A team currently maintains a forum-based application. All of the threads and their data are sent to a DynamoDB table. Below is the structure of the DynamoDB table
Thread ID- Partition Key
Author – Sort Key
Number of Replies
LastReplytimestamp
The table currently has around 600 million rows of data. There is a requirement now to get those threads which are not getting any traction and no response has been made in 6 months. This needs to be done on an on-going basis. Which of the following would be the most efficient way to achieve this?

Answer options:

A.Query the data based on the ThreadID
B.Query the data based on the Author
C.Scan the entire table and search for the older records
D.Create a global secondary index on LastReplytimestamp.

Answer – D
Such a similar use case is given in the AWS Documentation:
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For example, consider the Thread table that is defined in Creating Tables and Loading Sample Data. This table is useful for an application such as the AWS Discussion Forums. The following diagram shows how the items in the table would be organized. (Not all of the attributes are shown.)
DynamoDB stores all of the items with the same partition key value contiguously. In this example, given a particular ForumName, a
Query
operation could immediately locate all of the threads for that forum. Within a group of items with the same partition key value, the items are sorted by sort key value. If the sort key (Subject) is also provided in the query, DynamoDB can narrow down the results that are returned—for example, returning all of the threads in the "S3" forum that have a Subject beginning with the letter "a".
Some requests might require more complex data access patterns. For example:
Which forum threads get the most views and replies?
Which thread in a particular forum has the largest number of messages?
How many threads were posted in a particular forum within a particular time period?
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Options A and B are incorrect since here we need to query based on the timestamp
Option C is incorrect because even though this is possible, this is the least efficient way to manage the query
For more information on this use case, please visit the url
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/LSI.html