引言：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
引言：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 |
----------------------------------------------------------------------------------------
predicate information (identified by operation id):
---------------------------------------------------
2 - access(object_id>100)
statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
10735 consistent gets 10735次一致性读
0 physical reads
0 redo size
8241805 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