As you already know (we've discussed it throughout Chapter 1, Introduction to Elasticsearch), every Elasticsearch index is built out of one or more shards and can have zero or more replicas. You also know that each of the shards and replicas are actual Apache Lucene indices that are built of multiple segments (at least one segment). If you recall, the segments are written once and read many times, and data structures, apart from the information about the deleted documents that are held in one of the files, can be changed. After some time, when certain conditions are met, the contents of some segments can be copied to a bigger segment, and the original segments are discarded and thus deleted from the disk. Such an operation is called segment merging.

You may ask yourself, why bother about segment merging? There are a few reasons. First of all, the more segments the index is built of, the slower the search will be and the more memory Lucene will need. In addition...