为什么 postgresql 没有使用多列索引

当我想使用这个 sql 来检查是否使用索引时：

explain SELECT id AS id, sub_source_id AS sub_source_id 
FROM article 
WHERE sub_source_id IN (
select sub_source_id 
    from sub_relation sr 
    where user_id =14
    and sub_status = 1

) ORDER BY created_time DESC LIMIT 50

显示这样的信息：

    Workers Planned: 2
    ->  Sort  (cost=74344.09..74697.64 rows=141422 width=24)
          Sort Key: article.created_time DESC
          ->  Hash Join  (cost=77.11..69646.15 rows=141422 width=24)
                Hash Cond: (article.sub_source_id = sr.sub_source_id)
                ->  Parallel Seq Scan on article  (cost=0.00..67792.05 rows=675805 width=24)
                ->  Hash  (cost=70.56..70.56 rows=524 width=8)
                      ->  Seq Scan on sub_relation sr  (cost=0.00..70.56 rows=524 width=8)
                            Filter: ((user_id = 14) AND (sub_status = 1))

我已经用 columnsub_source_id和建立了多列索引created_time，为什么 sql 没有使用这个article_sub_source_id_create_time_idx 索引？这是表结构：

-- public.article definition

-- Drop table

-- DROP TABLE public.article;

CREATE TABLE public.article (
    id int8 NOT NULL GENERATED ALWAYS AS IDENTITY,
    user_id int8 NOT NULL,
    title varchar(512) NOT NULL,
    author varchar(256) NOT NULL,
    guid varchar(512) NOT NULL,
    created_time int8 NOT NULL,
    updated_time int8 NOT NULL,
    link varchar(512) NULL,
    pub_time timestamptz NULL,
    sub_source_id int8 NOT NULL,
    cover_image varchar(1024) NULL,
    channel_reputation int4 NOT NULL DEFAULT 0,
    editor_pick int4 NULL DEFAULT 0,
    permanent_store int2 NOT NULL DEFAULT 0,
    CONSTRAINT article_id_seq_pk PRIMARY KEY (id),
    CONSTRAINT article_title_sub_source_id_key UNIQUE (title, sub_source_id)
);
CREATE INDEX article_sub_source_id_create_time_idx ON public.article USING btree (sub_source_id, created_time);
CREATE INDEX article_title_gin ON public.article USING gin (to_tsvector('english'::regconfig, (title)::text));
CREATE INDEX article_title_zh_gin ON public.article USING gin (to_tsvector('dolphinzhcfg'::regconfig, (title)::text));

当我删除子查询并用静态子源 ID 替换时，外部查询可以使用article_sub_source_id_create_time_idx 索引。