Re: Order-independent multi-field uniqueness constraint?

Kynn Jones wrote:
> I have a table used to store information about pairs of items.  This
> information is independent of the order of the two items in the pair,
> so having two records
>
>   X    Y    <info>
>   Y    X    <info>
>
> in the table would be redundant.  But as far as I can tell, this
> situation would not violate a uniqueness constraint involving the two
> fields.
>
> I could add the original constraint that enforces some canonical
> order, say X < Y (assuming that they are integer IDs), but I'm trying
> to avoid this because it would lead to a significant complication of
> many of my queries, which currently ascribe slightly different
> semantics to the first and second members of the pair.
>
> The only solution I could think of is to write a function that takes
> the two elements as input and returns them in some canonical order:
>
> CREATE OR REPLACE FUNCTION canonicalize( anyelement, anyelement )
>   RETURNS anyarray AS
> $$
> BEGIN
>   IF $1 < $2 THEN RETURN ARRAY[ $1, $2 ];
>   ELSE            RETURN ARRAY[ $2, $1 ];
>   END IF;
> END;
> $$ LANGUAGE plpgsql;
>
> and this function works as expected, but when I try to use it in a
> constraint I get the error:
>
> -> ALTER TABLE foo ADD CONSTRAINT foo_uniq_x_y UNIQUE(canonicalize(x,y));
> ERROR:  42601: syntax error at or near "("
> LINE 1: ...E foo ADD CONSTRAINT foo_uniq_x_y UNIQUE(canonicalize(x,y));
>                                                                 ^
> LOCATION:  base_yyerror, scan.l:795
>
> I found this puzzling; it's not clear to me why UNIQUE(UPPER(x)) is OK
> syntax but not UNIQUE(my_function(x)).
>
> But be that as it may, is there any way to enforce an
> order-independent uniqueness constraint without forcing a canonical
> ordering on the elements saved in the table.
>

I'm not sure that what you're doing is the best solution, but shouldn't
that be: "... foo_uniq_x_y UNIQUE(SELECT canonicalize(x,y))"?

brian