Capture which unique key was violated

As the version I am using is 9.5, the ideal solution is the GET STACKED DIAGNOSTICS command.

Example:

create table foo (a int not null, b int not null, unique(a), unique(b));
insert into foo (a,b) values(1,1);
insert into foo (a,b) values(2,2);
create function ffoo(x integer, y integer) returns void as $$
declare
    tabela text;
    restricao text;
begin
insert into foo (a,b) values(x,y);
exception when unique_violation then
    get stacked diagnostics tabela = TABLE_NAME,
                restricao = CONSTRAINT_NAME;
    raise notice 'tabela: % ; constraint: %', tabela, restricao;
end;
$$ language plpgsql;
select ffoo(2,3);
select ffoo(5,1);

Credits for Euler Taveira from the Brazilian PostgreSQL Community ([email protected])

When an error occurs, next to the code representing the type of it, PostgreSQL also returns a text containing error message; in the case of unique constraints, the text of the error message itself contains which constraint has been violated.

create table t (
    email text unique,
    login text unique
);

with d (email, login) as ( values
    ('[email protected]','fulano')
), e as (
    select
        exists (select 1 from t inner join d using(email)) as email,
        exists (select 1 from t inner join d using(login)) as login
), i as (
    insert into t (email, login)
    select email, login
    from d
    where not(select email or login from e)
    returning 'email inexistente' as email, 'login inexistente' as login
)
select
    case when email then 'violação de email único' else 'email inexistente' end as email,
    case when login then 'violação de login único' else 'login inexistente' end as login
from e
where email or login
union all
select email::text, login::text
from i

It may be easier to parse the error message if the driver used returns the message.