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• Tags will have to be explicitly created by a user of the warehouse (through a command)
• Commands like show tables and show partitions will need changes to understand tags.
• Capacity tracking and management will be more complex than using databases for this purpose.
• Data migration is more complex since the data is not contained with the root folder of one database

None of these are insurmountable problems, but using databases to model namespaces is a cleaner approach.
(Taking this idea further, a database in Hive could itself have been implemented using tags in a single global namespace which would have not been as elegant as the current implementation of a database being a first class concept in Hive.)

Modeling Namespace as a database but using views for imports

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