1) Multiple Databases
Let's go over the terminology used by Rdb/VMS as compared to parallel terminology often used in discussions of relational theory:
Rdb/VMS Term Relational Technology Term
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Relation Table
Record Row (Tuple)
Field Column (Attribute)
System relations Metadata
In Rdb/VMS, you can choose to store relations (tables) in a single file or in separate files. A database that stores relations in one file with file type RDB is a single-file database. You can also have a database in which all metadata and system information are stored in a root file (file type RDB) and the data are stored in one or more storage area files (default file type RDA). You can place each relation in a separate file, several relations in one file, or several relations in separate files. A database with a root file and one or more storage area is a multifile database.
As you can see, each database can store one or more relations (tables) in one or more files.
Data Access Language will open more than one database using the OPEN dbbrand DATABASE "databasename"statement or command. For example:
open rdb database "daldemo"
will open the "daldemo" database that contains one or more tables.
You can use another OPEN rdb DATABASE to open a new database. The newly opened database becomes the default database for the session. This means all subsequent unqualified table references refer to this database. You can, however, use the USE DATABASE dbalias to establish a previously-opened database as the current default database. Each Open Database statement requires a USE LOCATION statement if the Rdb databases are not all in the same location.
2) CPU Use
There are no rough figures on how much CPU power DAL uses because many factors affect the performance of the system. In theory, DAL overhead should be minimum compared to one imposed by the DBMS. If you monitor top CPU usage on the VAX, you will notice that percentage used by DAL server is quite low.