An index summary is an in-memory sample of keys from an SSTable’s index. It tells a reader where to begin searching for a target instead of scanning the entire index. section 8.3.3
A query can first use a Bloom filter to check whether an SSTable may contain a key, then use the index summary to jump near that key in the index. The indexing hierarchy stops at this second level rather than adding further index layers. section 8.3.3 section 8.3.3
