Introduction to Physical Design › Physical Database Structures

Physical Database Structures

Once you have created your design, you perform the necessary calculations to determine the amount of space required by your database, and then implement the database design using SQL or non-SQL data definition statements.

For further information on sizing the database, see Chapter 15, "Determining the Size of the Database". For further information on implementing the design, see the chapter Implementing Your Design.

No matter how you choose to define the database, certain physical database structures are used by CA IDMS/DB to implement your design.

For further information on physical database concepts, see the CA IDMS Concepts and Facilities Guide and CA IDMS Database Administration Guide.

Areas and pages

CA IDMS/DB subdivides the physical database into separate areas, each consisting of a set of contiguously numbered pages.

Areas are stored in operating system files, each page corresponding to one or more direct access blocks. CA IDMS/DB usually transfers an entire page of data in a single input/output operation.

While some database pages are reserved for space management, the majority of pages are used to hold user data in the form of entity occurrences. Each entity occurrence corresponds to a single row of an SQL-defined table or an instance of a record defined by a non-SQL schema.

A page can contain as many entity occurrences as space availability permits.

Segments

A segment defines the areas and files that contain the data in the database. A segment represents a physical database usually defined by a single schema. For the database to access the segment at runtime, the segment must be included in the definition of a DMCL.

DMCL

A DMCL is a collection of segment definitions that can be accessed in a single execution of CA IDMS/DB. The DMCL also specifies buffer characteristics, describes the buffer and files for journaling database activity, and identifies a database name table that the database uses at runtime to map a logical (or schema) definition of the database to specific segments.

A DMCL exists as a load module in a load (core-image) library and is used at runtime to determine where data required by an application is physically stored.

More Information

For more information on segments and the DMCL, see the CA IDMS Database Administration Guide.

Database keys

CA IDMS/DB assigns a database key (db-key) to each record occurrence when it is entered into the database. The database key is the concatenation of the number of the page on which a record occurrence is stored and a line number. A line number is an index to an eight-byte structure called a line index. The line index is used to locate the record occurrence within the page. The database key uniquely identifies the record with which it is associated and never changes as long as the record remains in the database.

Structure of the physical database

The diagram below shows how areas, pages, and entity occurrences appear in the database.

The EMPDATA database area contains four pages and five entity occurrences. Each of the entity occurrences is uniquely identified by a database key. For example, the database key for the Mary Bliss occurrence is 1001:1.