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Oracle Snapshots - How They Work

Oracle Tips by Burleson Consulting

The Data Warehouse Development Life Cycle

Distributed Oracle Data Warehouses


A snapshot is created on a destination system with the CREATE SNAPSHOT command, and the remote table is immediately defined and populated from the master table.

After creation, a snapshot may be refreshed periodically. There are two types of refreshing: complete and fast. A complete refresh can be done in several ways, but most savvy Oracle developers drop and re-create snapshots using a Unix cron job to achieve full refreshes, especially if the table is small and easily re-created. Optionally, a fast refresh can be used, which refreshes tables with only the changes made to the master table. This requires additional work on the slave database to create an Oracle refresh process (in the init.ora) and the definition of a snapshot log on the master database (see Figure 9.9).

Figure 9.9 A high-level overview of Oracle snapshots.

Several steps need to be completed before your Oracle data warehouse is ready to use snapshots. First you need to run CATSNAP.SQL, which can be found in your $ORACLE_HOME/rdbms/admin directory. This script will populate the Oracle dictionary with the necessary system tables to manage the snapshots. You’ll also need to run DBMSSNAP.SQL, which can also be found in the $ORACLE_HOME/rdbms/admin directory. This script creates the stored procedures that can be used to manipulate the snapshots. In addition, the following parameters must be added to the init.ora file before your Oracle data warehouse can use snapshots:

* SNAPSHOT_REFRESH_INTERVAL=60--This sets the interval (in minutes) for the refresh process to wake up.

* SNAPSHOT_REFRESH_PROCESSES=1--This is the number of refresh processes on the instance (the minimum is 1 refresh per instance).

* SNAPSHOT_REFRESH_KEEP_CONNECTIONS=FALSE--This specifies whether the database should keep remote connections after refreshing the tables. Always use FALSE.

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