Performance 为什么Oracle在视图上采用另一个执行路径

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。这其中很好的一点是，我们需要在视图中使用一个数字（12），以便.NET应用程序将其视为一个整数。因此，在select中有一个

强制转换（字段为数字（12））

。到目前为止，如果我们在某些字段上使用where子句（0.9k），成本是很高的。但现在有趣的是，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增加到18k

在解释计划中，突然跳过了所有索引，这导致了大量的全表扫描。为什么在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  

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  

没有问题，索引命中

如果我们将上述查询放在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: 1  Bytes: 7  Cardinality: 1     
    1 INDEX UNIQUE SCAN INDEX (UNIQUE) TAB1_IDX Cost: 1  Bytes: 7  Cardinality: 1  

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  

---

问题是您正在将函数应用于列（在本例中为强制转换）。Oracle无法将您的索引用作查询状态。要解决此问题，您需要从视图中删除强制转换函数，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)    

创建表tablea（数字整数）；
插入表A
从dual connect by level中选择rownum很可能是因为这种情况消除了Oracle使用索引的可能性。但是如果看不到实际的SQL和执行计划，就很难判断这一点