Determining the Size of the Database › Sizing a Megabase › Varying the Database Key Format

Varying the Database Key Format

A database key is the concatenation of an entity's page number and its line index, for a total of four bytes. The format for a database key is variable. The page number can make up 20 to 30 bits of the database key; the line index can make up 2 to 12 bits. You determine the database key format by specifying the MAXIMUM RECORDS PER PAGE clause of the CREATE SEGMENT statement.

Since database key format is variable, you can structure the database to allow for either:

• More pages with fewer entities per page—The number of pages in an area can be from 2 to 1,073,741,824.
• More entities per page with fewer pages—Each page in a database can have from 2 to 2,727 entities.

To accommodate a very large database, you need to make sure that the highest page in an area can be expressed in the database key format. You also need to ensure that the line index is large enough to identify the highest entity occurrence on a specific page.

Note: The number specified in the MAXIMUM RECORDS PER PAGE clause indicates the maximum number of entity occurrences that the run-time system will place on a single page. The actual number of occurrences on a given page depends on the page size and the size of individual entity occurrences placed on the page.