It has race conditions. The only work-around that works (unless perhaps you have a 2-phase locking system, and are using serializable isolation level) involves

It has race conditions. The only work-around that works (unless perhaps you have a 2-phase locking system, and are using serializable isolation level) involves subtransactions. For example: http://www.postgresql.org/docs/current/static/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE http://www.postgresql.org/docs/current/static/plpgsql-contro... This is ugly an inefficient.