If your Shopify app runs fine at 500 merchants and starts degrading at 5,000, the database layer is almost always the first thing to crack, not your app server, not your CDN, and not your queue. Here are the five patterns that cause the most production database failures in Shopify apps, and the exact fixes for each. Missing Composite Indexes on shop_id Every table in a multi-tenant Shopify app needs shop_id as the leftmost column in every composite index. A query filtering on shop_id + status + created_at gets no index benefit if your index only covers status. sql-- Correct composite index order CREATE INDEX idx_orders_shop_status_created ON orders (shop_id, status, created_at DESC); -- Partial index to reduce index size for filtered queries CREATE INDEX idx_jobs_shop_pending ON background_jobs (shop_id, created_at) WHERE status = 'pending'; Run EXPLAIN (ANALYZE, BUFFERS) on every query touching tables over 100k rows. A Seq Scan on a large table means a missing or unused index.…