Oracle Tips  by Burleson Consulting

The Data Warehouse Development Life Cycle

Aggregating Data For The Oracle Warehouse
In order to create the illusion of fast calculation time, most data warehouses are loaded in batch mode after the online system has been shut down and all the common aggregate values are rolled up into summary tables. In this sense, a data warehouse is bimodal, with a highly intensive loading window during off-hours and an intensive query window during the day. Because many data warehouses collect data from non-relational databases such as IMS or CA-IDMS, no standard methods for extracting data are available for loading into a warehouse and pre-aggregating summary data. Even when the warehouse is fed from an OLTP Oracle database, there are numerous techniques for loading in new data. However, we do have a few common techniques for extracting and loading a data warehouse from operational Oracle databases.

* Log “sniffing”--Applies the archived redo logs from the OLTP system to the data warehouse.

* Update, insert, and delete triggers--Fires off a distributed update to the data warehouse.

* Snapshot logs used to populate the data warehouse--Updates the replicated table changes.

* Nightly extract/load programs--Extracts and retrieves the operational data, and loads it into the warehouse.

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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