kup*_*upa 13 oracle indexing optimization hints sql-execution-plan
我有一个视图,我想查询我的视图,从基表提示一些索引,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....
我在table1.col1上有一个名为" index1 " 的索引.
我有一个问题:
--query
select *
from temp_view
where col1=12;
当我看到这个查询的解释计划时,它告诉我查询不使用"index1",我想表明它..
所以我希望它是,例如:
--query with hint
select /*+ index(temp_view index1)*/*
from temp_view
where col1=12;
我可以指出视图的提示吗？(如果我不想在创建此视图期间指出它)
Jus*_*ave 15
您可以对视图使用查询提示,以强制Oracle在基表上使用索引.但是您需要知道基础视图中基表的别名(如果有).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个相同行的表,并在表上创建索引.该索引不具有选择性,因此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)验证索引未正常使用,但Oracle将使用提示
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.验证针对视图的普通查询是否不使用索引,但通过在查询中指定视图别名和从视图定义中指定表别名来强制使用索引
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>
然而,所有这一切,在尝试调整查询时,一般来说都是最后的手段 - 通常更好的是找出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.这是一个更加稳定的解决方案.当你被简化为指定涉及多层别名的提示时 - 例如,通过更改表名的别名,触摸视图定义的人太容易破坏查询.