16. The method according to claim 14 , further comprising: assigning 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 calculating the processing priority levels includes calculating 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.
14. The method according to claim 13 , further comprising: broadening a range until a lexical item having not less than a frequency of occurrence is present within the range; and assigning a lexical identifier of a lexical item which has a highest frequency of occurrence within the broadened range as a relevant lexical identifier of the lexical item.