允许两种方式组合的 Postgresql 唯一约束

Question

I have a unique constraint in postgresql to join the Id's from two tables

TableA
IdA
1
2
3

TableB
IdB
1
2
3

TableJoin
IdA   IdB
1     1   --good
1     2   --good
1     3   --good

But if I want to Insert the another Id's

TableJoin
IdA   IdB
1     1   --good
1     2   --good
1     3   --good
2     1   --return error, because already exist 1   2 (I need save 2   1)
2     2   --good
2     3   --good
3     1   --return error, because already exist 1   3 (I need save 3   1)

My unique is:

ALTER TABLE TableJoin ADD CONSTRAINT "UX_Join" UNIQUE ("IdA", "IdB");

How can I insert two ways (1 2 AND 2 1) ?

Another alter unique?

Ty for help me !!!

Answer 1

It was answered somewhere.

basically for integers, the easiest would be creating a UK like this:

create unique index uk on tablejoin (least(ida,idb), greatest(ida,idb))

for not integers you can add to array and unnest with order in index. and even another way was with jsonb functions I believe. But greatest/least is the best option for two keys integer index...

here's a example:

t=# create table a (i int, e int);
CREATE TABLE
Time: 27.528 ms
t=# create unique index u on a (i,e);
CREATE INDEX
Time: 25.309 ms
t=# insert into a select 1,2;
INSERT 0 1
Time: 13.972 ms
t=# insert into a select 2,1;
INSERT 0 1
Time: 8.759 ms
t=# select * from a;
 i | e
---+---
 1 | 2
 2 | 1
(2 rows)

Time: 14.570 ms
t=# create unique index u1 on a (least(i,e),greatest(i,e));
ERROR:  could not create unique index "u1"
DETAIL:  Key ((LEAST(i, e)), (GREATEST(i, e)))=(1, 2) is duplicated.
Time: 15.241 ms