In a large bowl, cut butter into flour until mixture resembles coarse crumbs; stir in pecans. Press into an ungreased 13-in. x 9-in. baking pan.
Bake at 350° for 15 minutes. Combine rhubarb, sugar and flour; spoon over the crust. Bake for 15 minutes.
Meanwhile, in a large bowl, beat the cream cheese, sugar and vanilla until smooth. Add eggs, beat on low speed just until combined.
Pour over hot rhubarb layer. Bake for 30-35 minutes or until center is almost set. Cool on a wire rack for 10 minutes. Meanwhile, combine the sour cream, sugar and vanilla; spread over hot cheesecake. Sprinkle with additional pecans if desired. Cool on wire rack for 1 hour.
Refrigerate overnight. Refrigerate leftovers.
If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander, but do not press liquid out.