Postgres 9.5 ON CONFLICT DO SELECT

Question

While doing UPSERT in Postgres 9.5, is it possible to return null on INSERT success and return something ON CONFLICT?

I would like to something like this:

insert into "user" (timestamp, user_id, member_id) 
values ($1, $2, $3)
ON CONFLICT (user_id, member_id) 
DO select id from "user" where user_id = $2 returning user_id

The only purpose of selecting user_id is to return something (anything other than null) ON CONFLICT, and without writing to disk. I know that this can be done with ON CONFLICT DO UPDATE but it would involve writing to disk.

Answer 1

You could use CTE:

WITH cte AS (
   INSERT INTO "user"(timestamp, user_id, member_id)
   values ($1, $2, $3)
   ON CONFLICT (user_id, member_id) DO NOTHING
   RETURNING user_id
)
SELECT NULL AS result
WHERE EXISTS (SELECT 1 FROM cte)          -- success
UNION ALL
SELECT id 
FROM "user" 
WHERE user_id = $2 
  AND NOT EXISTS (SELECT 1 FROM cte);     -- conflict

DBFiddle Demo