postgresql - Postgres 使用 LIMIT 1 选择性能

Question

我在 PostgreSQL 中有一个包含大约 5000 万条记录的表。试图通过“标签”过滤选择“喜欢”最多的帖子。两个字段都有 b-tree 索引。对于“爱”标签，我得到

EXPLAIN analyse select user_id from posts where tags @> array['love'] order by likes desc nulls last limit 12

Limit  (cost=0.57..218.52 rows=12 width=12) (actual time=2.658..14.243 rows=12 loops=1)
  ->  Index Scan using idx_likes on posts  (cost=0.57..55759782.55 rows=3070010 width=12) (actual time=2.657..14.239 rows=12 loops=1)
        Filter: (tags @> '{love}'::text[])
        Rows Removed by Filter: 10584
Planning time: 0.297 ms
Execution time: 14.276 ms

14 毫秒很好，但如果我尝试为“tamir”获取它，它突然变成超过 22 秒!!显然查询规划器做错了什么。

EXPLAIN analyse select user_id from posts where tags @> array['tamir'] order by likes desc nulls last limit 12

Limit  (cost=0.57..25747.73 rows=12 width=12) (actual time=17552.406..22839.503 rows=12 loops=1)
  ->  Index Scan using idx_likes on posts  (cost=0.57..55759782.55 rows=25988 width=12) (actual time=17552.405..22839.484 rows=12 loops=1)
        Filter: (tags @> '{tamir}'::text[])
        Rows Removed by Filter: 11785083
Planning time: 0.253 ms
Execution time: 22839.569 ms

看完the article我在 ORDER BY 中添加了“user_id”，“tamir”的速度非常快，0.2 毫秒!现在它正在执行排序和位图堆扫描，而不是索引扫描。

EXPLAIN analyse select user_id from posts where tags @> array['tamir'] order by likes desc nulls last, user_id limit 12

Limit  (cost=101566.17..101566.20 rows=12 width=12) (actual time=0.237..0.238 rows=12 loops=1)
  ->  Sort  (cost=101566.17..101631.14 rows=25988 width=12) (actual time=0.237..0.237 rows=12 loops=1)
        Sort Key: likes DESC NULLS LAST, user_id
        Sort Method: top-N heapsort  Memory: 25kB
        ->  Bitmap Heap Scan on posts  (cost=265.40..100970.40 rows=25988 width=12) (actual time=0.074..0.214 rows=126 loops=1)
              Recheck Cond: (tags @> '{tamir}'::text[])
              Heap Blocks: exact=44
              ->  Bitmap Index Scan on idx_tags  (cost=0.00..258.91 rows=25988 width=0) (actual time=0.056..0.056 rows=126 loops=1)
                    Index Cond: (tags @> '{tamir}'::text[])
Planning time: 0.287 ms
Execution time: 0.277 ms

但是“爱”会发生什么？现在它从 14 毫秒变为 2.3 秒...

EXPLAIN analyse select user_id from posts where tags @> array['love'] order by likes desc nulls last, user_id limit 12

Limit  (cost=7347142.18..7347142.21 rows=12 width=12) (actual time=2360.784..2360.786 rows=12 loops=1)
  ->  Sort  (cost=7347142.18..7354817.20 rows=3070010 width=12) (actual time=2360.783..2360.784 rows=12 loops=1)
        Sort Key: likes DESC NULLS LAST, user_id
        Sort Method: top-N heapsort  Memory: 25kB
        ->  Bitmap Heap Scan on posts  (cost=28316.58..7276762.77 rows=3070010 width=12) (actual time=595.274..2171.571 rows=1517679 loops=1)
              Recheck Cond: (tags @> '{love}'::text[])
              Heap Blocks: exact=642705
              ->  Bitmap Index Scan on idx_tags  (cost=0.00..27549.08 rows=3070010 width=0) (actual time=367.080..367.080 rows=1517679 loops=1)
                    Index Cond: (tags @> '{love}'::text[])
Planning time: 0.226 ms
Execution time: 2360.863 ms

有人可以阐明为什么会发生这种情况以及解决方法是什么。

更新

“tag”字段有 gin 索引，不是 b-tree，只是拼写错误。

Answer 1

B 树索引对于在数组字段中搜索元素不是很有用。您应该从 tags 字段中删除 b-tree 索引并改用 gin 索引:

drop index idx_tags;
create index idx_tags using gin(tags);

并且不要按 user_id 添加顺序——当有很多行带有您搜索的标签时，这会破坏使用您的 idx_likes 进行排序的可能性。

---

此外，likes 字段应该not null default 0。