4. The document search method according to claim 1 , wherein combining further comprises: registering the search result generated from executing the first search expression at the private level as a cache item using a hash value generated from the first search expression.
4. The document search method according to claim 1 , wherein combining further comprises: registering the search result generated from executing the first search expression at the private level as a cache item using a hash value generated from the first search expression. 5. The document search method according to claim 4 , further comprising: generating the search result set at the private level by generating a union based on a search result at the shared level and the search result at the private level.