在数据仓库中，有事实表、维度表两个概念。
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事实表是数据仓库结构中的中央表，它包含联系事实与维度表的数字度量值和键。事实数据表包含描述业务（例如产品销售）内特定事件的数据。
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维度表是维度属性的集合。是分析问题的一个窗口。是人们观察数据的特定角度，是考虑问题时的一类属性，属性的集合构成一个维。
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如图示
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我们以sh用户下的sales表和times表来看，
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SALES为事实表
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SQL> desc sales
Name Null? Type
----------------------------------------- -------- ----------------------------
PROD_ID NOT NULL NUMBER
CUST_ID NOT NULL NUMBER
TIME_ID NOT NULL DATE
CHANNEL_ID NOT NULL NUMBER
PROMO_ID NOT NULL NUMBER
QUANTITY_SOLD NOT NULL NUMBER(10,2)
AMOUNT_SOLD NOT NULL NUMBER(10,2)
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TIMES为维度表
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SQL> desc times
Name Null? Type
----------------------------------------- -------- ----------------------------
TIME_ID NOT NULL DATE
DAY_NAME NOT NULL VARCHAR2(9)
DAY_NUMBER_IN_WEEK NOT NULL NUMBER(1)
DAY_NUMBER_IN_MONTH NOT NULL NUMBER(2)
CALENDAR_WEEK_NUMBER NOT NULL NUMBER(2)
FISCAL_WEEK_NUMBER NOT NULL NUMBER(2)
WEEK_ENDING_DAY NOT NULL DATE
WEEK_ENDING_DAY_ID NOT NULL NUMBER
CALENDAR_MONTH_NUMBER NOT NULL NUMBER(2)
FISCAL_MONTH_NUMBER NOT NULL NUMBER(2)
CALENDAR_MONTH_DESC NOT NULL VARCHAR2(8)
CALENDAR_MONTH_ID NOT NULL NUMBER
FISCAL_MONTH_DESC NOT NULL VARCHAR2(8)
FISCAL_MONTH_ID NOT NULL NUMBER
DAYS_IN_CAL_MONTH NOT NULL NUMBER
DAYS_IN_FIS_MONTH NOT NULL NUMBER
END_OF_CAL_MONTH NOT NULL DATE
END_OF_FIS_MONTH NOT NULL DATE
CALENDAR_MONTH_NAME NOT NULL VARCHAR2(9)
FISCAL_MONTH_NAME NOT NULL VARCHAR2(9)
CALENDAR_QUARTER_DESC NOT NULL CHAR(7)
CALENDAR_QUARTER_ID NOT NULL NUMBER
FISCAL_QUARTER_DESC NOT NULL CHAR(7)
FISCAL_QUARTER_ID NOT NULL NUMBER
DAYS_IN_CAL_QUARTER NOT NULL NUMBER
DAYS_IN_FIS_QUARTER NOT NULL NUMBER
END_OF_CAL_QUARTER NOT NULL DATE
END_OF_FIS_QUARTER NOT NULL DATE
CALENDAR_QUARTER_NUMBER NOT NULL NUMBER(1)
FISCAL_QUARTER_NUMBER NOT NULL NUMBER(1)
CALENDAR_YEAR NOT NULL NUMBER(4)
CALENDAR_YEAR_ID NOT NULL NUMBER
FISCAL_YEAR NOT NULL NUMBER(4)
FISCAL_YEAR_ID NOT NULL NUMBER
DAYS_IN_CAL_YEAR NOT NULL NUMBER
DAYS_IN_FIS_YEAR NOT NULL NUMBER
END_OF_CAL_YEAR NOT NULL DATE
END_OF_FIS_YEAR NOT NULL DATE
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如果我们创建一个物化视图
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create materialized view sales_month_sum
enable query rewrite as
SELECT t.calendar_month_id,
prod_id,
channel_id,
promo_id,
SUM (quantity_sold) quantity_sold,
SUM (amount_sold) amount_sold
FROM sales s, times t
WHERE s.time_id = t.time_id
GROUP BY prod_id,
channel_id,
promo_id,
t.calendar_month_id;
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如果我们做如下按月的分组查询
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SQL> alter session set query_rewrite_enabled=true;
SQL> alter session set query_rewrite_integrity=trusted;
SQL> set autotrace traceonly
SQL> set line 200
SQL> SELECT t.calendar_month_id,
prod_id,
channel_id,
promo_id,
SUM (quantity_sold) quantity_sold,
SUM (amount_sold) amount_sold
FROM sales s, times t
WHERE s.time_id = t.time_id
GROUP BY prod_id,
channel_id,
promo_id,
t.calendar_month_id;
9068 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3287305789
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