Software developers are increasingly required to write big data code. However, they find big data software development challenging. To help these developers it is necessary to understand big data topics that they are interested in and the difficulty of finding answers for questions in these topics. In this work, we conduct a large-scale study on Stackoverflow to understand the interest and difficulties of big data developers. To conduct the study, we develop a set of big data tags to extract big data posts from Stackoverflow; use topic modeling to group these posts into big data topics; group similar topics into categories to construct a topic hierarchy; analyze popularity and difficulty of topics and their correlations; and discuss implications of our findings for practice, research and education of big data software development and investigate their coincidence with the findings of previous work.