问题
I have a Core data model with a 32bit hash value. I need to look up specific hash values quickly. Should I use the indexed property? I have no idea what it does and the documentation is no help (am I looking in the wrong place?)
So what does indexed do exactly?
回答1:
I would recommend to read this on indexes: http://en.wikipedia.org/wiki/Index_(database).
Simply put, a database engine creates a new structure which keeps the indexed column (which corresponds to a property) sorted and a link to the corresponding row for each entry (primary key). This allows for faster searches (since search in ordered lists is faster than in unordered lists). But this increases used storage (for the data structure), and insertion times (to keep the structure sorted).
So yes, you should use indexes in such cases.
回答2:
If you check the box, Core Data will build an index of the values, which will make searching faster and more efficient. It's like what Spotlight uses. Without the index it'll have to travel through the database every time. You say you need to look up the values quickly—then you should index them.
来源：https://stackoverflow.com/questions/7935348/what-does-the-indexed-property-of-a-coredata-attribute-do