🔗 Stop loading large Ecto fields by default

Let's say you want to store a map in an Ecto.Schema. You add a :map field which Ecto converts into an optimized jsonb column in Postgres. You trust the awesomeness of Postgres and start dumping data into it. Everything works fine.

Until your PM pings you.

Hey. Why is our website so slow now?

Weird.

Everything works fine locally, but your production website now takes seconds to load when it worked just fine yesterday. You scratch your head. Is it maybe the new :map field you added?

Spoiler: It is! 🎉

You realize that only pages that list many records are affected and pages that show a single record are not. Then it hits you: jsonb fields can hold up to ~268MB of data and json fields even up to 1GB! Whenever you list your records, Ecto fetches all this data from the database. Most times, you don't even need it!

So, how can you tell Ecto to not load it by default?