底层 sql 查询中的 oracle 索引

Question

我有一个有 2 列的 table

item_name varchar2 品牌 varchar2

他们都有位图索引

假设我为特定品牌创建了一个视图并将列重命名为 item_name，类似这样

创建视图 my_brand 作为 select item_name 作为项目来自 table x where brand='x'

我们无法在普通视图上创建索引，但 Oracle 在发出该视图的基础查询时正在做什么？如果我们从 my_brand where item='item1'?

写入 select item，是否正在使用 item_name 列的索引

谢谢

Answer 1

We can create an index on a normal view

不，你不能

SQL> create table as_idx_view (item_name varchar2(10), brand varchar2(10));

Table created.

SQL> create view as_view as select item_name item from as_idx_view where brand = 'X';

View created.

SQL> create index as_idx_view_idx1 on as_view (item);
create index as_idx_view_idx1 on as_view (item)
                                 *
ERROR at line 1:
ORA-01702: a view is not appropriate here

Answer 2

答案是“视情况而定”。索引访问路径当然是对优化器开放的一个选项；但请记住，优化器会做出基于成本的决定。所以本质上它会评估所有可用计划的成本并选择成本最低的计划。

这是一个例子：

    create table tab1 ( item_name varchar2(15), brand varchar2(15) );
    
    insert into tab1
    select 'Name '||to_char( rownum), 'Brand '||to_char(mod(rownum,10))
    from dual
    connect by rownum < 1000000
    
    commit;
    
    exec dbms_stats.gather_table_stats( user, 'TAB1' );
    
    create bitmap index bm1 on tab1 ( item_name );
    create bitmap index bm2 on tab1 ( brand );
    
    create or replace view my_brand 
    as select item_name as item from tab1 where brand = 'Brand 1';
    
    explain plan for
    select item from my_brand where item = 'Name 1001'
    
    select * from table( dbms_xplan.display )


    --------------------------------------------------------------------------------------------
    | Id  | Operation                           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
    --------------------------------------------------------------------------------------------
    |   0 | SELECT STATEMENT                    |      |     1 |    20 |     3   (0)| 00:00:01 |
    |*  1 |  TABLE ACCESS BY INDEX ROWID BATCHED| TAB1 |     1 |    20 |     3   (0)| 00:00:01 |
    |   2 |   BITMAP CONVERSION TO ROWIDS       |      |       |       |            |          |
    |*  3 |    BITMAP INDEX SINGLE VALUE        | BM1  |       |       |            |          |
    --------------------------------------------------------------------------------------------
     
    Predicate Information (identified by operation id):
    ---------------------------------------------------
     
       1 - filter("BRAND"='Brand 1')
       3 - access("ITEM_NAME"='Name 1001')

底层 sql 查询中的 oracle 索引

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