*：对应步骤有驱动或过滤条件。可以从predicate information中找到
SQL> set autotrace traceonly 
SQL> select * from scott.emp where empno=1122;
SQL> set linesize 150;
SQL> /
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2949544139
 
--------------------------------------------------------------------------------------
| Id  | Operation                    | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |             |           1 |          87 |           2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP    |           1 |          87 |           2   (0)| 00:00:01 |
|*  2 |   INDEX UNIQUE SCAN            | PK_EMP |           1 |             |           1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   2 - access("EMPNO"=1122)
 
 
Statistics
----------------------------------------------------------
  0  recursive calls
  0  db block gets
  2  consistent gets
  0  physical reads
  0  redo size
862  bytes sent via SQL*Net to client
512  bytes received via SQL*Net from client
  1  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed
 
*号表示id为2的一步，过滤条件为"EMPNO"=1122

参考文档：《基于Oracle的SQL优化》