Question

I’m not sure I’ve phrased the question correctly, so I’ll try a longer explanation. I have this kind of table:

CREATE TABLE x (a int, b int);

I want to consider the pair (a,b) to be identical to (b,a), and to disallow insertion of duplicates. If PostgreSQL had a set data type, I might declare the table like this:

CREATE TABLE x (
    ab set,
    UNIQUE (ab)
);

But it doesn’t, so what’s the best way of doing this?

Answer 1

create unique index idx_unique_ab 
    on x (least(a,b), greatest(a,b));