Need help in understanding these two historical metrics- query/segmentAndCache/time and query/segment/time. From the description query/segment/time is time to query individual segment including time taken to fetch it from disk. Then what is the utility of query/segmentAndCache/time metric

My understanding is that this is related to the segment level query results cache ... which stores segment level results for a specific query. segmentAndCache/time includes segments that are found in the results cache and therefore the results can be retrieved and returned more quickly without having to process the segment ... segment/time does not include cached segment results. The difference between the two should tell you how effective your segment level results caching is. Note -- this is not related the handling of segment file data in the Linux file system page cache ... that happens regardless of whether segment-level results caching is used or not.