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by Burleson Consulting

The Data Warehouse Development Life Cycle

Distributed Oracle Data Warehouses

Table Replication With Oracle Snapshots

Oracle snapshots are used to create read-only copies of tables in other Oracle databases. This is a highly effective way to avoid expensive cross-database joins of tables. As you probably know, an SQL join with a table at a remote server is far slower than a join with a local table because SQL*Net overhead increases as it retrieves and transfers data across a network.
Note: For more information about table replication utilities see Chapter 10, Oracle Data Warehouse Utilities.

It is interesting to note that the general attitude about data replication has shifted dramatically in the past 10 years. In the 1980s, replication was frowned upon. Database designers believed that there was no substitute for the third-normal form database. Today, the practical realities of distributed processing have made replication a cheap and viable alternative to expensive cross-database joins.

Table replication is so stable and has been so successful within Oracle version 7 that Oracle is now introducing the concept of updatable snapshots with Oracle version 7.3. However, replication is not to be used indiscriminately, and the following guidelines exist for using replicated tables to the best advantage:

* Replicated tables should be read-only. Obviously, a table snapshot cannot be updated because the master copy of the table is on another server.

* Replicated tables should be relatively small. Ideally, a replicated table is small enough that the table can be dropped and re-created each night, or the REFRESH COMPLETE option can be used. Of course, large tables can be replicated with the REFRESH FAST option, but this involves a complicated mechanism for holding table changes and propagating them to the replicated table.

* Replicated tables should be used frequently. It does not make sense to replicate a table if it is only referenced a few times per day, and the cost of the replication would outweigh the cost of the cross-database join.

Despite any claims by Oracle to the contrary, snapshots are not to be used indiscriminately. Only tables that meet the above criteria should be placed in snapshots. In practice, snapshots are not maintenance-free, and many points of failure are possible--especially if the snapshot is created with the REFRESH FAST option. Problems can occur while writing to the SNAPSHOT_LOG table, and SQL*Net errors can cause update failures to transfer to the replicated tables.

This is an excerpt from "High Performance Data Warehousing". 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:


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