create temp table fact as select generate_series(1,999) f1, generate_series(1,999) f2;
insert into fact select s,null from generate_series(1,999) s;
insert into fact select null,s from generate_series(1,999) s;
create index f_f1 on fact(f1);
create index f_f2 on fact(f2);
analyze fact;

create temp table dim as select s, s::text t from generate_series(1,999) s;
alter table dim add primary key(s), add unique (t);
create temp view denorm as
    select f.*, d1.t t1, d2.t t2, array[f1,f2] ident_ids
    from fact f
    left join dim d1 on f1=d1.s
    left join dim d2 on f2=d2.s
;

-- Fast
explain analyze select count(*) from denorm where 1 in (f1,f2);
explain analyze select count(*) from denorm where '1' in (t1);

-- Slow
explain analyze select count(*) from denorm where '1' in (t1,t2);

CREATE FUNCTION ident_ids(
    idents text[]
) RETURNS int[] STABLE LANGUAGE sql AS $$
SELECT array( SELECT s FROM dim WHERE t = ANY(idents) )
$$;
CREATE FUNCTION ident_ids(
    idents text
) RETURNS int[] STABLE LANGUAGE sql AS $$
SELECT ident_ids( ('{' || idents || '}')::text[] )
$$;

explain analyze select count(*) from denorm where ident_ids && ident_ids('42,84,128');

\echo ERROR OK here on version >= 10
create index fact__f_array on fact using gin ((array[f1,f2]) _int4_ops); -- pre 10.0
\echo ERROR OK here on version < 10
create index fact__f_array on fact using gin ((array[f1,f2]) array_ops); -- 10.0

explain analyze select count(*) from denorm where ident_ids && ident_ids('42,84,128');