My grandmother taught me to sprinkle flour in a fruit pie, apple, rhubarb, blueberry, to try to get it to thicken, and not be a runny mess. Sometimes it worked, often it didn't, at worst the flour just sat there and baked into lumps. The idea behind flour, was to get the starch to absorb the liquid from the fruit, the starch granules expand, and form a sauce. Good idea, but not the most reliable in my experience. Cornstarch is much more reliable. For fruit like rhubarb, blueberries, strawberries I cut the fruit, add a little liquid (brandy, bourbon or water) sugar, and a heaping soup spoon of corn starch and pre-cook it while the pie shell is blind baking. You cook it over medium low or medium heat, until the fruit starts to release it's juice, and the starch starts to thicken. Put in the partially baked pie or tart shell and bake as you normally would. When cooled the liquid sets to a gel, allowing a slice to come out relatively clean, heaven on a plate.
Fruit pectin will also thicken a sauce. Cranberries, often have enough pectin to thicken without adding anything. An alternative would be to add pectin to thicken a pie - has anyone done that?