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Oracle 8 Tips  

by Burleson Consulting

The Data Warehouse Development Life Cycle

Parallelism And Oracle Data Warehousing

Oracle Parallel Query

We can now query the ALL_CUSTOMER view as if it were a single database table, and Oracle parallel query will automatically recognize the UNION ALL parameter, firing off simultaneous queries against each of the three base tables. It is important to note that the distributed database manager will direct each query to be processed at the remote location, while the query manager waits until each remote node has returned its result set. For example, the following query will assemble the requested data from three table segments in parallel, with each query optimized separately. The result set from each sub-query is then merged by the query manager.

SELECT customer_name
FROM ALL_CUSTOMER
SEE CODE DEPOT FOR FULL SCRIPT


Instead of having a single query server manage the I/O against the table, parallel query allows the Oracle query server to dedicate many processors to simultaneously access data.

In order to be most effective, the table should be partitioned onto separate disk devices, such that each process can do I/O against its segment of the table without interfering with the other simultaneous query processes. However, the client/server environment of the 1990s relies on RAID or a logical volume manager (LVM), which scrambles data files across disk packs in order to balance the I/O load. Consequently, full utilization of parallel query involves striping a table across numerous data files, each on a separate device.


This is an excerpt from "High Performance Data Warehousing", copyright 1997. 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

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