The following question was closed. The question naively asks which Java data structure is used to implement large internet databases. If we replace "Java data structure" with "data structures", then the question makes sense: How are internet databases implemented?
Raphael comments that "we can't know". But database people can probably enlighten us even if they never worked for Oracle. While specific tricks and optimizations are trade secrets, general principles are probably public domain. Databases not being one of my specialties, I cannot answer the question, but others probably can.