1. A computer executable document search method for searching for a document, comprising: generating, based on user information represented as a group hierarchy having a user identifier and one or more groups joined by one or more operators, an inclusion relationship expression including an operator that designates an inclusion relationship for access authority; combining the inclusion relationship expression with a received search expression to generate a combined search expression; for searching at a private level, using the combined search expression to generate a first search expression using the received search expression and specifying the user identifier without specifying the one or more groups; and for searching at a shared level, using the combined search expression to generate a second search expression using the received search expression and specifying the one or more groups without specifying the user identifier.
1. A computer executable document search method for searching for a document, comprising: generating, based on user information represented as a group hierarchy having a user identifier and one or more groups joined by one or more operators, an inclusion relationship expression including an operator that designates an inclusion relationship for access authority; combining the inclusion relationship expression with a received search expression to generate a combined search expression; for searching at a private level, using the combined search expression to generate a first search expression using the received search expression and specifying the user identifier without specifying the one or more groups; and for searching at a shared level, using the combined search expression to generate a second search expression using the received search expression and specifying the one or more groups without specifying the user identifier. 2. The document search method according to claim 1 , further comprising: in response to a search request at the shared level, when a cache item is found at the shared level, returning a search result for the cache item; and when there is no cache item found at the shared level, executing an index search of an index based on the combined search expression to generate a new search result at the shared level.