Composite Key
A composite key, also known as a multi-column key, is a primary or alternate key in a database table (or an index in a file) that consists of two or more columns (or fields) whose combined values uniquely identify each row (or record). It ensures uniqueness and provides a means to quickly access specific data based on the combination of these attributes within the mainframe environment. A A composite key, also known as a multi-column key, is a primary key or unique key in a database table that consists of two or more columns. The combination of values across these columns uniquely identifies each row in the table, rather than any single column doing so individually. It is essential when a single attribute cannot guarantee uniqueness for a record.
Key Characteristics
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- Uniqueness Constraint: The *combination* of values across all columns in the key must be unique for each row/record; individual columns within the key may not be unique on their own.
- Data Integrity Enforcement: Ensures entity integrity by guaranteeing that each row in a
DB2table or record in aVSAMfile can be uniquely identified by the composite key's values. - Indexing Foundation: Often forms the basis for a
composite index(e.g., inDB2orVSAM KSDS alternate indexes) to optimize data retrieval performance for queries involving these columns. - Order Significance: The order of columns within a composite key (and its corresponding index) can significantly impact
DB2access path selection and overall query performance. - Referential Integrity: Can be used as a
foreign keyin another table to establish relationships between entities, where the foreign key also consists of multiple columns referencing the composite primary key.
Use Cases
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- Relational Database Primary Keys: Defining a
PRIMARY KEYfor aDB2table where a single column is insufficient to guarantee uniqueness (e.g., aTRANSACTION_DETAILtable might use(TRANSACTION_ID, ITEM_NUMBER)as its primary key). - IMS Secondary Indexing: Creating a
secondary indexin anIMSdatabase to allow direct access to segments based on a combination of fields that are not part of the primarysequence field. - VSAM KSDS Alternate Indexing: Building an
alternate indexover aVSAM KSDSfile using acomposite keyto provide an additional access path to records based on multiple data fields. - Partitioning Keys: In
DB2partitioned tablespaces, acomposite keymight be used as thepartitioning keyto logically distribute data across multiple physical partitions for manageability and performance. - Efficient Join Conditions: Optimizing
SQLqueries that join multiple tables where the join predicate involves multiple columns that together form a logical key, leveraging acomposite index.
- Relational Database Primary Keys: Defining a
Related Concepts
A composite key is a fundamental concept in relational database design and data modeling on z/OS, contrasting with a simple key which uses only a single column. It is crucial for maintaining data integrity and establishing referential integrity between tables in DB2. Its effective implementation often relies on the creation of composite indexes to ensure optimal query optimization and efficient data access, particularly in high-volume transaction processing environments like CICS and Batch jobs.
- Careful Column Ordering: For
DB2indexes, place the most selective column first, or columns most frequently used inWHEREclauses,GROUP BY, orORDER BYconditions, to optimize access paths. - Minimize Key Columns: Include only the necessary columns to ensure uniqueness. Adding unnecessary columns increases storage overhead, can degrade index maintenance performance, and impact
I/O. - Choose Efficient Data Types: Select appropriate and efficient data types for key columns to minimize storage requirements and improve comparison performance during index lookups.
- Monitor Performance and Usage: Regularly monitor the performance of queries utilizing composite keys and their associated indexes, especially after data volume changes or application updates, using
DB2monitoring tools. - Document Key Definitions: Clearly document the purpose, composition, and ordering of all composite keys to aid in maintenance, understanding of the data model, and future development efforts.