[postgressomethi]

> You might think adding a UNIQUE index would cause the "losing" xact to get constraint errors, but instead both xacts succeed and no longer have a race condition.

This is not true. What happens is that the (sub)transaction that loses the race to the index is aborted:

  =# INSERT INTO foo (bar) (SELECT max(bar) + 1 FROM foo);
  ERROR:  duplicate key value violates unique constraint "foo_bar_idx"
  DETAIL:  Key (bar)=(2) already exists.

[hot_gril]

I probably got the wrong query. There was some other one where the unique index removes a race condition, lemme find it...