7 Things I Wish I Knew Before Scaling Next.js + Supabase to 100K Users

7 Things I Wish I Knew Before Scaling Next.js + Supabase to 100K Users Six months ago, we launched our SaaS with Next.js and Supabase. The stack was perfect for our MVP: fast development, great DX, and it just worked. Then we hit 10K users. Then 50K. Then 100K. Everything that worked beautifully at small scale started breaking. Database queries that took 50ms now took 5 seconds. Our Supabase bill went from $25/month to $800/month. Users complained about slow page loads. Here's what I wish someone had told me before we started. 1. RLS Policies Are Not Optional (Even in Development) We skipped RLS in development. "We'll add it before launch," we said. Launch day came. We enabled RLS on all tables. The app broke in 47 different places. Queries that worked suddenly returned empty arrays. Inserts failed with permission errors. We spent 12 hours fixing RLS policies while users waited. What I'd do differently: Enable RLS from day one. Write policies as you create tables: CREATE TABLE posts ( id UUID PRIMARY KEY DEFAULT uuid_generate_v4 (), title TEXT NOT NULL , user_id UUID REFERENCES auth . users ( id ) ); -- Enable RLS immediately ALTER TABLE posts ENABLE ROW LEVEL SECURITY ; -- Write policies now, not later CREATE POLICY "Users can view own posts" ON posts FOR SELECT USING ( auth . uid () = user_id ); Test with RLS enabled. If it works in development, it'll work in production. 2. Database Indexes Are Not Premature Optimization "We'll add indexes when we need them." We needed them on day 3. Our posts feed query went from 50ms to 8 seconds as we hit 10K posts. Users complained. We scrambled to add indexes during peak traffic. The query: const { data } = await supabase . from ( ' posts ' ) . select ( ' *, profiles(*) ' ) . eq ( ' published ' , true ) . order ( ' created_at ' , { ascending : false }) . limit ( 20 ) The fix: CREATE INDEX posts_published_created_at_idx ON posts ( published , created_at DESC ) WHERE published = true ; Query time dropped to 12ms. What I'd do di