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OCP Instructors Guide for Oracle DBA Certification

Chapter 5 - Oracle Database Objects

Single Table Hash Clusters

Oracle allows users to store multiple tables in a hash cluster. Table rows that have matching cluster key values are stored in the same data block.  As a result, applications that retrieve rows from clustered tables based on the tables’ cluster key values reduce I/O consumption by being able to find rows from both tables in the same data block.

The Oracle9i Performance and Tuning Guide states, “ In an ordinary hash cluster, Oracle scans all the rows for a given table in the block, even if there is only one row with the matching key. Oracle8i introduced single-table hash clusters. If there is a one-to-one mapping between hash keys and data rows (like a primary key or unique key index), Oracle is able to locate a single row without scanning all rows in the block.”  As a result, hash clusters should only be used in very specific circumstances. But under those circumstances, hash clusters provide high performance access to Oracle data.

Oracle9i External Tables

Seasoned data warehouse administrators know that getting data out of the data warehouse is not the only challenging issue they must address. Extracting, transforming and loading data into the data warehouse can also be quite formidable (and quite expensive) tasks.

Before we begin our discussion on data warehousing, we need to understand that the data warehouse always contains data from external sources. The data is extracted from the source systems, transformed from operational data to decision support data using transformation logic, and ultimately, loaded into the data warehouse tables. This process of extracting data from source systems and populating the data warehouse is called Extraction, Transformation and Loading or ETL. Shops deploying data warehouses have the options of purchasing third-party ETL tools or writing scripts and programs to perform the transformation process manually.

The above text is an excerpt from:

OCP Instructors Guide for Oracle DBA Certification
A Study Guide to Advanced Oracle Certified Professional Database
Administration Techniques

ISBN 0-9744355-3-8

by Christopher T. Foot

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