Query dynamoDB based on HASH key only efficiently and cost effectively

I have a dynamoDB table with a HASH key and a SORT key.

**Now I have a situation where I need to get table contents based on the HASH key only and it will return an array of items. **

I am able to do this with .query() operation

    const params = {
      KeyConditionExpression: `${key} = :${key}`,
      ExpressionAttributeValues: {
        [`:${key}`]: value,
      },
      TableName: tableName,
    }
    const result = await this.dynamodb.query(params).promise()

Now the question I have is around 'best practices' and 'cost'.

GOAL: I need to get many items based on one key.

QUESTION: Is this the best way to do it? Or am I performing a full scan every time I query?

NOTE: I have knowledge on GSI/LSI but since I'm querying on the HASH key only, I think making another Index is redundant?

    const params = {
      KeyConditionExpression: `${key} = :${key}`,
      ExpressionAttributeValues: {
        [`:${key}`]: value,
      },
      TableName: tableName,
    }
    const result = await this.dynamodb.query(params).promise()

A Query is an efficient operation. It does not do a full table scan, it only reads the items related to the provided hash key.

If you want it to be more efficient you can provide a condition on the sort key to help filter the result set, such as SK begins_with(ABC).

And you are correct that an index is not required, as you are already as efficient as you can be.