Drawbacks of Running PostgreSQL as AWS RDS

/ blopsts / By

Introduction:

While Amazon RDS for PostgreSQL offers a fully managed database service that automates many administrative tasks, it comes with several limitations and drawbacks. These restrictions arise primarily due to AWS’s management model, which prioritizes ease of use, automation, and security over full administrative control. Below are key reasons why one should not implement PostgreSQL as AWS RDS, focusing on restricted features, loss of control, and operational limitations.

1. Loss of Superuser (postgres) Privileges

❌ No Access to the PostgreSQL Superuser Account

  • In self-managed PostgreSQL, the postgres superuser has full control over all database operations, including security settings, performance tuning, and maintenance tasks.
  • In AWS RDS: The postgres role is not available. Instead, AWS provides a limited RDS admin user with restricted privileges.
  • Consequences:
    • Cannot run certain system-level commands (ALTER SYSTEM, pg_terminate_backend() on other users’ sessions, etc.).
    • Cannot modify critical configuration settings in postgresql.conf beyond what AWS allows.
    • Cannot perform manual database tuning beyond what AWS exposes via parameter groups.

2. Limited Customization of PostgreSQL Configuration

❌ Restrictions on postgresql.conf Parameters

  • On a self-managed PostgreSQL server, the postgresql.conf file can be modified to fine-tune performance settings such as:
    • Shared Buffers (shared_buffers)
    • Work Memory (work_mem)
    • Background Writer Settings (bgwriter_delay, bgwriter_lru_maxpages)
    • Autovacuum Parameters (autovacuum_vacuum_threshold, autovacuum_vacuum_cost_limit)
  • In AWS RDS:
    • These configurations are set via Parameter Groups, which provide limited control.
    • Certain performance-critical parameters cannot be modified (e.g., max_connections, random_page_cost).
    • Some changes require instance restarts, causing downtime.

3. Restricted Access to Extensions

❌ Only AWS-Approved PostgreSQL Extensions

  • PostgreSQL’s extensibility is one of its strongest features, but AWS RDS restricts which extensions can be installed.
  • Common PostgreSQL extensions NOT available in RDS include:
    • pg_partman (for advanced partitioning management)
    • pldebugger (debugging stored procedures)
    • hypopg (hypothetical index testing for query optimization)
    • pglogical (logical replication beyond native PostgreSQL streaming)
  • Impact: Users relying on unsupported extensions must re-architect their solutions or use Amazon Aurora PostgreSQL (which supports some additional extensions).

4. No Native Replication (Only AWS-Managed Replication)

❌ Cannot Use pg_basebackup or Replication Slots Fully

  • Self-Managed PostgreSQL: Supports various replication methods, including:
    • Streaming Replication with pg_basebackup
    • Logical Replication for specific tables
    • Physical Replication Slots
  • AWS RDS:
    • AWS only allows Read Replicas (AWS-managed replication).
    • Cannot set up custom failover architectures.
    • Cannot configure hot standbys with custom synchronous/asynchronous replication settings.

5. Backup and Restore Limitations

❌ No Full Physical Backups (Only AWS Snapshots)

  • On Self-Managed PostgreSQL: You can take full physical backups using:
    • pg_basebackup
    • Custom incremental backups
    • Point-in-time recovery (PITR) using WAL archiving
  • AWS RDS Restrictions:
    • Only AWS-native backups (daily automated snapshots and PITR via AWS).
    • No direct control over WAL retention policies.
    • AWS-enforced backup retention policies can result in higher storage costs.

6. High Availability (HA) and Failover Limitations

❌ No Control Over HA & Failover Mechanisms

  • Self-Managed PostgreSQL: Allows custom failover mechanisms using:
    • Patroni
    • pg_auto_failover
    • repmgr
  • AWS RDS:
    • Only Multi-AZ Failover is available (AWS decides failover timing).
    • Failover events are not user-controllable.
    • In high-traffic systems, failover time (~60 seconds) can cause major disruptions.

7. Performance Limitations

❌ No Fine-Tuned Query Execution Control

  • AWS RDS Limits Query Tuning Features:
    • Cannot modify optimizer settings (e.g., random_page_cost, effective_cache_size).
    • Limited ability to influence parallel query execution.
    • Cannot enable async I/O optimizations.

❌ Higher Latency in Large Workloads

  • RDS instances share AWS’s virtualized infrastructure, leading to performance variability.
  • I/O bottlenecks arise when running disk-intensive workloads.
  • Network latency issues when accessing RDS from AWS Lambda, EC2, or external services.

8. Higher Cost Over Time

❌ Expensive Compared to Self-Managed PostgreSQL

  • AWS RDS Charges for:
    • Compute (vCPUs)
    • Storage (GB per month)
    • IOPS (if using Provisioned IOPS)
    • Data transfer (between AZs or VPC peering)
    • Backups (beyond the free 100% of instance storage)
  • Self-Managed PostgreSQL on EC2 or On-Prem: Provides more predictable costs with full control over hardware choices.

9. Lack of Full Root OS Access

❌ Cannot Perform Low-Level OS Optimizations

  • Self-Managed PostgreSQL: Can optimize OS settings for PostgreSQL workloads:
    • Kernel tuning (e.g., vm.dirty_ratio)
    • Disk configuration (e.g., ext4, XFS, RAID arrays)
    • Dedicated read/write caching mechanisms
  • AWS RDS: No access to the underlying OS or storage configuration.

Why NOT Use AWS RDS for PostgreSQL?

AWS RDS for PostgreSQL may not be suitable if:

✔️ You require full superuser control.
✔️ You need unsupported extensions.
✔️ You rely on fine-tuned replication or backup methods.
✔️ You need low-latency, high-performance workloads.
✔️ You want predictable long-term costs without AWS overhead.
✔️ You require full failover and disaster recovery customization.

When to Consider Self-Managed PostgreSQL Instead?

📌 Performance-Critical Applications (Data warehouses, real-time analytics).
📌 Highly Available, Custom Failover Setups (Multi-region or DR clusters).
📌 Advanced Query Optimization & Debugging.
📌 Custom Backup, Archiving, and Replication Strategies.
📌 Workloads Requiring Fine-Tuned Security & Compliance Controls.

Final Verdict

AWS RDS is an excellent choice for users who need convenience, automation, and managed services. However, it imposes significant limitations in terms of control, customization, and cost. If your workload demands full administrative access, fine-tuned performance optimizations, and advanced replication capabilities, you should consider self-managing PostgreSQL on EC2, on-premises, or in a hybrid-cloud architecture.