![](https://csdnimg.cn/release/download_crawler_static/88306294/bg4.jpg)
hint, uses multiblock i/o, can be executed in parallel, can be used to access second
column of concatenated indexes. This is because we are selecting all of the index.
eg:
SQL> explain plan for select empno,ename from big_emp;
index skip scan --索引跳跃扫描,where 条件列是非索引的前导列情况下常发生
Index skip scan finds rows even if the column is not the leading column of a
concatenated index. It skips the first column(s) during the search.
eg:
SQL> create index i_emp on emp(empno, ename);
SQL> select /*+ index_ss(emp i_emp)*/ job from emp where ename='SMITH';
3.Rowid 物理 ID 扫描
This is the quickest access method available.Oracle retrieves the specified block and
extracts the rows it is interested in. --Rowid 扫描是最快的访问数据方式
六、表连接方式
有三种连接方式:
1、Sort Merge Join (SMJ) --由于 sort 是非常耗资源的,所以这种连接方式要避免;
Rows are produced by Row Source 1 and are then sorted Rows from Row Source 2 are then
produced and sorted by the same sort key as Row Source 1. Row Source 1 and 2 are NOT
accessed concurrently.
SQL> explain plan for
select /*+ ordered */ e.deptno,d.deptno
from emp e,dept d
where e.deptno = d.deptno
order by e.deptno,d.deptno;
Query Plan
-------------------------------------
SELECT STATEMENT [CHOOSE] Cost=17
**MERGE JOIN
****SORT JOIN
******TABLE ACCESS FULL EMP [ANALYZED]