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

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.

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?

Maybe try combining them into a single array then performing array
comparisons...

On Wednesday, March 22, 2017, Glen Huang <hey.hgl@gmail.com> wrote:

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 Schl�sselwert verletzt Unique-Constraint
�index_unique_foo�
DETAIL: Schl�ssel �(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

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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/UPDATE to 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.

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Yes, the order doesn't matter, and this approach sounds like a good idea. I'll try it out, thanks.