Q:

How to get most recent data from DynamoDB for each primary partition key?

I have a table in dynamodb. It stores account stats. It's possible that the account stats will be updated several times per day. So table records may look like:
+------------+--------------+-------+-------+
| account_id | record_id    | views | stars |
+------------+--------------+-------+-------+
| 3          | 2019/03/16/1 | 29    | 3     |
+------------+--------------+-------+-------+
| 2          | 2019/03/16/2 | 130   | 21    |
+------------+--------------+-------+-------+
| 1          | 2019/03/16/3 | 12    | 2     |
+------------+--------------+-------+-------+
| 2          | 2019/03/16/1 | 57    | 12    |
+------------+--------------+-------+-------+
| 1          | 2019/03/16/2 | 8     | 2     |
+------------+--------------+-------+-------+
| 1          | 2019/03/16/1 | 3     | 0     |
+------------+--------------+-------+-------+

account_id is a primary partition key.
record_id is a primary sort key
How I can get only latest records for each of the account_ids? So from the example above I expect to get:
+------------+--------------+-------+-------+
| account_id | record_id    | views | stars |
+------------+--------------+-------+-------+
| 3          | 2019/03/16/1 | 29    | 3     |
+------------+--------------+-------+-------+
| 2          | 2019/03/16/2 | 130   | 21    |
+------------+--------------+-------+-------+
| 1          | 2019/03/16/3 | 12    | 2     |
+------------+--------------+-------+-------+

This data is convenient to use for a reporting purposes. 

A:

This can be done quite efficiently if you know the list of account_ids that you have stored in the table.
In that case, all you need to do is to query primary keys, one by one, sorting values using ScanIndexForward=False and limiting the result to 1 item using Limit=1.
Here is code in python
import boto3
import json

client = boto3.client('dynamodb')

account_ids = ['1', '2', '3']
results = []

for aid in account_ids:
    result = client.query(
        TableName='test-table',
        KeyConditionExpression="#aid = :aid",
        ExpressionAttributeNames={
            '#aid': 'account_id'
        },
        ExpressionAttributeValues={
            ':aid': {
                'N': aid
            }
        },
        ScanIndexForward=False,
        Limit=1,
    )
    results.append(result['Items'])

print(json.dumps(results, indent=2))

