In my xcdatamodel two new fields, byTitleIndex and compoundIndex, are showing under Fetch Index Elements. I have not seen this section before in Xcode. What is a Fetch Index Element?
Fetch Index Elements are part of Apple’s new indexing API, announced at WWDC 2017. They allow you to specify one or more Index Elements; properties that are used to create an index for faster database searches. In your case, the "title" property will be used to create an index called “compoundIndex” under your "Passage" entity, allowing for faster title searches.
Announcement: https://developer.apple.com/videos/play/wwdc2017/210/?time=628
Demo: https://developer.apple.com/videos/play/wwdc2017/210/?time=997
Documentation (scroll down to "Working with Indexes"): https://developer.apple.com/documentation/coredata/nsmanagedobjectmodel
来源：https://stackoverflow.com/questions/47664780/what-are-fetch-index-elements-in-the-coredata-model