41. The system of claim 19 , wherein at least one constraint for preserving statistical consistency specified by the constraint specification is based on an equivalence between a field of the second dataset and a field of the first dataset.
41. The system of claim 19 , wherein at least one constraint for preserving statistical consistency specified by the constraint specification is based on an equivalence between a field of the second dataset and a field of the first dataset. 42. The system of claim 41 , wherein the field of the first dataset and the field of the second dataset are keys in a join operation.