FYI the -bugs thread in question:
https://www.postgresql.org/message-id/flat/CAEHBEOBCweDWGNHDaUk4%3D10HG0QXXJJAGXbEnFLMB30M%2BQw%2Bdg%40mail.gmail.com
seems to imply the primary blocker was a unique constraint.
Cheers,
Greg
--
Crunchy Data - https://www.crunchydata.com
Enterprise Postgres Software Products & Tech Support