Maybe my 10000 rows are 5 tables, 2000 rows each, and I want to have some regular SQL features like joins? Can I do that without having to do joins in code? Because if I have to do joins in code, I can just store data in flat files and process everything in memory. But what if I want to have an existing app designed to run against a db system?

If I have to start writing joins by hand, what's the point of even having a database?

Dynamodb doesn't do well with complex queries. Imagine you've got 4 integer fields, any combination of which can be filtered by a range.

Using dynamoDb, you're going to have a hard time querying if field A, B, and D are included. If you add a fifth field, it's going to be a pain to add to historical data.