Faz parte do material de Cloud / AWS que eu montei a partir das minhas entrevistas. Abaixo, a resposta em inglês e, quando existe, a pegadinha mais comum.
RDS manages relational engines such as PostgreSQL and MySQL, providing familiar joins, constraints, and transactions. You still design indexes, capacity, backup/restore, and availability behavior.
DynamoDB is a managed key-value and document database. Design partition keys and indexes around expected access patterns, capacity, and hot keys. It supports transactions and multiple read-consistency options, subject to feature-specific constraints.
You can add some new access paths later, for example with a global secondary index, but that changes cost and operational behavior. A Scan reads through items and is not a free substitute for an efficient key query. Latency is not guaranteed to remain a fixed number under every workload.
Sources: DynamoDB secondary indexes and query/scan guidance.
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