优化拥有谓词or的子查询

于节点2上发现一个sql，其结构如下：
select distinct t.attr, t.item
from a t
where attr = :1
  and (pro = :2 or exists
       (select 1
          from b hps
         where hps.pro = :3
           and t.pro = hps.sub_pro))
鉴于以往的经验，一开始就觉得子查询中的连接谓词or有问题。
注:
表a有50多万条记录，attr和pro上分别有索引；表b有10万条记录，pro上有单独索引，同时(pro,sub_pro)组成unique索引；
同时表a上的attr,pro字段的分布情况如下,对于表a，pro字段的选择性比attr高出了很多
SQL> select count(distinct pro),count(distinct attr) from a;

COUNT(DISTINCT pro)           COUNT(DISTINCT attr)
------------------------- ---------------------------
          164067                         716
          

先通过set autotrace traceonly查看一把其执行计划和consistent gets
--------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name                   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                        |    63 |   945 |    83   (2)| 00:00:01 |
|   1 |  HASH UNIQUE                  |                        |    63 |   945 |    83   (2)| 00:00:01 |
|*  2 |   FILTER                      |                        |       |       |            |          |
|   3 |    TABLE ACCESS BY INDEX ROWID| a                      |  1268 | 19020 |    82   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | idx_a_attr             |  1268 |       |     8   (0)| 00:00:01 |
|*  5 |    INDEX UNIQUE SCAN          | idx_b_un_pro_sub       |     1 |    12 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------

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