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对视图使用提示?

hints sql-execution-plan indexing optimization oracle

kupa · 技术社区 · 15 年前

我有一个视图,我想这样查询我的视图以提示来自基表的一些索引,可以这样做吗?

我是说:

--view
create or replace view temp_view
as select col1,col2,col3
from table1,table2....

我有一个指数在 表1.col1 打电话来” 索引1 ".

我有一个查询 :

--query
select * 
from temp_view 
where col1=12;

当我看到这个查询的explain plan时,它告诉我这个查询不使用“index1”,我想指出它。。

所以我希望它是,例如:

--query with hint
select /*+ index(temp_view  index1)*/* 
from temp_view 
where col1=12;

我能给你一些意见提示吗??(如果我不想在创建此视图时指出它)

2 回复 | 直到 12 年前

Justin Cave 15 年前

您可以对视图使用查询提示来强制Oracle在基表上使用索引。但您需要知道基础视图中基表(如果有的话)的别名。一般语法是 /*+ index(<<alias of view from query>> <<alias of table from view>> <<index name>>) */

一个例子

1)创建具有10000行相同行的表,并在该表上创建索引。索引没有选择性,所以Oracle不想使用它

SQL> ed
Wrote file afiedt.buf

  1  create table foo
  2  as
  3  select 1 col1
  4    from dual
  5* connect by level <= 10000
SQL> /

Table created.

SQL> create index idx_foo on foo(col1);

Index created.

2)确认索引未正常使用,但Oracle将在使用时提供提示

SQL> set autotrace traceonly;
SQL> select * from foo where col1 = 1;

10000 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1245013993

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      | 10000 |   126K|     7   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| FOO  | 10000 |   126K|     7   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("COL1"=1)

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          9  recursive calls
          0  db block gets
        713  consistent gets
          5  physical reads
          0  redo size
     172444  bytes sent via SQL*Net to client
       7849  bytes received via SQL*Net from client
        668  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      10000  rows processed

SQL> select /*+ index(foo idx_foo) */ *
  2    from foo
  3   where col1 = 1;

10000 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 15880034

----------------------------------------------------------------------------
| Id  | Operation        | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT |         | 10000 |   126K|    25   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| IDX_FOO | 10000 |   126K|    25   (0)| 00:00:01 |
----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("COL1"=1)

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          7  recursive calls
          0  db block gets
        715  consistent gets
         15  physical reads
          0  redo size
     172444  bytes sent via SQL*Net to client
       7849  bytes received via SQL*Net from client
        668  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      10000  rows processed

3)现在创建视图。通过指定查询中的视图别名和视图定义中的表别名,验证针对视图的常规查询不使用索引,而是强制使用索引

SQL> create view vw_foo
  2  as
  3  select col1
  4    from foo f;

View created.

SQL> select col1
  2    from vw_foo
  3   where col1 = 1;

10000 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1245013993

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      | 10000 |   126K|     7   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| FOO  | 10000 |   126K|     7   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("COL1"=1)

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
         16  recursive calls
          0  db block gets
        715  consistent gets
          0  physical reads
          0  redo size
     172444  bytes sent via SQL*Net to client
       7849  bytes received via SQL*Net from client
        668  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      10000  rows processed

SQL> select /*+ index(vf f idx_foo) */ col1
  2    from vw_foo vf
  3   where col1 = 1;

10000 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 15880034

----------------------------------------------------------------------------
| Id  | Operation        | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT |         | 10000 |   126K|    25   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| IDX_FOO | 10000 |   126K|    25   (0)| 00:00:01 |
----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("COL1"=1)

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
         14  recursive calls
          0  db block gets
        717  consistent gets
          0  physical reads
          0  redo size
     172444  bytes sent via SQL*Net to client
       7849  bytes received via SQL*Net from client
        668  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      10000  rows processed

SQL>

尽管如此,在试图优化查询时,提示通常是最后的手段——通常最好找出优化器缺少的信息并提供适当的统计数据,以便它能够自己做出正确的选择。这是一个更稳定的解决方案。双重原因是,当您被简化为指定涉及多个别名层的提示时——例如,对于触摸视图定义的人来说,通过更改表名的别名来中断查询太容易了。

3per 8 年前

我试过贾斯汀·卡夫的语法(旁边的答案)

select /*+ index(vf f idx_foo) */ col1
from vw_foo vf
where col1 = 1;

,但这对我不起作用。下一个是工作

select /*+ index(vf.f idx_foo) */ col1
from vw_foo vf
where col1 = 1;

我试过了 Oracle Database 11g企业版11.2.0.1.0版-64位生产