Perioperative neurocognitive disorders is a common postoperative complication in elderly surgical patients. The role of gut microbiota in cognitive function has been concerned in recent years. Studies suggests that gastrointestinal surgery may affect the gut microbiota, and the effect varies between surgical procedures. In this study, the investigators will compare the differences of gut microbiota between total gastrectomy and double-tract reconstruction, to investigate the effect of gastric acid on the gut microbiota colonizing, and the effect of different surgical procedures on the postoperative cognitive function of proximal gastric cancer patients.