How to efficiently query a list column in dynamodb?

Question

I have a table that maps a parent key into multiple foreign keys. For example:

domain (hash)	parent key (sort)	foreign keys
1	A	[B1, Y2, Z3]
1	X	[B4, G6, Y9]

This structure is optimized for the most common workload, which is to lookup items based on a batch of parent keys using BatchGetItem with.eg. Keys = [{domain:1, parentKey:A}, {domain:1, parentKey:X}]

However, I also need to lookup parent keys with batches of foreign keys, eg in a single round-trip, find the parent keys for the foreign keys (Y2, B4, G6) => (A, X, X)

I cannot seem to find an efficient way to support this second workload in DynamoDB. The only idea I have is to run a Query with partition key (domain = 1), with no key condition on the sort key, and then use a Filter Expression to reduce down to relevant rows -- the problem is that this would consume the same capacity as if I were to retrieve all rows for (domain = 1), which would not work with my capacity model.

Any ideas on how I can achieve this second query format?

Bonus points: I also need a third, far less common workload, where I "getsert" parent keys. Similar to the second workload, however I insert a new table item if a parent key is not found.

Answer 1

You can copy the data.

FK (hash)   Parent (Sort)
B1          A
Y4          B

This way you will be able to efficiently look up the parents one by one. I will try to think of a way to make this into a single query

EDIT: I am not sure if it is possible but Dynamo has Batch Get Item that can query multiple partition keys at the same time