index_ss hint 使用的执行计划变化对比
其中 buffer 代表：当前操作中发生的内存读次数，包含一致性读和当前读
虽然 emp 表记录数不多，但是buffer 读内存的次数差别还是有点大的
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 |
------------------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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.