If you only need one index, then just name your s3 document by the compound index value and call it a day.
Otherwise, just use RDS for everything.
> In order to maximize your workload’s throughput on Amazon Aurora, we recommend building your applications to drive a large number of concurrent queries.
Perrrrfect. Thanks!
https://aws.amazon.com/rds/aurora/faqs/
If you only need one index, then just name your s3 document by the compound index value and call it a day.
Otherwise, just use RDS for everything.