1. A method performed by a server device, the method comprising: receiving, by the server device, information identifying documents selected in response to a search query; generating, by a processor of the server device, a plurality of substrings from the received search query, each of the plurality of substrings including at least two words; detecting, by the processor of the server device, an actual occurrence of one or more substrings, of the plurality of substrings, in one or more of the documents; calculating, by the processor of the server device, 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 substring is detected; determining, by the processor of the server device, based on the calculated value for a particular substring of the one or more substrings, that the particular substring consists of words that form a single compound unit, the determining including: determining that the calculated value exceeds a threshold associated with compound units of words; and identifying, by the processor of the server device, a set of relevant documents based on determining that the particular substring forms the single compound unit.
3. The method of claim 1 , where the calculated values are based on a ranking defined by relevance of the documents to the search query, where substrings, of the plurality of substrings, that occur in more relevant ones of the documents are assigned higher calculated values than other substrings, of the plurality of substrings, that occur in less relevant ones of the documents.