MVCC (Multi-Version Concurrency Control) Quick Start¶

Overview¶

MVCC allows multiple transactions to access the same data simultaneously without blocking each other. Each transaction sees a consistent snapshot of the database at a specific point in time.

Key Concepts¶

What is MVCC?¶

Isolation without locking: Readers don't block writers and vice versa
Snapshot consistency: Each transaction sees a consistent view of the database
Historical data: Multiple versions of rows are maintained temporarily

When to Use MVCC¶

High-concurrency applications
Long-running transactions (reports, batch jobs)
Systems requiring snapshot isolation
Applications with frequent read-write conflicts

Quick Start¶

1. Enable MVCC (Usually Default)¶

-- Check MVCC status
SHOW mvcc_enabled;

-- Configure MVCC settings
SET mvcc_retention_period = '1 hour';
SET mvcc_gc_interval = '5 minutes';

2. Understanding Transactions¶

-- Start transaction (implicit in most cases)
BEGIN;
  SELECT * FROM accounts WHERE id = 1;
  UPDATE accounts SET balance = balance - 100 WHERE id = 1;
COMMIT;

-- Each transaction gets a snapshot at START

3. Isolation Levels¶

-- Snapshot Isolation (default)
SET TRANSACTION ISOLATION LEVEL SNAPSHOT;

-- Serializable (strictest)
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

-- Read Committed (looser)
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

4. Conflict Detection¶

-- HeliosDB automatically detects write conflicts
BEGIN;
  UPDATE products SET price = 99 WHERE id = 1;
COMMIT;  -- Succeeds or fails depending on conflicts

-- Check transaction status
SELECT pg_blocking_pids();  -- See blocking info

Common Use Cases¶

1. Concurrent Updates¶

-- Transaction 1 & 2 run simultaneously
-- Both see their own snapshot
-- Changes don't interfere
MVCC handles this automatically

2. Long-Running Reports¶

BEGIN;
  -- Report queries see consistent snapshot
  -- Other transactions modify data
  -- Report isn't blocked or affected
SELECT * FROM large_table;
COMMIT;

3. Real-Time Analytics¶

-- Real-time queries run alongside OLTP
-- Each gets consistent view
-- No interference or locking

Performance Tips¶

Periodic Vacuum: Clean up old versions

VACUUM ANALYZE;
VACUUM FULL;  -- When needed

Monitor Version Chains: Long version chains slow queries

SELECT pg_stat_user_tables.relname,
       n_live_tup,
       n_dead_tup
FROM pg_stat_user_tables;

Adjust Retention: Balance freshness with performance
```
SET mvcc_retention_period = '30 minutes';
```
Use Appropriate Isolation Levels
Use READ COMMITTED for most applications
Use SERIALIZABLE only when needed
Snapshot Isolation for balance

Troubleshooting¶

Q: Why are queries slow?¶

A: Long version chains. Run VACUUM ANALYZE.

Q: Conflicts between transactions?¶

A: Use SERIALIZABLE isolation if needed, but impacts performance.

Q: Database grows quickly?¶

A: Increase VACUUM frequency or reduce retention period.

Best Practices¶

Use explicit transactions for related operations
Avoid very long transactions
Run VACUUM during low-traffic periods
Monitor version tuple statistics
Use appropriate isolation levels per use case

Next Steps¶

Review /docs/features/mvcc/MVCC_CONFIGURATION_GUIDE.md for detailed configuration
Check implementation details in /docs/implementation/features/mvcc/
Read transaction isolation guide for advanced scenarios

Transactions: /docs/features/transactions/
Distributed Transactions: /docs/features/transactions/distributed/
Deadlock Detection: /docs/guides/user/DEADLOCK_PREVENTION_OPERATIONS_GUIDE.md

Document Version: 1.0 Last Updated: December 30, 2025 Audience: Database developers, DBA operations Reading Time: 5 minutes