public int count(DynamoDBScanExpression scanExpression) {
        return mapper.count(model.targetType(), scanExpression);
    }
Evaluates the specified scan expression and returns the count of matching
items, without returning any of the actual item data.
@param scanExpression The scan expression.
@return The count.
@see com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapper#count
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/DynamoDBTableMapper.java