I think the example for SAMPLE is wrong
In this example, a scalar expression is used (it will sample approximately 1000 records from the input).
a = load 'a.txt';
b = group a all;
c = foreach b generate COUNT(a) as num_rows;
e = sample d 1000/num_rows;
There is no relation d and I get a "Projected field [num_rows] does not exist in schema" error, I think it need to be
e = sample a 1000/c.numrows