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Oracle Tips 

by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Data Warehouse Design
DE-NORMALIZING ONE-TO-MANY DATA RELATIONSHIPS

With Oracle and the rule-based optimizer, this type of complex join guarantees that at least one table is read front to back using a full-table scan. This is a shortcoming of Oracle’s rule-based optimizer because an SQL optimizer should always avoid a full-table scan whenever indexes are present--and full-table scans are very expensive. This situation might be avoided by using Oracle hints with the cost-based optimizer to determine the optimal path to this data. A hint is an extension of Oracle’s SQL that directs the SQL optimizer to change its normal access path. For more detailed information on optimizing full-table scans and using hints, refer to Chapter 13, Tuning Oracle SQL.

What if your goal is to simplify the data structure by removing several of the one-to-many relationships? Adding redundancy imposes two requirements. You need additional space for the redundant item, and you need a technique to update the redundant item if it changes. One solution is to build a table of columns that rolls the CITY and STATE tables into the CUSTOMER table. For example, Table 4.1 assumes that the STATE table contains 50 rows, the CITY table contains 2,000 rows, and the CUSTOMER table contains 10,000 rows.
Table 4.1 Redundancy matrix to determine optimal normalization.

Column             Size              Duplication           Total Space                Change
state_bird        10                     10,000               100,000                  Rare
state_flower     10                     10,000               100,000                  Rare
region_name       2                     10,000                20,000                 Never
cost_of_living     8                     10,000                80,000              Quarterly
city_mascot      10                     10,000              100,000                   Rare

In Table 4.1, you can see that the CITY and STATE tables can be removed entirely for a total savings of 400,000 bytes (see Figure 4.4).

Figure 4.4 De-normalized E/R model sales database.


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:
http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm
 

 


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