sql - Postgres - Function to return the intersection of 2 ARRAYs?

Question

In postgresql, you can use the && operator to return t (true) if two arrays have common members, i.e. they overlap. Is there a function/operator that will return what those common members are?

i.e. something like this

select arrray_intersection(ARRAY[1, 4, 2], ARRAY[2, 3]);
ARRAY[2]

Answer 1

Since 8.4, there are useful builtins in Postgres which make the function from the first answer easier and possibly faster (that's what EXPLAIN tells me, anyway: "(cost=0.00..0.07 rows=1 width=64)" for this query vs. "(cost=0.00..60.02 rows=1 width=64)" for the original one).

The simplified code is:

SELECT ARRAY
    (
        SELECT UNNEST(a1)
        INTERSECT
        SELECT UNNEST(a2)
    )
FROM  (
        SELECT  array['two', 'four', 'six'] AS a1
              , array['four', 'six', 'eight'] AS a2
      ) q;

and yeah, you can turn it into a function:

CREATE FUNCTION array_intersect(anyarray, anyarray)
  RETURNS anyarray
  language sql
as $FUNCTION$
    SELECT ARRAY(
        SELECT UNNEST($1)
        INTERSECT
        SELECT UNNEST($2)
    );
$FUNCTION$;

which you can call as

SELECT array_intersect(array['two', 'four', 'six']
                     , array['four', 'six', 'eight']);

But you can just as well call it inline too:

 SELECT array(select unnest(array['two', 'four', 'six']) intersect
              select unnest(array['four', 'six', 'eight']));