Data migration becomes a lot easier the closer to "raw" data you store. The farther away you are, the tricker it will be to write those migration scrips. And,

Data migration becomes a lot easier the closer to "raw" data you store. The farther away you are, the tricker it will be to write those migration scrips. And, if what you're storing is rolled-up enough, you may not even have the information you want/need any more. Note that the source data doesn't have to be what you're serving queries from; it could be an archive with batch scripts to re-process that's only dusted off in case of disaster/migration.