sql - Unique constraint on combination of two columns?

Question

I have been trying to figure out if it is possible to have a unique constraint for a combination of two columns.

Specifically I have two columns A and B.

I have a row like below

A     B
1     2

Then I want the following combinations to fail when inserted

A     B
1     2
2     1

I have tried adding a simple constraint

ALTER TABLE test ADD CONSTRAINT test_constraint UNIQUE (a, b);

but this lets me insert (2, 1) when (1, 2) already exists.

Is this possible to do? Or will I have to check if the combination exists before I insert?

Answer 1

You can do this using an index on expressions:

create unique index unq_test_a_b on (test(least(a, b), greatest(a, b));

I don't think the unique constraint allows expressions (and don't have a convenient Postgres to test on right now), but this is essentially the same thing.