inser or update

pg.sql

create table test (a varchar(255), b varchar(255), c integer);
create unique index idx_test_a_b on test (a, b);
insert into test values ('a', 'foo', 1);
select * from test;
insert into test values ('a', 'foo', 1) on conflict (a, b) do update set a = 'c';
select * from test;
drop table test;