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  • Full Hint

    FULL Hint可以提升优化器对指定表走全表扫描,但是FULL提示一次只能对一个表起作用

    SQL> select  ename,dept.deptno from emp,dept where emp.deptno=dept.deptno;

    已选择14行。


    执行计划
    ----------------------------------------------------------
    Plan hash value: 3074306753

    ------------------------------------------------------------------------------
    | Id  | Operation          | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
    ------------------------------------------------------------------------------
    |   0 | SELECT STATEMENT   |         |    14 |   168 |     3   (0)| 00:00:01 |
    |   1 |  NESTED LOOPS      |         |    14 |   168 |     3   (0)| 00:00:01 |
    |   2 |   TABLE ACCESS FULL| EMP     |    14 |   126 |     3   (0)| 00:00:01 |
    |*  3 |   INDEX UNIQUE SCAN| PK_DEPT |     1 |     3 |     0   (0)| 00:00:01 |
    ------------------------------------------------------------------------------

    SQL> select /*+ full(dept) */ ename,dept.deptno from emp,dept where emp.deptno=dept.deptno;

    已选择14行。


    执行计划
    ----------------------------------------------------------
    Plan hash value: 615168685

    ---------------------------------------------------------------------------
    | Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
    ---------------------------------------------------------------------------
    |   0 | SELECT STATEMENT   |      |    14 |   168 |     7  (15)| 00:00:01 |
    |*  1 |  HASH JOIN         |      |    14 |   168 |     7  (15)| 00:00:01 |
    |   2 |   TABLE ACCESS FULL| DEPT |     4 |    12 |     3   (0)| 00:00:01 |
    |   3 |   TABLE ACCESS FULL| EMP  |    14 |   126 |     3   (0)| 00:00:01 |

    SQL> select /*+ full(dept,emp) */ ename,dept.deptno from emp,dept where emp.deptno=dept.deptno;

    已选择14行。


    执行计划
    ----------------------------------------------------------
    Plan hash value: 3074306753

    ------------------------------------------------------------------------------
    | Id  | Operation          | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
    ------------------------------------------------------------------------------
    |   0 | SELECT STATEMENT   |         |    14 |   168 |     3   (0)| 00:00:01 |
    |   1 |  NESTED LOOPS      |         |    14 |   168 |     3   (0)| 00:00:01 |
    |   2 |   TABLE ACCESS FULL| EMP     |    14 |   126 |     3   (0)| 00:00:01 |
    |*  3 |   INDEX UNIQUE SCAN| PK_DEPT |     1 |     3 |     0   (0)| 00:00:01 |
    ------------------------------------------------------------------------------
    可见FULL提升失效,full只能对一个表指定全表扫描,如果想要对多个表执行全表扫描,使用多个full提升。

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  • 原文地址:https://www.cnblogs.com/hehe520/p/6330657.html
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