Ask HN: Experienced devs, what advice for newcomers to relational databases?

Devs who are experienced with relational databases, I want to learn from your mistakes.

I have limited experience with relational databases and I'm setting up Postgres (Supabase) for my B2B2C SaaS product and desigining table schemas.

What advice would you like to give? Anything beyond the very basics is welcome.

Thanks!