Here 1-pass version, if you have improvement suggestions, you are welcome -
DROP FUNCTION IF EXISTS words_array_subtract(anyarray, anyarray);
CREATE OR REPLACE FUNCTION words_array_subtract(from_array anyarray, remove_array anyarray)
RETURNS anyarray AS
$func$
DECLARE
i integer;
j integer;
remove_ignore boolean[];
result_array from_array%TYPE := '{}';
BEGIN
IF NOT from_array @> remove_array THEN
RAISE EXCEPTION '% does not contain %', from_array, remove_array;
END IF;
remove_ignore := ARRAY_FILL(FALSE, ARRAY[ARRAY_LENGTH(remove_array, 1)]);
RAISE NOTICE 'remove_ignore = %', remove_ignore;
<<from_loop>>
FOR i IN ARRAY_LOWER(from_array, 1)..ARRAY_UPPER(from_array, 1) LOOP
FOR j IN ARRAY_LOWER(remove_array, 1)..ARRAY_UPPER(remove_array, 1) LOOP
IF remove_ignore[j] = FALSE AND
from_array[i] = remove_array[j] THEN
remove_ignore[j] := TRUE;
CONTINUE from_loop;
END IF;
END LOOP;
result_array := ARRAY_APPEND(result_array, from_array[i]);
END LOOP;
RETURN result_array;
END;
$func$ LANGUAGE plpgsql;
# select words_array_subtract(ARRAY[1,2,2,3,4,4, 5], ARRAY[2,4,4]);
NOTICE: remove_ignore = {f,f,f}
words_array_subtract
----------------------
{1,2,3,5}
(1 row)
# select words_array_subtract(ARRAY['A','A','B','B','C'], ARRAY['A','B']);
NOTICE: remove_ignore = {f,f}
words_array_subtract
----------------------
{A,B,C}
(1 row)
