如何优化这个 MySQL 计数算法？

Question

我有 2 张桌子；一个是用户，另一个记录用户操作。我想计算每个用户的操作数并将其记录在用户表中。大约有 10 万用户，下面的代码需要 6 个小时！一定有更好的办法！

def calculate_invites():

sql_db.execute("SELECT id, uid FROM users")

for row in sql_db:
    id = row['id']
    uid = row['uid']

    sql1 = "SELECT COUNT(1) FROM actions WHERE uid = %s"
    sql_db.execute(sql1, uid)
    count_actions = sql_db.fetchone()["COUNT(1)"]

    sql = "UPDATE users SET count_actions=%s WHERE uid=%s"
    sql_db.execute(sql, (count_actions, uid))

Answer 1

您可以将所有这些作为一个语句来完成：

update users
    set count_actons = (select count(*) from actions a where a.uid = users.uid)

没有 for 循环。没有多个查询。在 SQL 中执行您可以在 SQL 中执行的操作。通常循环遍历行是您希望在数据库中而不是在应用程序中执行的操作。

Answer 2

仅作为替代方案提供，因为 Gordon 的回答可能更快：

update users
from (
   select uid, count(*) as num_actions
   from actions
   group by uid
   ) x
set count_actions = x.num_actions
where users.uid=x.uid