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我们正在使用 oracle 10g 数据库上的视图向 .NET 应用程序提供数据。这样做的好处是我们在视图中需要一个数字 (12)，因此 .NET 应用程序将其视为一个整数。所以在选择中有一个cast(field as NUMBER(12)). 到目前为止，成本是如果我们在某些字段上使用 where 子句 0.9k。但是现在有趣的是，如果我们对此进行查看并使用 where 子句查询该视图，则成本从 0.9k 变为 18k。

在解释计划中突然跳过所有索引，这导致大量的全表扫描。为什么当我们使用视图时会发生这种情况？

问题的简化版本：

SELECT CAST (a.numbers AS NUMBER (12)) numbers
  FROM tablea a
 WHERE a.numbers = 201813754;

解释计划：

Plan
SELECT STATEMENT  ALL_ROWSCost: 1  Bytes: 7  Cardinality: 1     
    1 INDEX UNIQUE SCAN INDEX (UNIQUE) TAB1_IDX Cost: 1  Bytes: 7  Cardinality: 1

没有问题索引命中

如果我们将上述查询放在一个视图中并执行相同的查询：

SELECT a.numbers
  FROM index_test a
 WHERE a.numbers = 201813754;

没有使用索引。解释计划：

Plan
SELECT STATEMENT  ALL_ROWSCost: 210  Bytes: 2,429  Cardinality: 347     
    1 TABLE ACCESS FULL TABLE TABLEA Object Instance: 2  Cost: 210  Bytes: 2,429  Cardinality: 347
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1 回答 1

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问题是您正在将函数应用于列（在本例中为强制转换）。Oracle 不能使用您拥有的索引作为您的查询代表。要解决此问题，您需要从视图中删除 cast 函数，或创建基于函数的索引：

create table tablea (numbers integer);

insert into tablea
  select rownum from dual connect by level <= 1000;

create index ix on tablea (numbers);

-- query on base table uses index
explain plan for 
 SELECT * FROM tablea
 where  numbers = 1;

SELECT * FROM table(dbms_xplan.display(null,null, 'BASIC +PREDICATE'));

---------------------------------                                                                                       
| Id  | Operation        | Name |                                                                                       
---------------------------------                                                                                       
|   0 | SELECT STATEMENT |      |                                                                                       
|*  1 |  INDEX RANGE SCAN| IX   |                                                                                       
---------------------------------                                                                                       

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

   1 - access("NUMBERS"=1)                 

create view v as 
 SELECT cast(numbers as number(12)) numbers FROM tablea;

-- the cast function in the view means we can't use the index
-- note the filter in below the plan
explain plan for 
 SELECT * FROM v
 where  numbers = 1;

SELECT * FROM table(dbms_xplan.display(null,null, 'BASIC +PREDICATE'));

------------------------------------                                                                                    
| Id  | Operation         | Name   |                                                                                    
------------------------------------                                                                                    
|   0 | SELECT STATEMENT  |        |                                                                                    
|*  1 |  TABLE ACCESS FULL| TABLEA |                                                                                    
------------------------------------                                                                                    

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

   1 - filter(CAST("NUMBERS" AS number(12))=1)   

-- create the function based index and we're back to an index range scan
create index iv on tablea (cast(numbers as number(12)));

explain plan for 
 SELECT * FROM v
 where  numbers = 1;

SELECT * FROM table(dbms_xplan.display(null,null, 'BASIC +PREDICATE'));

---------------------------------                                                                                       
| Id  | Operation        | Name |                                                                                       
---------------------------------                                                                                       
|   0 | SELECT STATEMENT |      |                                                                                       
|*  1 |  INDEX RANGE SCAN| IV   |                                                                                       
---------------------------------                                                                                       

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

   1 - access(CAST("NUMBERS" AS number(12))=1)
于 2013-06-19T11:35:58.943 回答