Climbing for a second straight session, the Sensex at the Bombay Stock Exchange gained 106 points, or 0.6 percent, to 18,609 on Wednesday, on continued buying by investors, amid strong export growth in April. The Nifty at the National Stock Exchange advanced 32 points, 0.6 percent, to 5,592.Stock markets in Japan, China, Singapore, Hong Kong, and South Korea ended mixed.