Question

我在PostgreSQL中有一个包含大约5000万条记录的表。试图选择最多的帖子＆＃34;喜欢＆＃34;通过＆＃34;标记过滤＆＃34;。两个字段都有b-tree索引。对于＆＃34;爱＆＃34;我得到了标签

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毫秒很棒，但是如果我试图为＃t; tamir＆＃34;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

阅读the article后，我添加了＆＃34; user_id＆＃34;通过ORDER BY和＆＃34; tamir＆＃34;非常快，0.2ms！现在它正在进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＆＃34;爱情＆＃34;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

有人可以解释为什么会发生这种情况以及解决方法。

更新

＆＃34;标记＆＃34;字段有杜松子酒索引，而不是b-tree，只是错字。

Answer 1

B树索引对于搜索数组字段中的元素不是很有用。您应该从tags字段中删除b-tree索引并改为使用gin索引：

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

并且不要通过user_id添加订单 - 这种破坏可能会使用您的idx_likes进行订购，因为您搜索的代码中有很多行。

同样likes字段可能应为not null default 0。

Postgres使用LIMIT 1选择性能

1 个答案: