Here is the oracle version, tested on 10g.
declare v_exists integer;
begin
select count(*) into v_exists from user_indexes where lower(index_name)='acs_permissions_object_id_idx';
if v_exists = 0 then
execute immediate 'create index acs_permissions_object_id_idx on acs_permissions(object_id)';
end if;
end;
/
show errors
