Dealing with empty data bags

[Prashanth Pappu]

(a) I see that at a lot of places where PIG doesn't correctly deal with
results that are empty bags.

Here's an example - Counting Tuples. Let's say I want to count number of
tuples in 'b' ( a subset of 'a'). I can do the following -

a = load 'xyz' as (x,y,z);
b =  filter a by x==X;
c = group b all;
d = foreach c generate COUNT(b);

Ideally, we want d to be (0) if b has no tuples and non-zero otherwise.
Unfortuantely, if b is empty, c is also empty! This is buggy because it
causes d to be empty or null and not (0).

Whereas, if b is empty, c should ideally be, c = (all, {}). Which will make
d = (0).

(b) Is there a different way of computing the number of tuples in b that
will always (irrespective of whether b is empty or not) give the correct
answer?

(c) I also see that PIG supports data maps. But I haven't seen any examples
that illustrate how to create or manipulate data maps. Is there any such
documentation?

thanks,
Prashanth