dddh@gentoo64 ~ $ psql -U networks
psql (9.6devel)
Type "help" for help.

networks=> create table test_01(a int, b int);
CREATE TABLE
networks=> create unique index test_01__a__b__uq on test_01(a, b);
CREATE INDEX
networks=> insert into test_01 values(1, 2);
INSERT 0 1
networks=> insert into test_01 values(1, 3);
INSERT 0 1
networks=> insert into test_01 values(1, 4);
INSERT 0 1
networks=> insert into test_01 values(1);
INSERT 0 1
networks=> insert into test_01 values(1);
INSERT 0 1
networks=> insert into test_01 values(1);
INSERT 0 1
networks=> insert into test_01 values(1);
INSERT 0 1
networks=> insert into test_01 values(2, 1);
INSERT 0 1
networks=> insert into test_01 values(2, 1);
ERROR:  duplicate key value violates unique constraint "test_01__a__b__uq"
DETAIL:  Key (a, b)=(2, 1) already exists.
networks=> select * from test_01 order by 1, 2;
 a | b 
---+---
 1 | 2
 1 | 3
 1 | 4
 1 |  
 1 |  
 1 |  
 1 |  
 2 | 1
(8 rows)

networks=>