There are many questions already on meta-stackoverflow regarding the database schema, but here is another, that I don't think is a duplicate, so please read on.
Stack Overflow copied a key part of the Wikipedia database design. This turned out to be a mistake which will need massive and painful database refactoring to fix. The refactorings will be to avoid excessive joins in a lot of key queries.
As a newbie to database design, I thought it would be highly enlightening for somebody to expand on this statement a little, giving, for example, 1 or two examples of queries used by Stack Overflow that suck when it came to excessive joins.
Also, I'm thought I read somewhere (can't remember exactly where, so maybe I didn't) that the database had recently undergone some refactoring which, I am assuming, was to address the issues regarding the joins. Is the, what seems to be widely published, schema (in the link above) a description of the database before or after refactoring?
I'm not looking for a complete description of all the details about this, but some examples, general insights or links to other material would be a very useful resource for those of us who are new to this.