An identity graph database is built for the kind of many-to-many relationships an identity graph generates. Traditional relational databases struggle with deep traversal queries, like "show me every identifier ever connected to this user, even across three hops of merged profiles." Graph databases are designed for exactly that. Some vendors use a dedicated graph engine. Others run graph-style modeling on top of managed relational stores. Either approach can work, but the choice determines how much identifier history you can store and how quickly you can query it.