Carbs are given a score of 0-100 called GI (Glycaemic Index). The higher the number the faster the carb absorbs into the blood stream. Glucose has a GI of 100. This makes it the fastest absorbing carb.
For the most part you want the carbohydrates you eat from food during the day to be low GI. However, around training you may need to supplement with high GI carbs.
The best high GI carbs to supplement around training are glucose and maltodextrin because they are quickly absorbed, well tolerated and create a greater increase in insulin.
This boost in insulin helps create an anabolic muscle building state while increasing glycogen (muscle energy) recovery.
Insulin also helps increase the absorption of other nutrients and amino acids including Beta Alanine, Creatine, Citrulline, Glutamine etc.
Glucose and Maltodextrin are incredibly easily digested and absorbed so create very little, if any digestive issues. That said, if you are supplementing with simple carbs during training you should keep your solution to no greater than 7%. That means 7g of carbs per 100ml of water.