mysql - 使用 COUNT 时改进 MySQL 删除查询

Question

我发现以下查询导致了一些瓶颈（执行时间超过 40 秒！）

DELETE FROM doctors 
WHERE (SELECT COUNT(loc_id) 
       FROM locations 
       WHERE locations.loc_doctor = doctors.doc_id) = 0 AND 
       doctors.doc_user = $myVar

我想问题出在该(SELECT COUNT(loc_id) FROM locations WHERE locations.loc_doctor = doctors.doc_id) = 0部分，对吗？有没有办法改进它？

Answer 1

这应该快一点：

DELETE FROM doctors WHERE doctors.doc_user = $myVar AND NOT EXISTS (SELECT 1 FROM locations WHERE locations.loc_doctor = doctors.doc_id LIMIT 1)

因为你数 0 实际上是一张NOT EXISTS支票。您还应该考虑为 locations.loc_doctor 列创建索引（如果您还没有索引）。

Answer 2

我建议从外部连接中删除：

DELETE doctors
FROM doctors LEFT JOIN locations
   ON locations.loc_doctor = doctors.doc_id
WHERE locations.loc_id IS NULL
AND doctors.doc_user = $myVar

Answer 3

我想为@tpeczek 答案添加限制。

DELETE FROM doctors WHERE doctors.doc_user = $myVar AND NOT EXISTS (SELECT * FROM locations WHERE locations.loc_doctor = doctors.doc_id limit 1)