PostgreSQL优化:顺序扫描VS索引扫描

我有一个关于postgres表上select的有趣案例:

advert (~2.5 million records)
    id serial,
    user_id integer (foreign key),
    ...

这是我的选择:

select count(*) from advert where user_id in USER_IDS_ARRAY

如果 USER_IDS_ARRAY 长度<=100我接下来要解释分析:

Aggregate  (cost=18063.36..18063.37 rows=1 width=0) (actual time=0.362..0.362 rows=1 loops=1)
  ->  Index Only Scan using ix__advert__user_id on advert  (cost=0.55..18048.53 rows=5932 width=0) (actual time=0.030..0.351 rows=213 loops=1)
        Index Cond: (user_id = ANY ('{(...)}'))
        Heap Fetches: 213
Planning time: 0.457 ms
Execution time: 0.392 ms

但当 用户ID数组 长度>100:

Aggregate  (cost=424012.09..424012.10 rows=1 width=0) (actual time=867.438..867.438 rows=1 loops=1)
  ->  Seq Scan on advert  (cost=0.00..423997.11 rows=5992 width=0) (actual time=0.375..867.345 rows=213 loops=1)
        Filter: (user_id = ANY ('{(...)}'))
        Rows Removed by Filter: 2201318
Planning time: 0.261 ms
Execution time: 867.462 ms

无论user_ids_ARRAY中的user_id是什么,只有长度才重要。

有人知道如何为超过100个user_id优化这个选择吗?