Question

这是一个查询：

SELECT DISTINCT
    spentits.*,
    username,
    (SELECT count(*) from likes where likes.spentit_id = spentits.id) as like_count,
    (SELECT count(*) from comments where comments.spentit_id = spentits.id) as comment_count,
    (SELECT count(*) from wishlist_items where wishlist_items.spentit_id = spentits.id) as wishlist_count,
    (case when likes.id is null then 0 else 1 end) as is_liked_by_me,
    (case when wishlist_items.id is null then 0 else 1 end) as is_wishlisted_by_me
FROM spentits
LEFT JOIN users ON users.id = spentits.user_id
LEFT JOIN likes ON likes.user_id = 9 AND likes.spentit_id = spentits.id
LEFT JOIN wishlist_items ON wishlist_items.user_id = 9 AND wishlist_items.spentit_id = spentits.id
WHERE spentits.user_id IN
    (SELECT follows.following_id
     FROM follows
     WHERE follows.follower_id = 9)
ORDER BY id DESC 
LIMIT 15;

这需要执行平均43ms次。现在另一个查询（下面），没有where子句，更不用说第二个SELECT子查询执行速度慢5倍240ms）！

SELECT DISTINCT
    spentits.*,
    username,
    (SELECT count(*) from likes where likes.spentit_id = spentits.id) as like_count,
    (SELECT count(*) from comments where comments.spentit_id = spentits.id) as comment_count,
    (SELECT count(*) from wishlist_items where wishlist_items.spentit_id = spentits.id) as wishlist_count,
    (case when likes.id is null then 0 else 1 end) as is_liked_by_me,
    (case when wishlist_items.id is null then 0 else 1 end) as is_wishlisted_by_me
FROM spentits
LEFT JOIN users ON users.id = spentits.user_id
LEFT JOIN likes ON likes.user_id = 9 AND likes.spentit_id = spentits.id
LEFT JOIN wishlist_items ON wishlist_items.user_id = 9 AND wishlist_items.spentit_id = spentits.id
ORDER BY id DESC 
LIMIT 15;

为什么呢？第二个查询不应该快得多，因为没有条件，没有第二个查询。所有数据库需要做的是选择最后15条记录，与执行子查询后检查每条记录需要扫描的第一条记录进行比较，并检查其中是否包含ID。我真的很困惑一个应该执行速度快得多的查询实际上要慢5倍。

Answer 1

存在与统计信息和元数据相关的执行路径，为两者执行explain plan并查看执行路径。

此外，如果您没有按照该ID过滤投影行，则按ID过滤可能会对没有匹配行的表执行跳过。

为什么一个查询比另一个更快？

1 个答案: