Re: two-column primary key (not the typical question)

I don't think it's possible using PKeys. It can be done with unique expression index combined with NOT NULL constraints.

Here's a working example:

postgres=# create table test3( a varchar, b varchar);
CREATE TABLE
postgres=# create unique index on test3 ((case when a < b then a || b else b || a end));
CREATE INDEX
postgres=# alter table test3 alter a set not null, alter b set not null;
ALTER TABLE


postgres=# insert into test3 values('foo', 'bar');
INSERT 0 1
postgres=# insert into test3 values('foo', 'bar');
ERROR:  duplicate key value violates unique constraint "test3_case_idx"
DETAIL:  Key ((
CASE
    WHEN a::text < b::text THEN a::text || b::text
    ELSE b::text || a::text
END))=(barfoo) already exists.
postgres=# insert into test3 values('bar', 'foo');
ERROR:  duplicate key value violates unique constraint "test3_case_idx"
DETAIL:  Key ((
CASE
    WHEN a::text < b::text THEN a::text || b::text
    ELSE b::text || a::text
END))=(barfoo) already exists.
postgres=#