I think what I meant was that the join can be caused by the front end code, even if it happens on the back end server:

https://supabase.com/docs/guides/database/joins-and-nesting

"The data APIs automatically detect relationships between Postgres tables. Since Postgres is a relational database, this is a very common scenario."

There will be many people with the title Full Stack or Front End engineer who are using Supabase in this way and they may consider the code that triggers this to be front end code. There might or might not be another engineer who is in charge of the database and does some coding for it but always refers to him/herself as "backend" and is not aware that some of these joins are happening and need indices.