PostgreSQL Sharding: Scaling Relational Databases

Source: Dev.to

Why Shard?

Key Insight: Never pick a shard key based solely on convenience; it must guarantee even data distribution and align with your most common query patterns.

Sharding Strategies

Range‑Based Example (PostgreSQL pg_partman)

-- Create a parent table
CREATE TABLE users (
    id BIGINT NOT NULL,
    name TEXT NOT NULL,
    created_at TIMESTAMP NOT NULL,
    PRIMARY KEY (id)
) PARTITION BY RANGE (id);

-- Create partitions (shards)
CREATE TABLE users_0_9999 PARTITION OF users FOR VALUES FROM (0) TO (10000);
CREATE TABLE users_10000_19999 PARTITION OF users FOR VALUES FROM (10000) TO (20000);

Hash‑Based Example (Citus Extension)

-- Enable the Citus extension
CREATE EXTENSION IF NOT EXISTS citus;

-- Create a distributed (sharded) table
SELECT create_distributed_table('users', 'id');

Directory‑Based Configuration (YAML)

sharding:
  enabled: true
  shardKey: "userId"
  shards:
    - name: shardA
      range: "0-2499"
    - name: shardB
      range: "2500-4999"
    - name: shardC
      range: "5000-7499"
    - name: shardD
      range: "7500-9999"

Implementation Steps with Citus

1. Install Citus

   psql -c "CREATE EXTENSION IF NOT EXISTS citus;"

2. Create a Distributed Table

   SELECT create_distributed_table('users', 'id');

3. Monitor Shard Health

   • Track CPU, latency, and replication lag per node.
   • Use dashboards or Prometheus‑compatible exporters.

4. Automate Re‑balancing

   • Move hot ranges to new nodes via Citus’s rebalance_table_shards or custom scripts.

5. Backup per Shard

   • Perform logical backups (pg_dump) per shard to avoid cross‑shard restore complexity.

Pitfalls & Best Practices

• Shard Key Selection: Must ensure even distribution and align with frequent query predicates.
• Cross‑Shard Queries: Expect higher latency; design schema to minimize joins across shards.
• Operational Overhead: More nodes mean more monitoring, patching, and capacity planning.
• Data Re‑balancing: Plan for automated tools; manual moves can cause downtime.
• Testing: Validate performance and failure scenarios in a staging environment before production rollout.

Getting Started

1. Prototype: Deploy a small‑scale sharded cluster on a staging environment using Citus.
2. Measure: Record query latency, throughput, and node resource utilization.
3. Iterate: Refine shard key, partition strategy, and monitoring based on observed traffic patterns.

PostgreSQL sharding empowers teams to scale out beyond the limits of a single instance, but it demands disciplined design, vigilant monitoring, and thoughtful operational practices. Start with a clear shard key, prototype a single strategy, and iterate based on real‑world traffic.