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Using Oracle Read-Only Tablespaces

Oracle Tips by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Using Oracle’s Read-Only Tablespaces

When we examine the backup and recovery of an Oracle data warehouse, we generally find that the Oracle warehouse runs in two modes. As we know, most Oracle data warehouses are refreshed periodically, and we see different configurations for Oracle during load processing when compared to the configuration that is used for during query processing. During query time, the database is generally optimized for queries, and it is not uncommon to see Oracle’s recovery mechanisms disabled. Oracle provides several sophisticated mechanisms for read consistency and roll forward.

Read consistency is defined as the ability of Oracle to “fix” a query such that a 30 minute query will retrieve data as it existed when the query started, even if information is being changed while the query is running. Oracle implements read consistency by referring to the online redo logs whenever a requested data item has been changed during a query, and the “old” value for the data is read from the online redo log. Of course, using the online redo logs adds to the overhead for oracle.

Another source of update overhead occurs as Oracle archives the online redo logs. Redo logs store the before and after images of all rows that have been added, modified or deleted. In addition, the redo log contains log checkpoints such as begin-job, end-job, commit, and abort checkpoints for every transaction that runs under Oracle. As redo logs, fill, Oracle directs the online log to be archived to a disk file. Periodically, these archived redo logs are written to tape media in case the Oracle database administrator need to use them for a roll-forward.


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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