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.
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. 6. The apparatus according to claim 5 , wherein the parsing unit generates structure information of the query data, and the selection unit selects the similar structure similar to the structure model tree from the structured document processing apparatus based on the structure information.