Greenplum/Postgres - Why not backward index scan?

I have this query running on Greenplum 6(Postgres 9.4) too slow as it performing a sequential scan. But I am thinking why it's not performing an index backward scan to fetch the records since there is an index on the same column. explain analyse select * from tab1 where create_time > now() - interval '1d' order by create_time desc limit 20; https://explain.depesz.com/s/UTA7#html The index is a composite index (create_time,col2), could it be the reason behind sequential scan? I have already updated statistics of the table and also the bloat ≈5% only.