Primary Key

Caché provides two ways to uniquely identify a row in a table: the RowID and the Primary Key.

The optional primary key is a meaningful value an application can use to uniquely identify a row in the table (for example in joins). A primary key can be user-specified data field or can be a combination of more than one data fields. Primary key values must be unique, but are not required to be integer values. The RowID is an integer value used internally to identify a row in the table. Often the primary key is a value generated by the application, while the RowID is a unique integer value generated by Caché.

The system automatically creates a Master Map to access rows of data using the RowID field. If you define a primary key field, the system automatically creates and maintains a primary key index.

Obviously, the duality of having two different fields and indexes to identify rows might not necessarily be a good thing. You can resolve to a single row identifier and index in either of two ways:

Use the application-generated primary key value as the IDKEY. You can do this by identifying the primary key index in the class definition with both keywords PrimaryKey and IdKey (you can also do that from DDL if you set the PKey is IDKey flag for this purpose). This makes the primary key index the table's Master Map. Thus, the primary key will be used as the main internal address for the rows. This can be less efficient if the primary key consists of more than one field, or if the primary key values are not integers.
Do not use an application-generated primary key value, but instead use the system-generated RowID integer within the application as the application-used primary key (for example in joins). The advantage of doing this is that the integer RowID lends itself to more efficient processing, including use of bitmap indices.

Depending on the nature of the application, you may wish to resolve to a single row identifier and index or to have separate indexes for the application-generated primary key and the system-generated RowID.