We do not plan to implement secondary unique indexes. The philosophy behind RethinkDB is that if a feature cannot be efficiently scaled across multiple nodes we don't add it, and unfortunately unique secondary indexes are one such feature.

AFAIK most NoSQL databases don't implement it, and a few that do take two approaches -- forbid it on multiple shards, or just take a massive performance hit during sharding.

We chose to keep the feature out of the database. This way the application can be architected to account for it, so it remains fast as the database scales up.