Mix crushed pretzels, ½ cup sugar and butter; press into a 9-by-13-inch Pyrex dish. Bake at 325 degrees for 5 minutes and then let it cool.
Mix cream cheese, ½ cup sugar and Cool Whip until blended. Spread this mixture on top of the pretzel crust.
Drain the pineapple and reserve the juice. Heat pineapple juice with 2 tablespoons cornstarch until thick. Mix crushed pineapple with juice mixture and spread on top of creamed mixture and refrigerate until firm.