute the processes in descending order of the processing priority levels; and a result acquisition unit configured to acquire execution results of execution of the plurality of processes as search results.