Question

我有以下表格：

table1 - session_id，company_id，session_start_time
table2 - id，session_id，message_time，message_type，message
table3 - company_id，company_name

table1存储在公司上完成的会话。每个会话都有很多消息，这些消息存储在table2中。对于特定的message_type（比如message_type = 2），需要从消息中提取子字符串。该子字符串是公司名称。现在我需要从table1中找到下一个session_id，其中company_id与从message中提取的公司名称的session_id匹配，session_start_time＆gt; = message_time。

我正在使用以下查询。

select t1.session_id as session1,
       t1.company_id as company1,
       @transfer_time := t2.message_time as transfer_time,
       @company2 := trim(substring(t2.message, 38, locate(' abc', t2.message) - 38)) as company2,
       (select t1.session_id
        from table1 as t1
          inner join table3 as t3 on t1.company_id = t3.company_id
        where t1.session_start_time >= @transfer_time
          and t3.company_name = @company2
       order by t1.session_start_time
       limit 1) as session 2
from table1 as t1
  inner join table2 as t2 on t1.session_id = t2.session_id
  inner join table3 as t3 on t1.company_id = t3.company_id
where t2.message_type = 2

原始查询稍微复杂一点，有一些标量子查询。此查询执行时间过长。我检查了explain函数，标量子查询似乎需要很长时间才能执行。但是，我无法想到更好的方法。

Answer 1

这是查询：

select t1.session_id as session1, t1.company_id as company1,
       @transfer_time := t2.message_time as transfer_time,
       @company2 := trim(substring(t2.message, 38, locate(' abc', t2.message) - 38)) as company2,
       (select t1.session_id
        from table1 t1 inner join
             table3 t3
             on t1.company_id = t3.company_id
        where t1.session_start_time >= @transfer_time and
              t3.company_name = @company2
        order by t1.session_start_time
        limit 1
       ) s session 2
from table1 t1 inner join
     table2 t2
     on t1.session_id = t2.session_id inner join
     table3 t3
     on t1.company_id = t3.company_id
where t2.message_type = 2;

首先，使用变量是不正确的。 MySQL不保证SELECT中变量的评估顺序。所以，你需要输入原始定义：

select t1.session_id as session1, t1.company_id as company1,
       t2.message_time as transfer_time,
       trim(substring(t2.message, 38, locate(' abc', t2.message) - 38)) as company2,
       (select t1.session_id
        from table1 t1 inner join
             table3 t3
             on t1.company_id = t3.company_id
        where t1.session_start_time >= t2.message_time and
              t3.company_name = trim(substring(t2.message, 38, locate(' abc', t2.message) - 38)) 
        order by t1.session_start_time
        limit 1
       ) s session 2
from table1 t1 inner join
     table2 t2
     on t1.session_id = t2.session_id inner join
     table3 t3
     on t1.company_id = t3.company_id
where t2.message_type = 2;

接下来，我会尝试索引：table3(company_name, company_id)和table1(company_id, session_start_time, session_id)。

标量子查询需要花费大量时间来执行

1 个答案: