Writing Custom Projections

In most cases, the projections created by the Database Designer provide excellent query performance within physical constraints, but the accuracy of the Database Designer is limited to the information it can get from your input files, particularly the example queries. Should the projection created by the Database Designer not meet your needs, you can write custom projections as described in Defining Custom Projections in the Database Administrator's Guide (Advanced).

Vertica Systems, Inc. strongly recommends that you contact Technical Support before and after designing your physical schema. Technical Support can inspect your physical schema design for problems before you execute it.

Requirements for a K-Safe Physical Schema Design

If you choose to write custom projections, be aware that your physical schema design must meet the following requirements.

• Segmented projections must have K buddy projections (projections that have identical columns and segmentation criteria, except that corresponding segments are placed on different nodes).
• Unsegmented projections must be replicated on all nodes.

In general, only small dimension tables are unsegmented.