Hi Ke,
The patch for this has been posted to pgsql-hackers:
https://www.postgresql.org/message-id/SN7P223MB0600215618AD31B5ACE2109EDD642@SN7P223MB0600.NAMP223.PROD.OUTLOOK.COM
and registered in CommitFest (PG20-3) as #7507:
https://commitfest.postgresql.org/patch/7507/
Thanks again for the detailed report.Prashant