Question

I have a table with 4 array columns.. the results are like:

ids       signed_ids   new_ids   new_ids_signed
{1,2,3} | {2,1,3}    | {4,5,6} | {6,5,4}

Anyway to compare ids and signed_ids so that they come out equal, by ignoring the order of the elements?

Answer 1

The simplest thing to do is sort them and compare them sorted. See sorting arrays in PostgreSQL.

Given sample data:

CREATE TABLE aa(ids integer[], signed_ids integer[]);
INSERT INTO aa(ids, signed_ids) VALUES (ARRAY[1,2,3], ARRAY[2,1,3]);

the best thing to do is to if the array entries are always integers is to use the intarray extension, as Erwin explains in his answer. It’s a lot faster than any pure-SQL formulation.

Otherwise, for a general version that works for any data type, define an array_sort(anyarray):

CREATE OR REPLACE FUNCTION array_sort(anyarray) RETURNS anyarray AS $$
SELECT array_agg(x order by x) FROM unnest($1) x;
$$ LANGUAGE 'SQL';

and use it sort and compare the sorted arrays:

SELECT array_sort(ids) = array_sort(signed_ids) FROM aa;

There’s an important caveat:

SELECT array_sort( ARRAY[1,2,2,4,4] ) = array_sort( ARRAY[1,2,4] );

will be false. This may or may not be what you want, depending on your intentions.

---

Alternately, define a function array_compare_as_set:

CREATE OR REPLACE FUNCTION array_compare_as_set(anyarray,anyarray) RETURNS boolean AS $$
SELECT CASE
  WHEN array_dims($1) <> array_dims($2) THEN
    'f'
  WHEN array_length($1,1) <> array_length($2,1) THEN
    'f'
  ELSE
    NOT EXISTS (
        SELECT 1
        FROM unnest($1) a 
        FULL JOIN unnest($2) b ON (a=b) 
        WHERE a IS NULL or b IS NULL
    )
  END
$$ LANGUAGE 'SQL' IMMUTABLE;

and then:

SELECT array_compare_as_set(ids, signed_ids) FROM aa;

This is subtly different from comparing two array_sorted values. array_compare_as_set will eliminate duplicates, making array_compare_as_set(ARRAY[1,2,3,3],ARRAY[1,2,3]) true, whereas array_sort(ARRAY[1,2,3,3]) = array_sort(ARRAY[1,2,3]) will be false.

Both of these approaches will have pretty bad performance. Consider ensuring that you always store your arrays sorted in the first place.