Your team created a DynamoDB table called “Global-Temperature” to track the highest/lowest temperatures of cities in different countries. The items in the table were identified by a partition key (CountryId) and there was no sort key. The application is recently improved with more features and some queries need to be based on a new partition key (CityId) and sort key (HighestTemperature). How should you implement this?

Answer options:

A.Add a Local Secondary Index with a partition key as CityId and a sort key as HighestTemperature.
B.Modify the existing primary index with partition key as CityId and sort key as HighestTemperature.
C.Add a Global Secondary Index with a partition key as CityId and a sort key as HighestTemperature.
D.Add a Global Secondary Index with partition key as CityId and another Global Secondary Index with partition key as HighestTemperature. Because the primary index only uses a simple primary key (partition key), Secondary Index can only have one partition key as well.

Correct Answer – C
Commonly, applications might need to perform various queries via different attributes as query criteria. In this case, more global secondary indexes are required. Please refer to https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GSI.html for its introductions.
Option A is incorrect: Because Local Secondary Index can only be configured when the DynamoDB table is created, which means a local secondary index cannot be added to an existing table.
Option Bis incorrect: Because the existing primary index cannot be modified. In this scenario, Global Secondary Index should be considered.
Option C is CORRECT: Because Global Secondary Index can achieve this requirement. For example:
Option D is incorrect: Because the Global Secondary index key schema can be different from the base table schema. Even if the table contains a simple primary key (partition key), the global secondary index can still contain a composite primary key (partition key and sort key). This is clearly illustrated in
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GSI.html.