Ensuring Dynamo retrieves *exactly* n results, given a filter expression

Question

In DynamoDB is there a way to guarantee that exactly n results will be
returned if I specify a limit and a filter?

The problem I see is that the docs state:

In a response, DynamoDB returns all the matching results within the scope of the Limit value. For example, if you issue a Query or a Scan request with a Limit value of 6 and without a filter expression, DynamoDB returns the first six items in the table that match the specified key conditions in the request (or just the first six items in the case of a Scan with no filter). If you also supply a FilterExpression value, DynamoDB will return the items in the first six that also match the filter requirements (the number of results returned will be less than or equal to 6).

So this means 6 items will be retrieved and then the filter applied. How can I keep searching until I get exactly '6' items? (Ideally there is some setting in the query to keep going until the limit has been reached -- or exhaustion has been reached)

For example, Suppose I make a query to get 50 people, who's name is "john", Dynamo would return 50 people and then apply the "john" filter. Now only 3 people are returned. Is there a way I can ensure it will keep searching until the limit of 50 is satisfied?

I don't want to use a Scan since a Scan always searches every item in the table (regardless of limit -- correct me if I'm wrong on this).

How can I make the query's filter lazily until the Limit is satisfied? How can I keep searching until the Limit is satisfied?

Answer 1

If you can filter in the query itself, then that'll be best, since you wouldn't have to use a filter expression. But if you can't, the way dynamo works I suspect means the filter is just a scan over the results - basically a way to save on bandwidth, not much more. You can still use pagination to get more results; and if you're using Dynamo you probably care about the rate in which you're querying, so having that control over how many queries you're actually doing (and their size) is kind of a good thing.