Oracle Tips  by Burleson Consulting

The Data Warehouse Development Life Cycle

Aggregating Data For The Oracle Warehouse

The resulting region_item_type table might look like Table 6.1.

Date            Region               Type                    Monthly Sales
3/97          West               Clothes              $113,999
3/97          West               Hardware           $56,335
3/97          West               Food                 $23,574
3/97          East                Clothes              $45,234
3/97          East                Hardware           $66,182
3/97          East                Food                 $835,342
3/97          South              Clothes              $1,223
3/97          South              Hardware           $56,392
3/97          South              Food                 $9,281
3/97          North               Clothes             $826,463
3/97          North               Hardware           $77,261
3/97          North               Food                 $43,383

Table 6.1 A sample region_item_type table.

While this technique provides summaries for March 1997, what if we want to keep a rolling average of regions and items regardless of the date? In this case, we can use the same SQL that we used earlier, without the SQL WHERE clause. Because the values will change each time the warehouse is updated, new aggregate tables can be built in the middle of the night if need be--right after the master fact tables have been populated with the day’s sales. The next morning, the prior day’s sales will have been rolled-up into these summaries, giving management an accurate, fast, and easy-to-use tool for decision support.

Note: Parallel Create Table As Select (PCTAS) can be very useful in an Oracle data warehouse environment where tables are replicated across numerous servers or when pre-aggregating roll-up summary tables. Parallel Create Table As Select is also very useful when performing roll-up activities against your Oracle warehouse. For details on PCTAS, see Chapter 7, Parallelism And Oracle Data Warehousing.

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This is an excerpt from "High Performance Data Warehousing". To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:
http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm