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Oracle Tips 

by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

How to create a clustered index

For Oracle data warehouses updates are usually done on a time-based formula, so the issues becomes one of re-sequencing the index and insuring that additions to the index are managed.  The most obvious key for an Oracle data warehouse would be the date column.  Since Oracle data warehouses are updated in batch mode periodically, the physical sequence of the data with the index could be maintained if we insure that the new records are pre-sorted in date order. 

Note:  A Date sequence for a clustered index is only beneficial if your system performs a lot of range scans by date.

Here are the options for appending new rows onto the end of a physically sequenced Oracle table:

1. Pre-sort the extract files in index key order and load with SQL*Loader.

2.  Extract the data directly from an Oracle OLTP system using date predicates:

     INSERT INTO warehouse_table
           SELECT * FROM oltp_table@remote_instance
           ORDER BY trans_date;

Note: Oracle export/import utilities have no mechanism for changing the physical sequence of tables, and cannot be used to cluster an index.

Now that we understand the basic constructs of Oracle indexes, let’s look at a dictionary query (Listing 8.2) that will tell us the structure of our indexes. Note that this query assumes that your Oracle database is using the cost-based optimizer, and that your tables have been analyzed with the ANALYZE TABLE

command. Here we see that the indexes are grouped according to the tables that they are built upon.  We also see that the clustering factor for each index is computed as a percentage of the number of rows in the index.
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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