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

# MVCC (Multi-Version Concurrency Control)

Multi-Version Concurrency Control 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.

## Overview

HeliosDB implements PostgreSQL-compatible MVCC with Serializable Snapshot Isolation (SSI). Key capabilities:

- **Isolation without locking**: Readers don't block writers and vice versa
- **Snapshot consistency**: Each transaction sees a consistent view of the database
- **Conflict detection**: Automatic detection of write-write and read-write conflicts
- **PostgreSQL compatibility**: Same isolation levels and behavior

## Quick Start

### 1. Basic Transaction Usage

```sql
-- Start transaction (uses default isolation level)
BEGIN;
  SELECT * FROM accounts WHERE id = 1;
  UPDATE accounts SET balance = balance - 100 WHERE id = 1;
COMMIT;
```

### 2. Isolation Levels

```sql
-- READ COMMITTED (default) - Best performance
BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;

-- REPEATABLE READ - Consistent snapshots
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;

-- SERIALIZABLE - Strictest, detects conflicts
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
```

### 3. Serializable Conflict Detection

```sql
-- Transaction 1
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
UPDATE accounts SET balance = balance + 50 WHERE id = 1;
-- Don't commit yet...

-- Transaction 2 (concurrent)
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
UPDATE accounts SET balance = balance - 30 WHERE id = 1;
COMMIT;  -- Succeeds (first committer wins)

-- Transaction 1 continues
COMMIT;  -- Fails with serialization_failure
```

## When to Use MVCC

| Use Case | Recommended Isolation |
|----------|----------------------|
| High-concurrency applications | READ COMMITTED |
| Long-running reports | REPEATABLE READ |
| Financial transactions | SERIALIZABLE |
| Inventory management | SERIALIZABLE |
| Analytics queries | READ COMMITTED |
| Batch data exports | REPEATABLE READ |

## Performance Characteristics

| Isolation Level | Overhead | Use When |
|----------------|----------|----------|
| READ COMMITTED | ~0% | High throughput needed |
| REPEATABLE READ | <1% | Consistent reads required |
| SERIALIZABLE | 2-5% | Strict consistency critical |

## Maintenance

> **`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.

```sql
-- Clean up old row versions
VACUUM ANALYZE;

-- Monitor version chains
SELECT relname, n_live_tup, n_dead_tup
FROM pg_stat_user_tables;
```

## Documentation

| Document | Description |
|----------|-------------|
| [MVCC_CONFIGURATION_GUIDE.md](https://www.heliosdb.com/docs/full/features/mvcc/mvcc_configuration_guide/) | Detailed configuration and setup |
| [Quick Start](https://www.heliosdb.com/docs/full/quick-starts/features/mvcc_quick_start/) | Fast getting started guide |
| [Deadlock Prevention](https://www.heliosdb.com/docs/full/guides/user/deadlock_prevention_operations_guide/) | Handling deadlocks |

## Related Features

- **Transactions**: `/docs/features/transactions/`
- **Distributed Transactions**: Two-phase commit across nodes
- **Deadlock Detection**: Automatic deadlock resolution