Postgresql enforce unique two-way combination of columns

A variation on Neil's solution which doesn't need an extension is:

create table friendz (
  from_id int,
  to_id int
);

create unique index ifriendz on friendz(greatest(from_id,to_id), least(from_id,to_id));

Neil's solution lets you use an arbitrary number of columns though.

We're both relying on using expressions to build the index which is documented https://www.postgresql.org/docs/current/indexes-expressional.html

Do you consider the intarray extension to be magic?

You'd need to use int keys for the users instead of text though...

Here's a possible solution:

create extension intarray;

create table friendz (
  from_id int,
  to_id int
);

create unique index on friendz ( sort( array[from_id, to_id ] ) );

insert into friendz values (1,2); -- good

insert into friendz values (2,1); -- bad

http://sqlfiddle.com/#!15/c84b7/1