Setting constraint for two unique fields in PostgreSQL

Question

I'm new to postgres. I wonder, what is a PostgreSQL way to set a constraint for a couple of unique values (so that each pair would be unique). Should I create an INDEX for bar and baz fields?

CREATE UNIQUE INDEX foo ON table_name(bar, baz);

If not, what is a right way to do that? Thanks in advance.

Answer 1

You can do what you are already thinking of: create a unique constraint on both fields . This way, a unique index will be created behind the scenes, and you will get the behavior you need. Plus, that information can be picked up by information_schema to do some metadata inferring if necessary on the fact that both need to be unique. I would recommend this option. You can also use triggers for this, but a unique constraint is way better for this specific requirement.

Answer 2

If each field needs to be unique unto itself, then create unique indexes on each field. If they need to be unique in combination only, then create a single unique index across both fields.

Don't forget to set each field NOT NULL if it should be. NULLs are never unique, so something like this can happen:

create table test (a int, b int);
create unique index test_a_b_unq on test (a,b);
insert into test values (NULL,1);
insert into test values (NULL,1);

and get no error. Because the two NULLs are not unique.