My fully custom website (thousands of pages per minute) has an authentication query that runs on every page—and it has a subquery inside a subquery inside a sub

My fully custom website (thousands of pages per minute) has an authentication query that runs on every page—and it has a subquery inside a subquery inside a subquery. That's four layers of SQL inception. And it executes in under ten milliseconds. Subqueries are indeed a performance red flag when the query is returning more than a few rows AND the subquery references fields outside of the subquery. Or put more simply, a subquery that will need to be executed an unreasonable number of times per run. But so long as you aren't doing anything like that, and your database engine has a competent query planner, there should be zero difference in performance. For this reason I don't consider subqueries to be a code smell. For me a query is "smelly" when it does things which aren't obvious to a human parsing the query with their brain.