With one-to-many relationships, there’s one core problem: how do I fetch information about the parent entity when retrieving one or more of the related entities?

DynamoDB Streams enables solutions such as these, and many others. DynamoDB Streams captures a time-ordered sequence of item-level modifications in any DynamoDB table and stores this information in a log for up to 24 hours.

One way to put it is this: LSI - allows you to perform a query on a single Hash-Key while using multiple different attributes to "filter" or restrict the query. GSI - allows you to perform queries on multiple Hash-Keys in a table, but costs extra in throughput, as a result.

DynamoDB supports two types of secondary indexes: Global secondary index — An index with a partition key and a sort key that can be different from those on the base table. A global secondary index is considered "global" because queries on the index can span all of the data in the base table, across all partitions. A global secondary index is stored in its own partition space away from the base table and scales separately from the base table. Local secondary index — An index that has the same partition key as the base table, but a different sort key. A local secondary index is "local" in the sense that every partition of a local secondary index is scoped to a base table partition that has the same partition key value.

While SimpleDB has scaling limitations, it may be a good fit for smaller workloads that require query flexibility. Amazon SimpleDB automatically indexes all item attributes and thus supports query flexibility at the cost of performance and scale.