index_ss hint 使用的运行计划变化对照

当中 buffer 代表：当前操作中发生的内存读次数，包括一致性读和当前读

尽管 emp 表记录数不多，可是buffer 读内存的次数区别还是有点大的

SQL> select job from emp where ename='SMITH';

JOB
------------------
CLERK

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'))

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID at8ssqpn41css, child number 0
-------------------------------------
select /*+ index_ss(emp i_emp)*/ job from emp where ename='SMITH'

Plan hash value: 3956160932

------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
------------------------------------------------------------------------------------
|* 1 | TABLE ACCESS FULL| EMP | 1 | 1 | 1 |00:00:00.01 | 8 |
------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("ENAME"='SMITH')

17 rows selected.

----创建一个索引

SQL> create index i_emp on emp(empno, ename);

Index created.

SQL> select /*+ index_ss(emp i_emp)*/ job from emp where ename='SMITH';

JOB
------------------
CLERK

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'))
2 ;

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID ck2pc7bpbzdz8, child number 0
-------------------------------------
select /*+ index_ss(emp i_emp)*/ job from emp where ename='SMITH'

Plan hash value: 98078853

-----------------------------------------------------------------------------------------------
| Id | Operation                   | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-----------------------------------------------------------------------------------------------
|   1 | TABLE ACCESS BY INDEX ROWID| EMP   |      1 |      1 |      1 |00:00:00.01 |       3 |
|* 2 |                 INDEX SKIP SCAN           | I_EMP |      1 |      1 |      1 |00:00:00.01 |       2 |

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("ENAME"='SMITH')
filter("ENAME"='SMITH')

19 rows selected.