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

Thanks.

Didn't realize it could be implemented with a exclusion constraint. The comparing between any two row definitely sounds like the right direction. But I'm still having a hard time figuring out how i should write the `exclude_element WITH operator` part, which I think, should detect if specified columns consist of the same items, regardless the order? could `exclude_element` contains multiple columns? (from the syntax it looks like it's impossible) And is there such an operator to compare multiple columns?

> On 23 Mar 2017, at 1:04 AM, David G. Johnston <david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
>
> On Wed, Mar 22, 2017 at 9:54 AM, Glen Huang <hey(dot)hgl(at)gmail(dot)com <mailto: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?
>
> Sorry if the question is too basic, but I couldn't find the answer in the doc, at least not in the chapter on unique index.
>
> The most direct option to consider is a exclusion constraint.
>
> https://www.postgresql.org/docs/current/static/ddl-constraints.html <https://www.postgresql.org/docs/current/static/ddl-constraints.html> (bottom of page)
>
> David J.