Mysql子选择很慢

Question

I have a table like this :

-------------------------------
| id | valid_until |  username|
-------------------------------
| 1  | 2020-01-01  |   user1  |
-------------------------------  
| 1  | 2020-01-01  |   user2  |
-------------------------------  
| 1  | 2020-01-02  |   user3  |
-------------------------------  
| 1  | 2020-01-02  |   user4  |
-------------------------------  
| 1  | 2020-01-03  |   user5  |
-------------------------------  
| 1  | 2020-01-03  |   user6  |
-------------------------------  
| 1  | 2020-01-03  |   user7  |
-------------------------------  

This is the user subscription table, valid_until says when the subscription will end up.

I want to know active subscription in each day, So I have a range, for example, 2020-01-01 TO 2020-01-03 and here is my query :

SELECT 
    valid_until qva,
    (SELECT 
            COUNT(iod.id) AS ct
        FROM
            `order_detail` iod
        WHERE
            valid_until > qva)
FROM
    `order_detail`
WHERE
    valid_until >= '2020-01-01'
        AND valid_until <= '2020-01-03'
GROUP BY qva

But this query is too slow, What is the problem with my query? Response time (230 sec)

Answer 1

Your query is invalid according to the SQL standard. You have GROUP BY qva (and using the alias name here is usually not allowed, but in MySQL and MariaDB it is), but you don't apply an aggregate function on the subquery result. MySQL is known for violating the standard here and they silently apply ANY_VALUE on such unaggregated expressions.

This makes the query slow. For each order in the date range the count is evaluated, only to pick one of those counts per date at the very end.

So, let's build the query up from scratch. You want one result row for each day in the given date range. That is:

select distinct valid_until
where valid_until between date '2020-01-01' and date '2020-01-03'
from order_detail;

Then, for each of these dates you want to get the count. You can do this in a subquery as in your original query. I suppose it must be >= instead of > though, as an order valid until a day is still valid that day (or at least this is what I'd expect).

SELECT 
  dates.day,
  (
    select count(*) 
    from order_detail od
    where od.valid_until >= dates.day
  ) as ct
FROM
(
  select distinct valid_until as day
  where valid_until between date '2020-01-01' and date '2020-01-03'
  from order_detail
) dates
order by dates.day;

(This assumes that valid_until is a mere date. If it's a datetime, you'll have to adjust this query a little.)

UPDATE:

As ysth told you in the request comments, your query will get you only days in the given range that exist as valid_until in your table. So does mine. If you wanted the three days regardless, you'd have to replace the subquery and make it independent from the table, eg

FROM
(
  select date '2020-01-01' as day union all
  select date '2020-01-02' as day union all
  select date '2020-01-03' as day
) dates

Answer 2

I think you just want a cumulative count of valid_until from the latest date to the earliest:

select valid_until, count(*) day_count,
       sum(count(*)) over (order by valid_until desc) as count_as_of_day
from order_details od
group by valid_until;

If you want this for a range of dates, then use a subquery:

select od.*
from (select valid_until, count(*) day_count,
             sum(count(*)) over (order by valid_until desc) as count_as_of_day
      from order_details od
      group by valid_until
     ) od
where valid_until >= :from and valid_until <= :to