> An additional problem here is that there’s no standard way to represent EXPLAIN queries across systems so that makes an additional barrier to entry unfortunately.
The outputs from each DBMS are often specific to their implementation, even when there is a lot of overlap. The good news is they are usually similar enough and user friendly enough that an advanced user will
have no issue but for beginners it can be daunting.
Not necessarily. I often see people learn starting with SQLite because it's easy/installed (and quite a bit of tutorials start with it, especially for Python), and then move to MySQL/Postgres, but that is N=2 so it's not too bad.