DynamoDB Hierarchical, Sorted Queries

Question

I would like to be able to query data hierarchically and return the results ordered by another attribute. What is the most efficient way to store and query sorted hierarchical data?

For example, if I have a table with four attributes: `customer_id`, `country`, `location`, and `last_updated_date`, where `location` contains hierarchical information such as `state:county:city`, so a few records may look like:

```
------------|--------|-------------------|-------------|
customer_id |country |location           |last_updated |
------------|--------|-------------------|-------------|
123456      |USA     |WA:King:Seattle    |2022-03-18   |
789012      |USA     |WA:King:Kent       |2022-03-15   |
098765      |USA     |NY:Bronx:NYC       |2022-02-28   |
432109      |USA     |WA:Spokane:Spokane |2022-03-20   |
```
The `PK` of the table is the `customer_id` because most queries will pull information by `customer_id`, but there are other use cases that will want to (a) find all customers within a given location (e.g. `state` or `county`), and (b) return the results sorted (descending) by `last_updated`.  To accomplish (a), I have a `GSI`, with `country` as the `PK` and `location` as the `SK`, `query`ing the `GSI` using `location.begins_with`.

But I can't figure out how to accomplish (b). My understanding is that ordering operations are usually performed with `scanIndexForward`, but I'm already using the `GSI` for the hierarchical query. Is there a way to do both (a) and (b)?

Thanks!

Accepted Answer

Create a GSI where the PK is Country+Location concatenated and the SK is the last_updated. Then for any given Country+Location value you can find the last updated values (and corresponding customer ids in the projection) for that location. Note you can setup 20 GSIs.

Or are you hoping to do a begins_with against the location (like for the wider value of WA:King) while also getting only the latest X many? In which case, well, you can't. You can however insert new items for each hierarchical level (like WA:King, etc) so that that value can be used as the PK. Basically pre-materialize at each level. You only get one sort key per table/index and you can't use the sort key functionality twice.

Of course, if you intend to get ALL the matching items (not just the latest X many) and just want them sorted, you can sort on the client.

DynamoDB Hierarchical, Sorted Queries

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