Re: How to create unique index on multiple columns where the combination doesn't matter?

Glen Huang <hey(dot)hgl(at)gmail(dot)com> wrote:

> Hello,
>
> If I have a table like
>
> CREATE TABLE relationship (
> obj1 INTEGER NOT NULL REFERENCES object,
> obj2 INTEGER NOT NULL REFERENCES object,
> obj3 INTEGER NOT NULL REFERENCES object,
> ...
> )
>
> And I want to constrain that if 1,2,3 is already in the table, rows like 1,3,2 or 2,1,3 shouldn't be allowed.
>
> Is there a general solution to this problem?

Sure.

test=*# create extension intarray;
CREATE EXTENSION
test=*# create table foo(c1 int, c2 int, c3 int);
CREATE TABLE
test=*# create unique index index_unique_foo on
foo(sort(array[c1,c2,c3],'asc'));
CREATE INDEX
test=*# insert into foo values (1,2,3);
INSERT 0 1
test=*# insert into foo values (3,2,1);
FEHLER: doppelter Schlüsselwert verletzt Unique-Constraint
»index_unique_foo«
DETAIL: Schlüssel »(sort(ARRAY[c1, c2, c3], 'asc'::text))=({1,2,3})«
existiert bereits.
test=*#

(sorry for german messages, it means error, dublicate entry ...)

Regards, Andreas Kretschmer
--
Andreas Kretschmer
http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services