public static HashSet<String> buildGlobalKeywordSet() {
		String sql = "select distinct word from inverted_index";
		HashSet<String> wordSet = new HashSet<String>();
		
		try{
			conn = DBConnection.getConnectionInstance();
			
			PreparedStatement preparedStatement = conn.prepareStatement(sql);
			
			ResultSet rs = preparedStatement.executeQuery();
			while(rs.next()) {
				wordSet.add(rs.getString(1));
			}
				
		}
		catch(Exception e) {
			System.out.println("read word failure!");
			e.printStackTrace();
		}
		
		if(wordSet.size() == 0)
			return null;
		
		return wordSet;
	}