To rebuild all statistics to all the tables and indexes and building up all histograms, you should do the following:

 

SQL> EXECUTE dbms_stats.gather_schema_stats(

                        ownname => 'Username',
                        method_opt => 'FOR ALL INDEXED COLUMNS SIZE AUTO',
                        cascade=>TRUE);


But if you want to let Oracle decide which statistices are to be rebuild, you should the following statment:

 

SQL> EXECUTE dbms_stats.gather_schema_stats(

                        ownname => 'Username',
                        options => 'GATHER AUTO',  -- for Oracle8i use GATHER only
                        cascade=>TRUE);

Good Luck!