root@127.0.0.1:53772/defaultdb> create table t (a int, b int, s string, unique index (a) where b > 0);
CREATE TABLE

Time: 6.058ms

root@127.0.0.1:53772/defaultdb> insert into t values (1, 1, 'foo') on conflict (a) do update set s = 'conflict';
INSERT 1

Time: 1.82ms

root@127.0.0.1:53772/defaultdb> select * from t;
  a | b |  s
----+---+------
  1 | 1 | foo
(1 row)

Time: 613µs

root@127.0.0.1:53772/defaultdb> insert into t values (1, 0, 'foo2') on conflict (a) do update set s = 'conflict';
INSERT 1

Time: 1.85ms

root@127.0.0.1:53772/defaultdb> select * from t;
  a | b |    s
----+---+-----------
  1 | 1 | conflict
(1 row)

Time: 599µs