Thread: [GENERAL] How to create unique index on multiple columns where the combination doesn't matter?

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.

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.g.johnston@gmail.com> wrote:

On Wed, Mar 22, 2017 at 9:54 AM, Glen Huang <hey.hgl@gmail.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 (bottom of page)

David J.

Glen Huang <hey.hgl@gmail.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

> On 22 Mar 2017, at 17:54, Glen Huang <hey.hgl@gmail.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?

Does the order of the values of (obj1, obj2, obj3) in relationship matter? If not, you could swap them around on
INSERT/UPDATEto be in sorted order. I'd probably go with a BEFORE INSERT OR UPDATE trigger. 

In addition, to prevent unsorted entry, on obj2 add CHECK (obj2 > obj1) and on obj3 add CHECK (obj3 > obj2).

Now you can create a normal PK or unique key on (obj1, obj2, obj3) as the order of their values is not variable
anymore.

Alban Hertroys
--
If you can't see the forest for the trees,
cut the trees and you'll find there is no forest.