Josh Berkus: Fancy SQL Friday: subtracting arrays

Here's one which just came up:  how to see all of the elements in a new array which were not in the old array.  This isn't currently supported by any of PostgreSQL's array operators, but thanks to UNNEST() and custom operators, you can create your own:

    create or replace function diff_elements_text (
        text[], text[] )
    returns text[]
    language sql
    immutable
    as $f$
    SELECT array_agg(DISTINCT new_arr.elem)
    FROM
        unnest($2) as new_arr(elem)
        LEFT OUTER JOIN
        unnest($1) as old_arr(elem)
        ON new_arr.elem = old_arr.elem
    WHERE old_arr.elem IS NULL;
    $f$;

    create operator - (
        procedure = diff_elements_text,
        leftarg = text[],
        rightarg = text[]
    );

Now you can just subtract text arrays:

    ktsf=# select array['a','n','z'] - array['n','z','d','e'];
    ?column?
    ----------
    {d,e}
    (1 row)

Unfortunately, you'll need to create a new function and operator for each base  type; I haven't been able to get it to work with "anyarray".  But this should save you some time/code on array comparisons.  Enjoy!