OLAP Hybrids

It is interesting to note that many products are beginning to create hybrid technologies that use both ROLAP and MOLAP. Oracle Express, for example, was originally implemented with an internal multidimensional database and required data to be pre-summarized and loaded. Today, Oracle Express has been extended to allow ROLAP functionality, and an Express user may query either from the multidimensional database or directly from Oracle relational tables. Oracle Express stores both the definition of the multidimensional data model as well as the mapping between the OLAP model and the relational data. Unfortunately, the mapping between the OLAP model and the relational database is still difficult and usually requires the assistance of an IS professional to use Oracle's Express 4GL tool to manually maintain the mappings. Oracle is addressing this problem by creating a graphical mapping interface that will make it easier for the end user to map their OLAP requirements to the Oracle RDBMS engine.

There are upper limits on the size of multidimensional databases, and Express is suitable for storing summarized data up to about 50 GB. These types of hybrid tools are popular for applications where there are small standard summarizations that can be pre-summarized and stored in the multidimensional engine, while very large summarizations use the ROLAP component of Express. Also, the availability of Oracle’s parallel server has allowed SMP and MPP technology to be applied to operational Oracle databases, greatly improving the speed of ROLAP extracts and summarization.

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

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