insert into pinned_tweets (user_handle, tweet_id, pinned_at)
  values (
    'rey',
    1,
    clock_timestamp()
  )
on conflict (user_handle)
do update set (tweet_id, pinned_at) = (1, clock_timestamp())
where pinned_tweets.user_handle = 'rey'; -- <--------------------------?

-- INSERT 0 1

select * from pinned_tweets where user_handle = 'rey';
--  tweet_id | user_handle |          pinned_at
-- ----------+-------------+------------------------------
--         1 | rey         | 2016-01-11 17:09:55.41554-06
-- (1 row)