oracle hints简介

hints翻译成中文就是提示，暗示的意思，它在数据库中作用就是更改SQL语句的执行方式，你可以使用hints强制sql按照你所设置的方式执行sql，一般用来做性能诊断和调优，不建议在开发中使用。

1.写一条SQL，使它通过全表扫描方式的效率优于索引访问，分别给出各自的执行计划。

LEO1@LEO1> create table leo1 as select * from dba_objects;      创建leo1表

Table created.

LEO1@LEO1> create index idx_leo1 on leo1(object_id);            在这个object_id列上创建索引

Index created.

LEO1@LEO1> execute dbms_stats.gather_table_stats('LEO1','LEO1',cascade=>true);  分析表和索引

PL/SQL procedure successfully completed.

LEO1@LEO1> select count(*) from leo1;         表上有71958行记录

COUNT(*)

---------------

71958

LEO1@LEO1> select /*+ full(leo1) */ * from leo1 where object_id>100;

71859 rows selected.

Execution Plan

----------------------------------------------------------

Plan hash value: 2716644435

--------------------------------------------------------------------------

| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |

--------------------------------------------------------------------------

|   0 | SELECT STATEMENT  |      | 71862 |  6807K|   287   (1)| 00:00:04 |

|*  1 |  TABLE ACCESS FULL| LEO1  | 71862 |  6807K|  287   (1)| 00:00:04 |

--------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("OBJECT_ID">100)

Statistics

----------------------------------------------------------

1  recursive calls

0  db block gets

5762  consistent gets                 5762次一致性读

0  physical reads

0  redo size

3715777  bytes sent via SQL*Net to client

53214  bytes received via SQL*Net from client

4792  SQL*Net roundtrips to/from client

0  sorts (memory)

0  sorts (disk)

71859  rows processed

LEO1@LEO1> select /*+ index(leo1 idx_leo1) */ * from leo1 where object_id>100;

71859 rows selected.

Execution Plan

----------------------------------------------------------

Plan hash value: 1434365503

----------------------------------------------------------------------------------------

| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |

----------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT            |          | 71862 |  6807K|  1232   (1)| 00:00:15 |

|   1 |  TABLE ACCESS BY INDEX ROWID| LEO1     | 71862 |  6807K|  1232   (1)| 00:00:15 |

|*  2 |   INDEX RANGE SCAN         | IDX_LEO1 | 71862 |       |  160   (0)| 00:00:02 |

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