Comment by jakozaur

Most likely DynamoDB is BigTable like database, Amazon may even used forked version of Cassandra or HBase.

All of this databases, got hot partition issue. If you cause to many read/write to one server you run into issues. That's why key schema is very important in non-trivial use-cases.

Classic anti-pattern is to use date or other increasing number as prefix. It is also problem when you use S3.