Keyed
Enhanced Definition
Key Characteristics
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- Direct Access: Allows for rapid retrieval of a specific record by providing its key value, bypassing the need to read preceding records.
- Sequential Access: Records can also be processed sequentially in the logical order of their key values, which is particularly useful for batch processing and reporting.
- Key Field: A specific field or combination of fields within a record that uniquely (or non-uniquely) identifies it and is used for indexing and ordering.
- File Types: Primarily associated with VSAM Key-Sequenced Data Sets (KSDS), where the primary key determines the physical and logical order of records. It's also fundamental to database indexing in
DB2andIMS. - Alternate Keys: Some keyed file organizations (like
VSAM KSDSwithPATHs) support alternate keys, allowing records to be accessed via different fields in addition to the primary key. - Performance Optimization: Proper key design is crucial for optimizing I/O performance and reducing search times in large datasets and databases.
Use Cases
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- Customer Record Retrieval: Quickly fetching a customer's entire record from a
VSAM KSDSusing their unique customer ID as the primary key in aCICStransaction. - Inventory Management: Accessing product details in a
DB2table orIMSdatabase by using the product code as an indexed key for fast lookups. - Batch Reporting: Processing a
VSAM KSDSsequentially by its primary key to generate reports ordered by that key (e.g., employee records by employee ID). - Data Validation: Checking for the existence of a record (e.g., a valid account number) using a key before processing a transaction.
- Transaction Processing: In
COBOLprograms, performingREAD ... KEY ISoperations onVSAM KSDSfiles to locate specific records based on a key value.
- Customer Record Retrieval: Quickly fetching a customer's entire record from a
Related Concepts
The concept of "keyed" is fundamental to VSAM KSDS (Key-Sequenced Data Sets), where the primary key dictates the physical and logical order of records and enables both direct and sequential access. It is also central to database management systems like DB2 and IMS, which heavily rely on indexing (using keys) to efficiently locate and manage data within tables or segments. In COBOL, operations like READ ... KEY IS directly leverage the keyed nature of files, while JCL defines the attributes of keyed datasets, including their key length and offset in IDCAMS DEFINE CLUSTER commands.
Best Practices:
- Key Selection: Choose keys that are stable, meaningful, and, if intended for direct access, as unique as possible. Avoid using volatile fields as primary keys.
- Key Length and Type: Design keys to be as short as possible while maintaining uniqueness and functionality, as longer keys can impact storage and I/O performance. Use appropriate data types (e.g.,
PACKED-DECIMALfor numeric keys). - Uniqueness Enforcement: For primary keys, ensure uniqueness is enforced either by the file system (e.g.,
VSAM KSDSwithUNIQUEKEY) or the application logic to prevent data integrity issues. - Alternate Key Usage: When multiple access paths are required, judiciously use alternate keys (
VSAM PATHs or database secondary indexes) but be mindful of the overhead they introduce during updates and maintenance. - Performance Monitoring: Regularly monitor the performance of keyed access, especially for
VSAM KSDSand database indexes, and reorganize files/tables as needed to maintain optimal access times