Current Row - Active database row

Enhanced Definition

In the context of mainframe database access (e.g., DB2, IMS), the **current row** refers to the specific database record or segment that a program is actively positioned on and processing at a given moment. It represents the data that has been retrieved into the program's work area or is the target for subsequent update or deletion operations.

Key Characteristics

- Program's Focus: It is the single record or segment currently in scope for a program's data manipulation operations (e.g., FETCH in DB2, GET NEXT in IMS, UPDATE, DELETE).
- Cursor/Position Dependent: For relational databases like DB2, the current row is typically determined by the position of an SQL cursor. For hierarchical databases like IMS, it's determined by the current position established by DL/I calls.
- Transient State: The concept of a current row is transient; it changes as the program iterates through a result set or navigates a database structure.
- Locking Implications: When a program is positioned on a current row, especially for update or delete operations, the row (or the page/block containing it) is often locked to prevent other concurrent transactions from modifying it until the current transaction commits or rolls back.
- Work Area Association: The data of the current row is typically moved into host variables (COBOL PIC clauses) within the application program's working storage for processing.

Use Cases

- Iterating through a Result Set (DB2): A COBOL program uses a DECLARE CURSOR and OPEN CURSOR statement, then repeatedly executes FETCH to retrieve each row into host variables, making each fetched row the current row in sequence.
- Updating a Specific Record: After fetching a row and making it current, an UPDATE table-name SET column = value WHERE CURRENT OF cursor-name statement is used to modify the data of that specific current row.
- Deleting a Specific Record: Similarly, a DELETE FROM table-name WHERE CURRENT OF cursor-name statement removes the current row from the database.
- Navigating IMS Segments: An IMS GET NEXT (GN) call retrieves the next segment in a defined path, making it the current segment for the application program.
- Processing Batch Reports: A batch COBOL program reads records from a DB2 table or IMS database sequentially, processing each record as it becomes the current row.

Related Concepts

The current row is intrinsically linked to the concept of a cursor in DB2, which provides a mechanism to process a result set one row at a time. In IMS, it relates to the Program Specification Block (PSB) and Data Language Interface (DLI) calls that establish and maintain the program's position within the hierarchical database. It is fundamental to transaction management and concurrency control, as the current row is often the subject of database locking to ensure data integrity during updates or deletions. Understanding the current row is crucial for managing programmatic access to data and ensuring data consistency in multi-user environments.

Best Practices:

Minimize Lock Duration: When a current row is locked for update, process it quickly and COMMIT or ROLLBACK the transaction as soon as possible to release locks and improve concurrency.
Efficient Cursor Usage: For DB2, use the FOR UPDATE OF clause in DECLARE CURSOR if updates are intended, as it can help DB2 optimize locking. Close cursors promptly when no longer needed.
Error Handling: Always check SQLCODE or SQLSTATE after FETCH operations. A NOT FOUND condition (SQLCODE +100) indicates no more rows, preventing infinite loops.