r0 := DatabaseTable: test2
  key   string
  value float64

r1 := DatabaseTable: test1
  c int32
  f float64
  g string

r2 := Selection[r1]
  predicates:
    r1.f > 0

r3 := InnerJoin[r0, r2] r2.g == r0.key

Aggregation[r3]
  metrics:
    foo: Mean(r2.f - r0.value)
    bar: Sum(r2.f)
  by:
    g:   r2.g
    key: r0.key