yeah, Datomic is EAV (technically EAVT, it's event sourced so there is a notion of time and reads/writes need be acid with transactions and stuff) and EAVT can express even higher level queries than an ORM can, mostly because you can cache the indexes locally so you can do consistent read queries in your app process (Datomic's query engine is a library that runs inside your app, if cache is hot reads don't touch network... like git). From my understanding it doesn't have much to do with sparseness, though you certainly can support sparse objects performantly with EAVT.
EAVT/Datomic is a better fit for apps with complex structured data than an ORM over SQL, but migrating there is no trivial feat. Nor is convincing my customers in their due diligence phase, as they have surely been burned before by some kid pushing MongoDB, but they understand SQL and know the product can be successful with SQL, they know they can go get Oracle consultants to save their ass in 10 years when my company is sold, etc.
EAVT/Datomic is a better fit for apps with complex structured data than an ORM over SQL, but migrating there is no trivial feat. Nor is convincing my customers in their due diligence phase, as they have surely been burned before by some kid pushing MongoDB, but they understand SQL and know the product can be successful with SQL, they know they can go get Oracle consultants to save their ass in 10 years when my company is sold, etc.