Dr. John is the real thing: one of the most requested session men in New Orleans during the R & B glory days of the 50s and early 60s, he studied under the legendary pianist Professor Longhair. The Doctor’s command of over a quarter century of blues and R & B heritage makes his one of the most varied and polished shows anywhere. Any minute he’s liable to break into a gutbucket piano boogie, a sophisticated, Gershwin-like interlude, or a hard-driving New Orleans rhythm-and-blues classic, overlaid with his famous bayou croak and enhanced by his warm, low-key stage demeanor. He plays his popular standards–“Gris Gris,” “Right Place, Wrong Time”–with the same funky soulfulness he brings to classics by the likes of Huey Smith and the Clowns (“Don’t You Just Know It”), Chris Kenner (“Something You Got”), and Professor Longhair (“Big Chief”). Tonight and Saturday, Cotton Chicago, 3204 N. Wilton; 528-1651.