Oracle查询使用源表中的虚拟列重写

Question

我在Oracle 11g中有一个表demo_fact,它有几个虚拟列定义如下:

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, DEMO_AGG_MV, with table, DEMO_FACT, 
           not possible
QSM-01102: materialized view, DEMO_AGG_MV, requires join back to table, 
           DEMO_FACT, on column, 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？

Answer 1

我不知道这对您有多大帮助，但 Oracle 要求物化视图分组的所有列都包含在要重写的语句中。（至少与虚拟列一起编辑。这可能是“不是设计使然”......）

如果你explain_rewrite尝试

 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

它应该告诉您它已经重写了查询。

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

insert into tq84_virt_col values ('B', 'Y', 11, 1);
insert into tq84_virt_col values ('B', 'X', 12, 0);
insert into tq84_virt_col values ('B', 'X', 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：它按列分组a和b：

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 

正如您所观察到的，将不会使用物化视图：

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, requires join back to table, TQ84_VIRT_COL, on column, VIRT_COL
QSM-01082: Joining materialized view, TQ84_MAT_VIEW, with table, TQ84_VIRT_COL, not possible
QSM-01102: materialized view, TQ84_MAT_VIEW, requires join back to table, TQ84_VIRT_COL, on column, C

现在，两列a和b都被选择并分组（使用外部查询来确保相同的结果集）：

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, TQ84_MAT_VIEW, using text match algorithm
QSM-01219: no suitable materialized view found to rewrite this query