Unique constraint for permutations across multiple columns

Question

Given the following three columns in a Postgres database: first, second, third; how can I create a constraint such that permutations are unique?

E.g. If (\'foo

Answer 1

For only three columns this unique index using only basic expressions should perform very well. No additional modules like hstore or custom function needed:

CREATE UNIQUE INDEX t_abc_uni_idx ON t (
  LEAST(a,b,c)
, GREATEST(LEAST(a,b), LEAST(b,c), LEAST(a,c))
, GREATEST(a,b,c)
);

SQL fiddle

Also needs the least disk space:

SELECT pg_column_size(row(hstore(t))) AS hst_row
      ,pg_column_size(row(hstore(ARRAY[a,b,c], ARRAY[a,b,c]))) AS hst1
      ,pg_column_size(row(hstore(ARRAY[a,b,c], ARRAY[null,null,null]))) AS hst2
      ,pg_column_size(row(ARRAY[a,b,c])) AS arr
      ,pg_column_size(row(LEAST(a,b,c)
                        , GREATEST(LEAST(a,b), LEAST(b,c), LEAST(a,c))
                        , GREATEST(a,b,c))) AS columns
FROM t;

 hst_row | hst1 | hst2 | arr | columns
---------+------+------+-----+---------
      59 |   59 |   56 |  69 |      30

Numbers are bytes for index row in the example in the fiddle, measured with pg_column_size(). My example uses only single characters, the difference in size is constant.