Assumptions:
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V fits in ram
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E does not fit in ram
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Graph is sparse — E/V^2 << 1 (the typical degree is smear less than V)
TODO: move this to early in book TODO: reconcile with pig notation
{a, b} -- record with fields a and b
<P | a, b> -- reduce able record with partition key P
<P | S | a, b> -- reduceable record with partition key P and secondary sort key S (all objects with same P arrive to same reducer, sorted by P then S)
<P1, P2 | S1, S2 | a, b> -- partitioned by P1 and P2, sorted by P1 then by P2 then by S1 then by S2. Objects with the same P1 may go to different reducers.
{a, b, {(c})} -- bag field c