This commit also resolves #1646.
Porting only the function `tuplesort_get_stats` in tuplesort.c from the
Postgres commit
https://github.com/postgres/postgres/blob/9bd27b7c9e998087f390774bd0f43916813a2847/src/backend/utils/sort/tuplesort.c
Link to gpdb-dev discussion - https://groups.google.com/a/greenplum.org/forum/#!topic/gpdb-dev/V-zIshnNzyE