Room, indices and @Index?

Question

android studio

More than once I come across the fact that in some kind of training lesson (in this case, on startandroyd), they talk about something, but how to use it - read it on the Internet, I can’t find information for an hour, tell me how it works .
The documentation says @Column(index = boolean) - if you want this field to be indexed, set it to true. Field indexing speeds up query execution.
1. What kind of requests, by field?
2. How fast?
3.And if I request the entire column, and it has indexed fields, will their indexing affect the speed?

So I make a request for this field and do not see the difference whether it is indexed or not ...

indices = [Index("salary",unique = true,name = "MyIndex")
name - index name, how to get it? I want to query all index names, how can I do that? And if you do not specify it, what will be in place of the name?
Thank you.