If we only need, say, 10 tuples in total, then we certainly don't need
more than 10 tuples from any single process.  Pushing down the limit
lets workers exit early when possible.  For Gather Merge, there is
an additional benefit: a Sort immediately below the Gather Merge can
be done as a bounded sort if there is an applicable limit.

Robert Haas and Tom Lane

Discussion: http://postgr.es/m/CA+TgmoYa3QKKrLj5rX7UvGqhH73G1Li4B-EKxrmASaca2tFu9Q@mail.gmail.com