Database Engine Selection

Database engine selection can result in cost differences of up to 80% for the same instance size. Open-source engines (MySQL, PostgreSQL, MariaDB) are significantly cheaper than commercial engines (Oracle, SQL Server). Aurora offers premium performance but at a higher price point than standard RDS. Always match the engine to your workload requirements and budget constraints.

MySQL, PostgreSQL, MariaDB

• Lowest cost tier in RDS
• No licensing fees included
• Similar pricing across all three engines
• PostgreSQL slightly higher for larger instances
• Best for: Cost-conscious workloads, web applications, general-purpose databases

Typical On-Demand Pricing (db.r5.xlarge):

• MySQL: ~$0.48/hour
• PostgreSQL: ~$0.48/hour
• MariaDB: ~$0.48/hour

Oracle and SQL Server

• Highest cost tier in RDS
• License cost included in instance price
• Can be 80%+ more expensive than open-source
• BYOL option available to reduce costs
• Best for: Applications requiring specific Oracle/SQL Server features

Typical On-Demand Pricing (db.r5.xlarge):

• SQL Server Standard: ~$2.50/hour
• Oracle Standard: ~$1.50/hour (License Included)
• Oracle BYOL: ~$0.48/hour (same as open-source)

Aurora MySQL and Aurora PostgreSQL

• 10-20% higher than standard RDS MySQL/PostgreSQL
• Superior performance (5x MySQL, 3x PostgreSQL throughput)
• Built-in high availability (Multi-AZ by default)
• Storage charged per GB-month plus I/O operations
• Best for: High-performance OLTP, enterprise workloads

Cost Components:

• Instance hours (compute)
• Storage (per GB-month)
• I/O requests (per million)
• Backup storage (beyond free tier)

License Included Model

• License cost bundled into hourly rate
• No license management required
• Higher hourly cost but simpler
• Available for: Oracle, SQL Server

When to Use:

• No existing licenses
• Want simplified management
• Short-term or variable workloads
• Don't want to track license compliance

BYOL Model

• Use existing Oracle licenses on RDS
• Reduces cost to open-source levels
• Requires Oracle License Mobility
• Must track license compliance yourself

When to Use:

• Have unused Oracle licenses
• Enterprise Agreement with Oracle
• Long-term, stable workloads
• Cost reduction is priority

Free Tier Available:

• Oracle BYOL: db.t3.micro eligible
• SQL Server Express: db.t2.micro eligible

Aurora includes Multi-AZ replication by default at no extra compute cost. Data is automatically replicated across three Availability Zones. For standard RDS engines, enabling Multi-AZ doubles your instance costs.

Standard RDS Read Replicas:

• Billed as separate DB instances
• Same price as primary instance (if same size)
• No data transfer charges within same Region
• Cross-Region replicas: $0.02/GB data transfer

Aurora Read Replicas:

• Share underlying storage with primary
• Only pay for compute (instance hours)
• Storage not duplicated = significant savings
• Up to 15 replicas with millisecond lag

Choosing Oracle or SQL Server without verifying the need adds 80%+ to database costs. Always validate that Oracle/SQL Server-specific features are truly required before selecting commercial engines.

While Aurora has higher per-instance costs, its shared storage model, built-in Multi-AZ, and efficient read replicas often result in lower total cost of ownership for enterprise workloads.

Enabling Multi-AZ on all databases doubles costs for standard RDS engines. Reserve Multi-AZ for production workloads where high availability is critical. Development and test environments rarely need it.

Read replicas don't need to match the primary instance size. Size them based on actual read traffic requirements, which are often lower than write traffic needs.

Instance cost is just one factor. Consider storage, I/O, data transfer, backup storage, and operational overhead when comparing engines. Aurora's I/O charges can add up for I/O-intensive workloads.