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That would be one way, or

On Postgres, you could use arrays: http://www.postgresql.org/docs/9.3/static/arrays.html also has some great contrib modules as well like: http://www.postgresql.org/docs/9.3/static/intarray.html

Mongodb documents their multikey indexes for a similar approach: http://docs.mongodb.org/manual/core/index-multikey/

not to take away from rethinkdb, awesome to see a project move rapidly!

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Importantly, in Postgres the arrays are indexable with a GIN (inverted) index. So you can ask arbitrary questions along the lines of: show me all rows in this table that have any of the following integers: [5, 7, 9] somewhere in their array column. This will be performed very efficiently.

In recent versions, Postgres will even keep track of statistics for arrays (and other, similar non-scalar types), so that you can get a good selectivity estimate, which is often critical to getting a good plan from the query planner.

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