Subschema Considerations

Writing a Navigational DML Program › Subschema Considerations

Subschema Considerations

A subschema is a program's view of the database; it typically defines a subset of the records and record elements contained in the schema. The following rules apply to subschema usage:

Any number of subschemas can be associated with a single schema.
Any number of programs can share a subschema.
A program can have only one subschema.

Comparing Subschema and Schema

The table below compares the features and characteristics of subschemas and schemas.

Subschema	Schema
One or more per database	One per database
A program's view of the database (subset of records and record elements)	Complete database description (all records and record elements)
Source description resides in the DDLDML area of the dictionary	Source description resides in the DDLDML area of the dictionary
Source description used at DML program compile time	Source description not used at DML program compile time
A load module resides in the DDLDCLOD area of the dictionary or in a load (core-image) library	No load module
Load module used at run time	Not used at run time

Subschema Access Restrictions

The subschema may place restrictions on the DML statements that can be used to access database records. For example, you may be able to retrieve a record but not modify or erase it.

DBA-designated access restrictions, defined in the subschema, control program access to the database. Restrictions can be placed on:

Areas-Access restrictions placed on areas prevent programs from readying them in specified usage modes (see Area Usage Modes).
For example, a subschema with an update access restriction on the ORG-DEMO-REGION area can prevent programs from readying that area in any update mode.
Records-Access restrictions placed on records prohibit programs from performing one or more of the following DML functions against the specified record types:
- STORE .li ERASE
- CONNECT .li FIND
- MODIFY .li GET
- DISCONNECT .li KEEP
For example, ERASE IS NOT ALLOWED for the OFFICE record type prohibits a program using the subschema from erasing OFFICE record occurrences.

Note: The DML OBTAIN statement is a combination of FIND and GET; access restrictions on either FIND or GET will affect the use of OBTAIN.
Sets-Access restrictions placed on sets prohibit programs from performing one or more of the following DML functions against record occurrences in the specified set:
- CONNECT .li FIND
- DISCONNECT .li KEEP
For example, DISCONNECT IS NOT ALLOWED for the JOB-EMPOSITION set prohibits a program from disconnecting EMPOSITION occurrences from the JOB-EMPOSITION set.

If your program issues a DML statement that is prohibited in the subschema, the DBMS returns a status of nn10 in the ERROR-STATUS field in the IDMS communications block. The IDMSRPTS utility (discussed below) produces listings of any access restrictions that apply to a given subschema.

Program Registration

The DBA can specify in the subschema that each program that is to use the subschema must be defined in the dictionary before compilation under one of the precompilers. If program registration is in effect, you should ensure that the name listed in the PROGRAM-ID statement (for COBOL) matches the program name registered with IDD.

IDMSRPTS Utility

The IDMSRPTS utility produces listings that describe the database definition (that is, the schema and all associated subschemas). These reports are useful in all phases of program development; they provide the following information:

Names of all records, sets, and areas included in the subschema
Names, attributes, and positions of all elements included in each subschema record
Storage mode of each record
Access restrictions
Set characteristics

IDMSRPTS Parameters

The table below lists the parameters of the IDMSRPTS utility that are most useful to applications programmers.

Parameter	Requested information
RECDES	All records and record elements defined in the schema
SETDES	Set name, owner, membership options, and linkage options for all sets defined in the schema
SUBREC	All records and record elements defined in the subschema; access restrictions placed on records
SUBSET	Set name, owner, membership options, and linkage options for all sets defined in the subschema; access restrictions placed on sets
SUBAREA	Usage modes applicable to subschema areas, default usage modes; access restrictions placed on areas

For more information on the IDMSRPTS utility, see CA IDMS Utilities Guide.