Database Optimization — Commerce

Overview

E-commerce databases face distinct query patterns: high-cardinality product filtering (category + price + attributes), session-scoped cart lookups, write-heavy order creation, and read-heavy catalog browsing that must scale to concurrent users. This skill covers identifying slow queries, designing effective indexes for product filtering, partitioning order tables, and routing read traffic to replicas.

When to Use This Skill

• When product listing pages are slow due to unindexed filter combinations (category + price + brand)
• When checkout throughput is limited by order insertion latency
• When read load on the primary database is causing write latency to increase
• When a slow query log reveals queries doing sequential scans on large tables
• When planning a database schema for a new custom e-commerce platform

Core Instructions

Step 1: Determine your situation

Database optimization applies primarily to self-hosted setups. Understand your constraints first:

Platform	Database Control	What to Optimize
Shopify	None — Shopify manages all infrastructure	Focus on Liquid template rendering speed, app performance, and Shopify's built-in query optimization via Search & Discovery app
WooCommerce	Full — you manage MySQL/MariaDB on your host	Optimize WooCommerce queries with caching plugins (Redis Object Cache, WP Rocket), add database indexes via WP Optimize plugin, and configure your hosting MySQL settings
BigCommerce	None — BigCommerce manages all infrastructure	Focus on theme performance, image optimization, and reducing third-party app overhead
Custom / Headless	Full — you own PostgreSQL (or MySQL)	Apply all the techniques below; PostgreSQL is assumed in code examples

Step 2: Quick wins for WooCommerce (managed WordPress/WooCommerce)

Before touching database indexes directly, apply these WooCommerce-specific optimizations:

1. Install Redis Object Cache (free, wordpress.org):

   • Your host must support Redis (most managed WordPress hosts — WP Engine, Kinsta, Cloudways — do)
   • Install and activate the plugin; go to Settings → Redis and click Enable Object Cache
   • This caches all WooCommerce database queries in memory, dramatically reducing repeat query times

2. Install WP-Optimize (free, wordpress.org):

   • Go to WP-Optimize → Database and run Clean database to remove orphaned order meta, expired transients, and post revisions
   • WooCommerce stores build up millions of rows of orphaned meta over time — regular cleanup is essential
   • Schedule automatic cleanup weekly

3. Enable the WooCommerce HPOS (High-Performance Order Storage):

   • Go to WooCommerce → Settings → Advanced → Features
   • Enable High-Performance Order Storage — this moves orders from WP post tables to dedicated order tables with proper indexes
   • Critical for stores with 10,000+ orders

4. Upgrade to a host with MySQL 8.0+ — older MySQL versions lack important index improvements; WP Engine, Kinsta, and Cloudways all run MySQL 8.0+

Step 3: PostgreSQL optimization for custom storefronts

---

Identify slow queries

-- Enable pg_stat_statements to find the worst offenders
CREATE EXTENSION IF NOT EXISTS pg_stat_statements;

-- Top 20 slowest queries by total cumulative time
SELECT
  round(total_exec_time::numeric, 2) AS total_ms,
  round(mean_exec_time::numeric, 2) AS mean_ms,
  calls,
  round((total_exec_time / sum(total_exec_time) OVER()) * 100, 2) AS pct_of_total,
  left(query, 200) AS query
FROM pg_stat_statements
WHERE calls > 100
ORDER BY total_exec_time DESC
LIMIT 20;

-- Diagnose a specific slow query
EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT p.id, p.name, p.price
FROM products p
JOIN product_categories pc ON pc.product_id = p.id
WHERE pc.category_id = 42
  AND p.price BETWEEN 1000 AND 5000
  AND p.status = 'active'
ORDER BY p.created_at DESC
LIMIT 24;
-- Look for "Seq Scan" on large tables — this means a missing index

Design indexes for product filtering

-- Partial index on active products only (smaller, faster)
CREATE INDEX CONCURRENTLY idx_products_status
  ON products (status) WHERE status = 'active';

CREATE INDEX CONCURRENTLY idx_products_price
  ON products (price) WHERE status = 'active';

-- Composite index for the most common filter combination
-- INCLUDE adds non-key columns for index-only scans (no table heap access)
CREATE INDEX CONCURRENTLY idx_products_listing
  ON products (status, brand_id, price, created_at DESC)
  INCLUDE (name, slug, thumbnail_url);

-- GIN index for flexible JSONB attribute filtering
-- Enables: attributes @> '{"color": "blue", "size": "M"}'
CREATE INDEX CONCURRENTLY idx_products_attributes
  ON products USING gin(attributes);

-- ALWAYS index foreign keys (PostgreSQL does NOT do this automatically)
CREATE INDEX CONCURRENTLY idx_product_categories_product_id
  ON product_categories (product_id);
CREATE INDEX CONCURRENTLY idx_order_lines_order_id
  ON order_lines (order_id);

Partition the orders table by date

-- Create orders table with range partitioning on created_at
CREATE TABLE orders (
  id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  customer_id UUID NOT NULL,
  status      TEXT NOT NULL,
  total_cents INTEGER NOT NULL,
  created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
) PARTITION BY RANGE (created_at);

-- Quarterly partitions
CREATE TABLE orders_2025_q1 PARTITION OF orders
  FOR VALUES FROM ('2025-01-01') TO ('2025-04-01');
CREATE TABLE orders_2025_q2 PARTITION OF orders
  FOR VALUES FROM ('2025-04-01') TO ('2025-07-01');
-- (continue for Q3, Q4, 2026...)

-- Indexes on the parent propagate to all partitions
CREATE INDEX CONCURRENTLY ON orders (customer_id, created_at DESC);
CREATE INDEX CONCURRENTLY ON orders (status, created_at DESC);

Route reads to replicas

// lib/database.js — two connection pools
import { Pool } from 'pg';

const primaryPool = new Pool({ connectionString: process.env.DATABASE_URL, max: 20 });
const replicaPool = new Pool({ connectionString: process.env.DATABASE_REPLICA_URL, max: 50 });

export const db = {
  // Writes and anything requiring freshness — primary
  async write(sql, params = []) {
    const result = await primaryPool.query(sql, params);
    return result.rows;
  },
  // Catalog reads — replica (slight staleness is acceptable)
  async read(sql, params = []) {
    const result = await replicaPool.query(sql, params);
    return result.rows;
  },
  // Transactions — always primary
  async transaction(fn) {
    const client = await primaryPool.connect();
    try {
      await client.query('BEGIN');
      const result = await fn(client);
      await client.query('COMMIT');
      return result;
    } catch (e) {
      await client.query('ROLLBACK');
      throw e;
    } finally {
      client.release();
    }
  },
};

Route reads correctly:

• Catalog pages, product search, order history → db.read() (replica)
• Cart operations, checkout, inventory decrement → db.write() or db.transaction() (primary)

Use keyset pagination (never OFFSET for large catalogs)

-- OFFSET 10000 reads and discards 10,000 rows — slow at scale
-- Use keyset pagination instead: pass the last row's cursor values

-- First page
SELECT id, name, price, created_at FROM products
WHERE status = 'active'
ORDER BY created_at DESC, id DESC
LIMIT 24;

-- Next page (pass last row's created_at and id as cursor)
SELECT id, name, price, created_at FROM products
WHERE status = 'active'
  AND (created_at, id) < ('2025-03-01T12:00:00Z', 'uuid-of-last-row')
ORDER BY created_at DESC, id DESC
LIMIT 24;

Best Practices

• Use EXPLAIN (ANALYZE, BUFFERS) to validate index usage — EXPLAIN alone shows estimates; ANALYZE runs the query and shows actuals; "Seq Scan" on a large table means a missing index
• Create indexes CONCURRENTLY — without CONCURRENTLY, index creation locks the table for writes; always use it in production
• Index all foreign keys — PostgreSQL does not auto-index foreign keys; customer_id, order_id, and product_id in join tables must be explicitly indexed
• Set work_mem carefully — increasing work_mem speeds up sorting but multiplies with connection count; benchmark before raising it
• Run VACUUM ANALYZE regularly — table bloat from dead tuples slows all queries; configure autovacuum aggressively on high-write tables like carts and sessions

Common Pitfalls

Problem	Solution
Index not used for multi-column filters	Composite index column order matters: equality columns first (status, brand_id), range columns last (price, created_at)
Slow JSONB attribute filtering	Add a GIN index on the full attributes column for @> containment queries; use expression indexes for range queries on specific JSON keys
Read replica lag causing stale cart data	Route cart reads to primary; only route catalog and order history reads to replica where slight staleness is acceptable
Partition pruning not working	Ensure WHERE clause includes the partition key (created_at) so PostgreSQL can skip irrelevant partitions
Slow pagination on page 50+	Replace OFFSET with keyset pagination using the last row's values as a cursor

Related Skills

• @flash-sale-scaling
• @monitoring-alerting-commerce
• @ecommerce-caching
• @load-testing-commerce