The goal of this \[clinical trial\] is to \[explore the balance time point between the waiting time for intestinal preparation after gastric emptying and quality of bowel preparation.\] in \[patients with painless colonoscopy\]. The main question\[s\] it aims to answer are: \[whether the gastric is empty or not 2 hours after finishing drinking 1.5 L of MgSO4 solution within 2 hours\] \[whether the quality of patients' bowel preparation is superior in a time period of 2-4 hours after finishing drinking 1.5 L of MgSO4 solution\] Participants will \[ be asked finishing the last dose of 1.5 L MgSO4 solution within 2 hours and received ultrasound gastric assessment immediately 2 or 4 hours after finishing drinking MgSO4 solution in a 15 minutes time interval until the gastric is empty. As soon as the gastric is empty, patients receive painless colonoscopy within 2 hours or 2-4 hours. Researchers will compare \[patients receiving painless colonoscopy within 2 hours or 2-4 hours after finishing drinking 1.5 L of MgSO4 solution\] to see if \[the quality of patients' bowel preparation is superior in a time period of within 2 hours\].