Buster Posey homered in the first inning and scored the go-ahead run on an error in the third, and the San Francisco Giants beat the San Diego Padres 6-1 on Tuesday night for their ninth straight victory a day after becoming the first team in the big leagues to clinch a playoff berth. Brandon Belt and Darin Ruf each hit run-scoring doubles and Tommy La Stella added an RBI single to back Anthony DeSclafani (12-6), who won for the first time in five starts since defeating Colorado on Aug. 13. The right-hander had allowed six earned runs over his previous 14 2/3 innings.