DynamoDB Query Performance when Retrieving Latest Item with ScanIndexForward and Limit

Question

I have a DynamoDB table with a composite key (partition key and sort key). I'm trying to retrieve the latest item for a specific partition key given that sort key is timestamp using the following query:

```
const getLatestItem = {
    TableName: "MyTable",
    KeyConditionExpression: "pk = :pk",
    ExpressionAttributeValues: {
        ":pk": "somePartitionKeyValue",
    },
    Limit: 1,
    ScanIndexForward: false
}
```

I have two main questions about this query:

1) Will this query scan the entire partition to be able to return latest item, or will it efficiently return just the latest item based on the sort key?

2) As the number of items in the partition grows, will the performance of this query degrade?

Leeroy Hannigan · Answer

> Will this query scan the entire partition to be able to return latest item, or will it efficiently return just the latest item based on the sort key?

It will only evaluate a single item, specified by the `Limit=1` parameter. Therefore will have same efficiency and performance as a `GetItem`.

> As the number of items in the partition grows, will the performance of this query degrade?

No, performance is the equivalent of a constant time lookup, no matter how much data relates to a partition key.

re:Post Agent · Answer

To address your questions about the DynamoDB query performance when retrieving the latest item:

1) Your query will not scan the entire partition to return the latest item. DynamoDB is designed to efficiently handle this type of query. By setting ScanIndexForward to false, you're instructing DynamoDB to traverse the index in descending order based on the sort key (timestamp in your case). Combined with the Limit of 1, DynamoDB will efficiently return just the latest item for the specified partition key without scanning all items in the partition.

2) The performance of this query should remain consistent and should not significantly degrade as the number of items in the partition grows. DynamoDB's design allows it to quickly locate and retrieve items based on the primary key (partition key and sort key) regardless of the partition size.

However, it's important to note that while the query itself remains efficient, there are some considerations:

- If your partition key has a very high number of items (millions or more), you might need to ensure that your provisioned throughput or on-demand capacity is sufficient to handle the increased data volume.

- If you frequently perform this query on the same partition key, you might want to consider using DynamoDB Accelerator (DAX) for caching to further improve performance and reduce latency.

- Keep in mind that this query consumes read capacity units (RCUs) based on the size of the returned item, not the number of items in the partition.

Overall, your approach of using ScanIndexForward: false and Limit: 1 is an efficient way to retrieve the latest item for a given partition key in DynamoDB, and it should maintain good performance even as your data grows.  
**Sources**  
[Query - Amazon DynamoDB](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/API_Query_v20111205.html)  
[Query - Amazon DynamoDB](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Query.html)  
[Best practices for querying and scanning data in DynamoDB - Amazon DynamoDB](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/bp-query-scan.html)

DynamoDB Query Performance when Retrieving Latest Item with ScanIndexForward and Limit

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