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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: What is SQLCODE and what are common values?
Answer:
SQLCODE is a return code in SQLCA indicating the result of SQL execution.
Common SQLCODE values:
| 0 | Successful execution |
| 100 | No data found / End of cursor |
| -803 | Duplicate key on insert |
| -811 | Multiple rows returned for singleton SELECT |
| -904 | Unavailable resource |
| -911 | Deadlock/timeout |
| -922 | Authorization failure |
| -927 | DB2 not available |
Negative = Error, 0 = Success, 100 = No data
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Q: Explain predicate types
Answer:
Stage 1 predicates evaluated by storage engine. Stage 2 evaluated by DB2 (more expensive). Indexable predicates can use index. Non-indexable force scan. Design for stage 1/indexable predicates. EXPLAIN shows predicate staging.
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Q: How to tune DB2 queries?
Answer:
Check EXPLAIN output. Look at: access type (index vs scan), join method, sort operations. Add indexes for predicates. Rewrite with hints. Update statistics. Check for full tablespace scans. Monitor actual execution.
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Q: What is referential integrity?
Answer:
RI ensures foreign key values exist in parent. CREATE TABLE child ... REFERENCES parent(key). ON DELETE CASCADE/SET NULL/RESTRICT. DB2 enforces automatically. Constraint violations return -530/-531. Design carefully.
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Q: What is XML support in DB2?
Answer:
DB2 stores XML natively. XMLTYPE column. XMLPARSE, XMLSERIALIZE for conversion. XQuery for querying. XMLTABLE extracts relational from XML. XML indexes for performance. Powerful for document storage.
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Q: What is Distributed Data Facility (DDF)?
Answer:
DDF enables remote DB2 access. TCP/IP and SNA connectivity. DRDA protocol. Location name identifies target. Three-part names: location.schema.table. Network security considerations.
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Q: Explain EXEC SQL statements
Answer:
EXEC SQL marks embedded DB2 SQL. EXEC SQL SELECT col INTO :host-var FROM table WHERE key = :key-var END-EXEC. Colon prefix for host variables. SQLCODE in SQLCA indicates result. Precompiler converts to COBOL calls.
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Q: What is a DB2 plan vs package?
Answer:
Plan is executable form of program's SQL, bound from DBRM. Package is independent unit, can be shared. Plans contain packages or direct SQL. Packages allow separate rebind. Modern approach: package collections with small plans.
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Q: What is DB2 catalog?
Answer:
Catalog tables describe DB2 objects. SYSIBM.SYSTABLES for tables, SYSIBM.SYSCOLUMNS for columns, SYSIBM.SYSINDEXES for indexes. Query catalog for metadata. Read-only except through DDL. Essential for documentation and analysis.
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Q: Explain NULL handling in DB2
Answer:
NULL means unknown/missing value. NULL != NULL returns unknown. Use IS NULL, IS NOT NULL. COALESCE(col, default) substitutes. NULL in arithmetic yields NULL. Indicator variables detect NULL in COBOL. NVL function alternative.
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Q: Explain CICS DB2 connection
Answer:
CICS connects to DB2 via thread. EXEC SQL in CICS program. RCT (Resource Control Table) defines. Thread pool for efficiency. DB2 subsystem parameter.
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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 DB2 subsystem?
Answer:
Subsystem is DB2 instance. Has unique name (4 chars). Multiple subsystems on same LPAR. Each has own catalog, logs, data. SSID in application connection. Data sharing allows multi-subsystem access.
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Q: Explain DB2 triggers
Answer:
Trigger fires automatically on INSERT/UPDATE/DELETE. CREATE TRIGGER name AFTER/BEFORE/INSTEAD OF event ON table FOR EACH ROW/STATEMENT. Use for audit, validation, derived columns. Can impact performance.
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Q: Explain DB2 logging
Answer:
DB2 logs all changes for recovery. Active log (circular), archive log (offloaded). LOG YES/NO on DDL. COMMIT writes to log. Recovery uses logs. Log full causes issues - monitor usage.
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Q: What is data sharing?
Answer:
Data sharing allows multiple DB2s to access same data. Group of subsystems share data. Uses Coupling Facility for coordination. Provides availability, scalability. Complex but powerful for high availability.
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Q: How to monitor DB2 performance?
Answer:
Use OMEGAMON, DB2PM, or built-in monitor. Check: buffer pool hit ratios, lock waits, CPU time, elapsed time. DISPLAY commands show real-time. Statistics trace for analysis. EXPLAIN for query level.
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Q: Explain storage group
Answer:
Storage group defines volumes for data. CREATE STOGROUP name VOLUMES(vol1, vol2). Tablespaces assigned to stogroups. DB2 manages space within. PRIQTY/SECQTY control allocation. Foundation of storage management.
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Q: What is zparm?
Answer:
ZPARM (DSNZPARMs) are DB2 installation parameters. Control system behavior, limits, defaults. DSNZPARM module loaded at startup. Changes need restart usually. Critical for performance and security tuning.
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Q: Explain thread management
Answer:
Thread is DB2 connection. Allied thread for TSO/batch. DBAT (database access thread) for DDF. Pool thread for efficient reuse. Max threads controlled by zparm. Monitor active threads.
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Q: What is LINEAR dataset?
Answer:
LINEAR VSAM is byte-addressable. No record structure. Used by DB2 tablespaces, system software. DEFINE CLUSTER LINEAR. Accessed via DIV (Data-in-Virtual) or memory mapping.
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Q: Explain DB2 isolation levels
Answer:
UR (Uncommitted Read) reads dirty data. CS (Cursor Stability) locks current row. RS (Read Stability) locks all accessed rows. RR (Repeatable Read) locks range, prevents phantom. Higher isolation = more consistency but more locking. CS most common.
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Q: How to do paging with DB2?
Answer:
Use FETCH FIRST n ROWS ONLY with OFFSET or ROW_NUMBER(). Modern: SELECT * FROM (SELECT ROW_NUMBER() OVER(ORDER BY col) AS rn, t.* FROM table t) WHERE rn BETWEEN start AND end. Or cursor with FETCH FIRST.
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Q: What is DB2 buffer pool?
Answer:
Buffer pool caches data/index pages in memory. BP0, BP1, etc. Hit ratio critical for performance. GETPAGE vs SYNCIO shows effectiveness. Size appropriately. Virtual buffer pool for each pool. Monitor with statistics.