Question

我有两张桌子：

用户 id |名字..

拉请求 id | user_id | created_at | ...

我需要获取所有用户加入他们的特定年份的拉取请求数。所以我写了一个像这样的查询：

SELECT users.*, COUNT(pull_requests.id) as pull_requests_count
FROM "users" INNER JOIN
     "pull_requests"
     ON "pull_requests"."user_id" = "users"."id"
WHERE (EXTRACT(year FROM pull_requests.created_at) = 2013)
GROUP BY users.id

我最初有索引，

pull_requests.user_id（btree）。在做解释时我得到了这个：

                                                     QUERY PLAN
--------------------------------------------------------------------------------------------------------------------
 HashAggregate  (cost=18.93..18.96 rows=3 width=2775)
   ->  Hash Join  (cost=14.13..18.92 rows=3 width=2775)
         Hash Cond: (users.id = pull_requests.user_id)
         ->  Seq Scan on users  (cost=0.00..4.08 rows=108 width=2771)
         ->  Hash  (cost=14.09..14.09 rows=3 width=8)
               ->  Bitmap Heap Scan on pull_requests  (cost=4.28..14.09 rows=3 width=8)
                     Recheck Cond: (date_part('year'::text, created_at) = 2013::double precision)
                     ->  Bitmap Index Scan on pull_req_extract_year_created_at_ix  (cost=0.00..4.28 rows=3 width=0)
                           Index Cond: (date_part('year'::text, created_at) = 2013::double precision)

然后我添加了这样的索引：

CREATE INDEX pull_req_extract_year_created_at_ix ON pull_requests (EXTRACT(year FROM created_at));

现在我的解释是：

                                         QUERY PLAN
--------------------------------------------------------------------------------------------
 HashAggregate  (cost=63.99..64.02 rows=3 width=2775)
   ->  Hash Join  (cost=59.19..63.98 rows=3 width=2775)
         Hash Cond: (users.id = pull_requests.user_id)
         ->  Seq Scan on users  (cost=0.00..4.08 rows=108 width=2771)
         ->  Hash  (cost=59.16..59.16 rows=3 width=8)
               ->  Seq Scan on pull_requests  (cost=0.00..59.16 rows=3 width=8)
                     Filter: (date_part('year'::text, created_at) = 2013::double precision)

对于100行左右，我仍然得到6.6毫秒。我该如何进一步优化这个？

谢谢！

Answer 1

尝试将两个索引合并为一个：

CREATE INDEX pr_ix ON pull_requests(EXTRACT(year FROM created_at), user_id);

然后将查询命名为：

SELECT users.*, pull_requests_count
FROM "users" INNER JOIN
     (select user_id, count(*) as pull_requests_count
      from "pull_requests"
      WHERE (EXTRACT(year FROM pull_requests.created_at) = 2013)
      group by user_id
     ) pr
     ON pr."user_id" = "users"."id";

索引完全覆盖了子查询，因此不需要原始表，只需要索引扫描。然后可以将其连接回用户。

Postgresql中此连接查询的正确索引

1 个答案: