How to calculate gp_skew_coefficients.skccoeff manually in Greenplum

Per Greenplum Admin Guide: "The coefficient of variation (CV) is calculated as the standard deviation divided by the average.".
The "standard deviation" and "average" are based on (select gp_segment_id,count(*) from <table_name> group by gp_segment_id).
I did some tests and finally got a SQL to calculate it.

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testdb=# select STDDEV(cnt)/avg(cnt)*100 testdb-# from testdb-#    ( testdb(#      select gp_segment_id segid,count(*) cnt from testtable group by gp_segment_id testdb(#      union all testdb(#      ( testdb(#        select tmp2.content segid,0 cnt from testdb(#        gp_segment_configuration tmp2 testdb(#        where tmp2.content not in (select gp_segment_id segid from testtable group by gp_segment_id) testdb(#        and tmp2.role='p' testdb(#        and tmp2.content<>-1 testdb(#       ) testdb(#     ) tmpc;         ?column?         -------------------------  28.54846392774083598000 (1 row)    Time: 461.224 ms testdb=# select * from gp_toolkit.gp_skew_coefficients where skcrelname='testtable';  skcoid | skcnamespace | skcrelname |         skccoeff         --------+--------------+------------+--------------------------  273985 | inf          | testtable  | 28.548463927740835980000 (1 row)