Oracle Tips  by Burleson Consulting

The Data Warehouse Development Life Cycle

Parallelism And Oracle Data Warehousing

Coordination Of Concurrent Tasks

For an Oracle warehouse to function properly, Oracle needs to manage concurrent tasks, as well as serial tasks. One of the most common bottlenecks to the management of parallel processes is the need to synchronize certain tasks. Let's return to our previous scenario where a customer places an order for a product. While we can concurrently check a customer's credit rating and payment history, both of these tasks, as well as the check inventory task, must be completed before the next processing phase begins. In Oracle parallel server, the synchronization of concurrent tasks is maintained by the Distributed Lock Manager (the DLM). In some cases, the benefits of parallelism can be lost if the DLM spends too much time waiting for a single process to complete.

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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