SQLite query performance

I'm new to SQLite and I need a query as performant as possible. I have a database which stores positions of various astronomical objects and I want to access their positions through my t REAL PRIMARY KEY that is time. My query is something like

    SELECT * FROM positions ORDER BY ABS(t-t0) LIMIT 1;

where t0 is the time at which I want to see positions. This query work with primary key and I think this make it enough efficient but I don't know if the math of ORDER BY ABS(...) slow down the search, even though I suspect that this implies a full table scan.

First question: The REAL PRIMARY KEY is a problem for efficiency with respect to the ordinary INTEGER PRIMARY KEY?

Second question: If I could find the wanted PRIMARY KEY t0 from my android app code and do the query like:

    SELECT * FROM positions WHERE t = t0;

would I improve the efficiency? There is no a full table scan in this case, correct?