How to properly “excavate” a complex database structure on a large project?

Question

MySQL

Let's say the situation: I will need to support a large project with a database with an unclear structure - a large number of tables (several tens or even hundreds), foreign keys, etc. How to quickly and effectively understand its structure? What are the tools for visualizing relationships between tables, regardless of their number? Databases - MySQL and Postgres

Answer 1

building a diagram with relationships from the working base is the most effective option.
if there are many tables, group them into separate blocks if possible.
then print the whole thing and hang it on a stand / wall.