Seq Scan because of stats or because of cast?

I'm interested in the members of specific roles, providing the roles of
interest to the query via an array of integers (binary bind in code, not
textual array literal like I had to use to have the EXPLAIN work, see
below).

I figured that query would use the "pg_auth_members_role_member_index"
index,
but instead it's using a sequential scan.

And I'm wondering is this is because the cardinality of that catalog is
small (172),
which is just an artifact of my dev-testing, or whether that's because I
cast roleid
to an int4, preventing the use of the index?

In production, the cardinality will be much greator, which is why I worry a
bit.
Also, I don't really need the grantor and admin_option columns for now, thus
it could even be an index-only scan, IF the index was used by the plan.

I tried changing the cast around, or allowing an index-only scan,
but it's still a Seq Scan on the table (see below).

Is there a way to know why the index is not used, in any of my attempts?

I currently does not support (binary) binding Oids in my case, thus the
::int4 casts.
Would supporting binding actual Oid arrays instead of Int4 arrays help in
this case?

I'd appreciate some insights here. Thanks, --DD

PS: libpq uses unsigned int for Oid. So do OIDs go above 2GB in practice?
I'm asking, since I'm casting to ::int4, thus if they do, then that
case might overflow.

PPS: Are OIDs recycled / reused? Or are they monotonically increasing?
What happens when the Cluster runs out of OIDs?
Are they Cluster-wide unique or it depends on the OID type?

dd_pns2=> explain SELECT roleid::int4, member::int4, grantor::int4,
admin_option
dd_pns2-> FROM pg_auth_members
dd_pns2-> WHERE roleid::int4 = ANY($1);
ERROR: there is no parameter $1
LINE 3: WHERE roleid::int4 = ANY($1);
^
dd_pns2=> explain SELECT roleid::int4, member::int4, grantor::int4,
admin_option
dd_pns2-> FROM pg_auth_members
dd_pns2-> WHERE roleid::int4 = ANY(array[1,2,3]);
QUERY PLAN
----------------------------------------------------------------
Seq Scan on pg_auth_members (cost=0.00..5.33 rows=3 width=13)
Filter: ((roleid)::integer = ANY ('{1,2,3}'::integer[]))
(2 rows)

dd_pns2=> explain SELECT roleid::int4, member::int4, grantor::int4,
admin_option
dd_pns2-> FROM pg_auth_members
dd_pns2-> WHERE roleid = ANY(array[1::oid,2::oid,3::oid]);
QUERY PLAN
----------------------------------------------------------------
Seq Scan on pg_auth_members (cost=0.00..5.33 rows=3 width=13)
Filter: (roleid = ANY ('{1,2,3}'::oid[]))
(2 rows)

dd_pns2=> explain SELECT roleid::int4, member::int4
dd_pns2-> FROM pg_auth_members
dd_pns2-> WHERE roleid = ANY(array[1::oid,2::oid,3::oid]);
QUERY PLAN
---------------------------------------------------------------
Seq Scan on pg_auth_members (cost=0.00..5.33 rows=3 width=8)
Filter: (roleid = ANY ('{1,2,3}'::oid[]))
(2 rows)

dd_pns2=> \d pg_auth_members
Table "pg_catalog.pg_auth_members"
Column | Type | Collation | Nullable | Default
--------------+---------+-----------+----------+---------
roleid | oid | | not null |
member | oid | | not null |
grantor | oid | | not null |
admin_option | boolean | | not null |
Indexes:
"pg_auth_members_member_role_index" UNIQUE, btree (member, roleid),
tablespace "pg_global"
"pg_auth_members_role_member_index" UNIQUE, btree (roleid, member),
tablespace "pg_global"
Tablespace: "pg_global"

dd_pns2=> select count(*) from pg_auth_members;
count
-------
172
(1 row)