postgresql - 来自连接表的 select 语句中的子查询

Question

我有一个帐户表、单位表和报告表。一个账户有多个单位(单位外键为account_id)，一个单位有多个报表(报表外键为unit_id)。我要选择账户名称、该账户的总单位数和最后报告时间:

SELECT accounts.name AS account_name, 
COUNT(units.id) AS unit_count,  
(SELECT reports.time FROM reports INNER JOIN units ON units.id = reports.unit_id  ORDER BY time desc LIMIT 1) AS last_reported_time
FROM accounts 
INNER JOIN units ON accounts.id = units.account_id 
INNER JOIN reports ON units.id = reports.unit_id
GROUP BY account_name, last_reported_time 
ORDER BY unit_count desc;

此查询一直在运行，我不确定它是否按照我的预期进行。

一个账户有很多单位，一个单位有很多报告。我想显示每个给定帐户关联的所有单位的最新报告的时间。这个查询是否正确？如果没有，我该如何完成我的任务(如果可能，不使用脚本语言)？

解释的结果:

 Sort  (cost=21466114.58..21466547.03 rows=172980 width=38)
   Sort Key: (count(public.units.id))
   InitPlan 1 (returns $0)
     ->  Limit  (cost=0.00..12.02 rows=1 width=8)
           ->  Nested Loop  (cost=0.00..928988485.04 rows=77309416 width=8)
                 ->  Index Scan Backward using index_reports_on_time on reports  (cost=0.00..296291138.34 rows=77309416 width=12)
                 ->  Index Scan using units_pkey on units  (cost=0.00..8.17 rows=1 width=4)
                       Index Cond: (public.units.id = public.reports.unit_id)
   ->  GroupAggregate  (cost=20807359.99..21446321.09 rows=172980 width=38)
         ->  Sort  (cost=20807359.99..20966559.70 rows=63679885 width=38)
               Sort Key: accounts.name, public.units.last_reported_time
               ->  Hash Join  (cost=975.50..3846816.82 rows=63679885 width=38)
                     Hash Cond: (public.reports.unit_id = public.units.id)
                     ->  Seq Scan on reports  (cost=0.00..2919132.16 rows=77309416 width=4)
                     ->  Hash  (cost=961.43..961.43 rows=1126 width=38)
                           ->  Hash Join  (cost=16.37..961.43 rows=1126 width=38)
                                 Hash Cond: (public.units.account_id = accounts.id)
                                 ->  Seq Scan on units  (cost=0.00..928.67 rows=1367 width=28)
                                 ->  Hash  (cost=11.72..11.72 rows=372 width=18)
                                       ->  Seq Scan on accounts  (cost=0.00..11.72 rows=372 width=18)

Answer 1

大约 95% 的查询成本都在这里:

     ->  Sort  (cost=20807359.99..20966559.70 rows=63679885 width=38)
           Sort Key: accounts.name, public.units.last_reported_time
           ->  Hash Join  (cost=975.50..3846816.82 rows=63679885 width=38)
                 Hash Cond: (public.reports.unit_id = public.units.id)
                 ->  Seq Scan on reports  (cost=0.00..2919132.16 rows=77309416 width=4)

reports.unit_id 有索引吗？如果没有，您绝对应该添加一个。

除此之外，输出列 unit_count 似乎给出了每个帐户的单位数，但是在所有连接之后计算它，然后按它排序非常浪费。选择列表中的子查询对我来说有些神秘；我假设您想要每个单位的最近报告时间，但它只会给您最后一次报告所有单位的总和。试试这个:

SELECT a.account_name, u.unit_count, r.last_reported_time
FROM account a
JOIN (
    SELECT account_id, COUNT(*) AS unit_count
    FROM units
    GROUP BY 1) u ON u.account_id = a.id
LEFT JOIN ( -- allow for units that have not yet submitted a report
    SELECT u.account_id, max(r.time) AS last_reported_time
    FROM reports r
    JOIN units u ON u.id = r.unit_id
    GROUP BY 1) r ON r.account_id = a.id
ORDER BY 2 DESC;