Oracle查询使用源表中的虚拟列重写

ALTER TABLE demo_fact ADD (demo_measure_from_virtual NUMBER GENERATED ALWAYS AS
  (CASE WHEN  demo_category_column = 20 THEN demo_numericdata_column ELSE 0 END)
  VIRTUAL VISIBLE);

然后我将物化视图定义为

CREATE MATERIALIZED VIEW demo_agg_mv 
REFRESH FORCE ON DEMAND
ENABLE QUERY REWRITE
AS 
SELECT
  demo_dim_one,demo_dim_two,SUM(demo_measure_from_virtual) demo_measure_from_virtual
  FROM demo_fact
  GROUP BY demo_dim_one,demo_dim_two

现在我想要Query Rewrite来启用以下查询：

SELECT demo_dim_one,SUM(demo_measure_from_virtual) 
FROM demo_fact
GROUP BY demo_dim_one

但事实并非如此.我运行了EXPLAIN_REWRITE,这是输出：

QSM-01150: query did not rewrite
QSM-01102: materialized view,DEMO_AGG_MV,requires join back to table,DEMO_FACT,on column,DEMO_MEASURE_FROM_VIRTUAL
QSM-01082: Joining materialized view,with table,not possible
QSM-01102: materialized view,DEMO_NUMERICDATA_COLUMN

背景故事：我正在使用70M行和50个虚拟列(所有这些都具有相同的结构,上面的简单case语句,但具有不同的比较列和不同的结果列)

这个问题似乎只在事实表具有虚拟列时才会显现,但将它们更改为非虚拟列会消耗太多磁盘空间.为什么Oracle不会重写查询？我该怎么办才能修复它？

如果您尝试explain_rewrite

select
    demo_dim_one,sum(s)
 from (
    select 
      demo_dim_one,sum(demo_measure_from_virtual) s
    from 
       demo_fact
    group by
      demo_dim_one,demo_dim_two
 )
 group by demo_dim_one

它应该告诉你它已经重写了查询.

这可以这样证明：

将定义虚拟列的表：

create table tq84_virt_col (
  a  varchar2(2),b varchar2(2),c  number,d  number 
);


insert into tq84_virt_col values ('A','X',1,1);
insert into tq84_virt_col values ('A',2,'Y',3,0);
insert into tq84_virt_col values ('A',4,1);

insert into tq84_virt_col values ('B',11,1);
insert into tq84_virt_col values ('B',12,0);
insert into tq84_virt_col values ('B',13,1);

虚拟列的定义：

alter table tq84_virt_col add (
  virt_col number generated always as (
    case when d = 1 then c else 0 end
  )
  virtual visible
);

物化视图.注意：它在列a和b上分组：

create materialized view tq84_mat_view
refresh force on demand
enable query rewrite
as 
select 
  a,b,sum(virt_col) sum_virt_col
from
  tq84_virt_col
group by 
  a,b

您已经观察到物化视图将不会被使用：

begin
  dbms_mview.explain_rewrite(
    'select a,sum(virt_col) from tq84_virt_col group by a'
  );
end;
/

select message
from rewrite_table;

QSM-01150: query did not rewrite
QSM-01102: materialized view,TQ84_MAT_VIEW,TQ84_VIRT_COL,VIRT_COL
QSM-01082: Joining materialized view,C

现在,选择并分组列a和b(使用外部查询以确保相同的结果集)：

truncate table rewrite_table;

begin
  dbms_mview.explain_rewrite(
    'select a,sum(s) from (select a,sum(virt_col) s from tq84_virt_col group by a,b) group by a'
  );
end;
/

select message
from rewrite_table;

QSM-01151: query was rewritten
QSM-01209: query rewritten with materialized view,using text match algorithm
QSM-01219: no suitable materialized view found to rewrite this query

（编辑：李大同）

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