9. A computer-readable medium, comprising: a plurality of computer-executable instructions, which, when executed by one or more processors, cause the one or more processors to: receive a search query; receive information identifying documents selected in response to the search query; generate a plurality of substrings from the search query, each of the substrings including at least two words; detect, for one or more substrings, of the plurality of substrings generated from the search query, one or more actual occurrences of the one or more substrings in the identified documents; calculate, for each of the one or more substrings generated from the search query, a value that indicates a fraction of the documents in which an actual occurrence of the each substring is detected; determine, based on the calculated values, that at least one of the one or more substrings consists of words that, when taken together, form a single compound unit, the computer-executable instructions that cause the one or more processors to determine which of the one or more substrings consist of words that, when taken together, form a single compound unit, including computer-executable instructions that cause the one or more processors to: determine which calculated values, of the calculated values, exceed a threshold value associated with substrings consisting of words that, when taken together, form respective single compound units; and identify a set of relevant documents based on determining that the at least one of the one or more substrings form the single compound unit.
9. A computer-readable medium, comprising: a plurality of computer-executable instructions, which, when executed by one or more processors, cause the one or more processors to: receive a search query; receive information identifying documents selected in response to the search query; generate a plurality of substrings from the search query, each of the substrings including at least two words; detect, for one or more substrings, of the plurality of substrings generated from the search query, one or more actual occurrences of the one or more substrings in the identified documents; calculate, for each of the one or more substrings generated from the search query, a value that indicates a fraction of the documents in which an actual occurrence of the each substring is detected; determine, based on the calculated values, that at least one of the one or more substrings consists of words that, when taken together, form a single compound unit, the computer-executable instructions that cause the one or more processors to determine which of the one or more substrings consist of words that, when taken together, form a single compound unit, including computer-executable instructions that cause the one or more processors to: determine which calculated values, of the calculated values, exceed a threshold value associated with substrings consisting of words that, when taken together, form respective single compound units; and identify a set of relevant documents based on determining that the at least one of the one or more substrings form the single compound unit. 10. The computer-readable medium of claim 9 , where the search query includes a set of words, where a particular substring, of the plurality of substrings, includes only words included in the search query.