---
title: "MVCC (Multi-Version Concurrency Control) Quick Start"
---

# 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)

```sql
-- Check MVCC status
SHOW mvcc_enabled;

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

### 2. Understanding Transactions

```sql
-- 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

```sql
-- 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

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

### 2. Long-Running Reports
```sql
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
```sql
-- Real-time queries run alongside OLTP
-- Each gets consistent view
-- No interference or locking
```

## Performance Tips

> **`ANALYZE` in the current release:** `ANALYZE` (including `VACUUM ANALYZE`) is accepted for PostgreSQL compatibility and returns immediately; it does not collect planner statistics yet. Statistics collection is planned. `EXPLAIN ANALYZE`, which runs the query and reports actual rows and timing, works today.

1. **Periodic Vacuum**: Clean up old versions
   ```sql
   VACUUM ANALYZE;
   VACUUM FULL; -- When needed
   ```

2. **Monitor Version Chains**: Long version chains slow queries
   ```sql
   SELECT pg_stat_user_tables.relname,
          n_live_tup,
          n_dead_tup
   FROM pg_stat_user_tables;
   ```

3. **Adjust Retention**: Balance freshness with performance
   ```sql
   SET mvcc_retention_period = '30 minutes';
   ```

4. **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

1. Review `/docs/features/mvcc/MVCC_CONFIGURATION_GUIDE.md` for detailed configuration
3. Read transaction isolation guide for advanced scenarios

## Related Features

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

---

**Document Version**: 1.0
**Audience**: Database developers, DBA operations
**Reading Time**: 5 minutes