The melody of Who Says by John Mayer runs 257 notes across 45 bars, 2:27 long in D at 190 beats per minute. The tab sits low on the neck, mostly under the fourth fret, one note at a time — no chords to barre and no strumming pattern to keep up with. A piano backing track plays the chords and bass underneath, so the line you are learning sounds like the record even at half speed.
Prefer to strum and sing it instead? Who Says chords and lyrics.