How to check if a query is correct after refactoring?

Question

MySQL

I have a properly working query that selects a large number of records, but it is slow, I already have ideas how to optimize it, but I want to make sure that my changes do not break it and it will return the same data.
How can I check this using MySql 8? It will also be interesting for other versions of MySql 5.* and PostrgesSql

Answer 1

(query 1) except (query 2);
(query 2) except (query 1);