sql - 哪个查询计划更快/更好

Question

对于返回相同结果的不同查询，我有两个查询计划我想知道是否有人可以告诉我哪个“更好”，以及为什么。

SELECT * 
FROM bids order by (select ranking from users where users.id = runner_id) DESC limit 1


"Limit  (cost=17.62..17.62 rows=1 width=568)"
"  ->  Sort  (cost=17.62..17.62 rows=2 width=568)"
"        Sort Key: ((SubPlan 1))"
"        ->  Seq Scan on bids  (cost=0.00..17.61 rows=2 width=568)"
"              SubPlan 1"
"                ->  Index Scan using users_pkey on users  (cost=0.28..8.29 rows=1 width=4)"
"                      Index Cond: (id = bids.runner_id)"

第二个陈述和计划:

SELECT  "bids".* 
FROM "bids" inner join users u on bids.runner_id=u.id  ORDER BY u.ranking DESC LIMIT 1

"Limit  (cost=17.64..17.64 rows=1 width=572)"
"  ->  Sort  (cost=17.64..17.64 rows=2 width=572)"
"        Sort Key: u.ranking"
"        ->  Nested Loop  (cost=0.28..17.63 rows=2 width=572)"
"              ->  Seq Scan on bids  (cost=0.00..1.02 rows=2 width=568)"
"              ->  Index Scan using users_pkey on users u  (cost=0.28..8.29 rows=1 width=8)"
"                    Index Cond: (id = bids.runner_id)"

Answer 1

它不足以产生任何真正的不同。我会使用第二个，因为第一个的语法相当不规则，这使得它更难维护。略。

你也可以试试这个:

select b.*
from   (select   id
        from     users
        order by ranking desc
        limit    1) u
join   bids                b on b.runner_id = u.id
limit  1

最后的限制 1 可能是多余的。