In this chapter, we've looked at how to index tree-like structures and how to modify existing index mappings. In addition to that, we've used nested documents and parent-child relationships to index data that can have relationship and structure. Finally, we've looked at the river functionality, which gives us the possibility of fetching data from other systems. We've also learned how to speed up our indexing process by using the batch indexing API. In the next chapter, we will take a look at faceting—a mechanism that allows us to calculate the aggregated data for our query results – and we will see the possibilities that give us the _mlt endpoint. Finally, we will look at the percolator functionality that allows us to use prospective search.