oracle - 对 View 使用提示？

Question

我有一个 View ，我想像这样查询我的 View 以提示基表中的一些索引，我可以这样做吗？

我的意思是:

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

我有一个 索引在 table1.col1 称为“ index1 ”。

我有一个 查询 :

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

当我看到这个查询的解释计划时，它告诉我该查询不使用“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;

我可以指出意见的提示吗？？ (如果我不想在创建此 View 期间指出它)

Answer 1

您可以对针对 View 的查询使用提示，以强制 Oracle 使用基表上的索引。但是您需要知道基础 View 中基表(如果有)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) 创建一个包含 10,000 行相同行的表，并在该表上创建索引。索引不会有选择性，所以 Oracle 不会想要使用它

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) 验证索引没有正常使用但Oracle会在提示下使用

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 View 。通过指定查询中的 View 别名和 View 定义中的表别名，验证针对 View 的普通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，在尝试调整查询时，提示通常是最后的手段——通常最好弄清楚优化器缺少哪些信息并提供适当的统计信息，以便它可以自己做出正确的选择。这是一个更稳定的解决方案。当您减少到指定涉及多层别名的提示时，更是如此——例如，对于触摸 View 定义的人来说，通过更改表名的别名来破坏您的查询太容易了。