Oracle Database Performance Optimization

1. what oracle waiting for ???

   select * from
    (select count(*), event, sql_id, program, inst_id
     from gv$active_session_history
     where sample_time > sysdate-5/1440
     group by event, sql_id, program, inst_id
     order by 1 desc)
   where rownum <= 20;

2. query execution plan

   select plan_table_output from table(dbms_xplan.display_cursor('&sql_id')); --已知SQL_Id的情况下
   
   explain plan for (SQL_TEXT); select * from table(dbms_xplan.display);
   
   set autotrace on explain --显示值和执行计划
   set autotrace traceonly explain --只显示执行计划