The following bug has been logged on the website:
Bug reference: 13846
Logged by: Paul Hester
Email address: paul(at)salesintel(dot)com
PostgreSQL version: 9.5rc1
Operating system: Windows 10 Pro
Description:
When using an INSERT statement with an ON CONFLICT clause, if there is a
conflict, and the table being inserted into has a column defaulted to
nextval('seq'), the sequencer 'seq' is always incremented. This can quickly
and artificially consume all values of the sequencer; it behaves as if
conflict detection happens after inserts are attempted, rather than before.