r0 := UnboundTable: t1
  a int64
  b float64

r1 := UnboundTable: t2
  a int64
  b float64

r2 := InnerJoin[r0, r1] r0.a == r1.a

r3 := Selection[r2]
  selections:
    a:       r0.a
    b:       r0.b
    b_right: r1.b

CountStar(): CountStar(r3)