updated Systems Embedded: RocksDB is "used for the storage stack and metadata storage" (stated in StorageMgr.h) and PostgreSQL is used for the system catalog (stated in SystemCatalog.h)
Preliminary entry of insight gained from perusing the Paradigm4 website and reading "The Architecture of SciDB" by Michael Stonebraker, Paul Brown, Alex Poliakov, Suchi Raman.