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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.
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