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Q: When should I use COMMIT in DB2?
Answer:
COMMIT saves all changes since the last COMMIT and releases locks.
When to COMMIT:
- After processing a logical unit of work
- Periodically in long-running batch (every N records)
- Before ending the program successfully
Frequency Guidelines:
- Too frequent: Performance overhead
- Too rare: Lock contention, large log
- Typical: Every 100-1000 records in batch
Best Practice:
MOVE 0 TO WS-COMMIT-COUNT.
PERFORM PROCESS-RECORD.
ADD 1 TO WS-COMMIT-COUNT.
IF WS-COMMIT-COUNT >= 500
EXEC SQL COMMIT END-EXEC
MOVE 0 TO WS-COMMIT-COUNT
END-IF.
Note: In CICS, syncpoint (COMMIT) happens automatically at task end. Explicit SYNCPOINT is rarely needed.
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Q: Explain DB2 locking
Answer:
DB2 locks at row, page, table, tablespace level. X (Exclusive) for writes, S (Share) for reads. IX/IS for intent. Lock escalation moves to higher level. LOCK TABLE statement forces mode. Timeout if wait too long.
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Q: What is REPLICATE option?
Answer:
REPLICATE duplicates sequence set on each track. Each track has own copy of sequence set index. Reduces contention. Obsolete with modern systems. Uses more space.