Question

如何优化此查询以获得相同的结果，而不需要花费太长时间？ NOT IN子查询需要很长时间。

SELECT DISTINCT EmployeeId FROM employees
    WHERE 
    status = 'Active' 
    && BranchId = '2' 
    && NOT EXISTS (
      SELECT * FROM attendance
      WHERE
      employees.EmployeeId = attendance.EmployeeId 
      && attendance.AttendanceDate = '2015-01-20'
    )
  )

SELECT EmployeeId FROM employees 
    WHERE 
    status = 'Active' 
    && BranchId = '2' 
    && NOT IN (
      SELECT EmployeeId FROM attendance WHERE AttendanceDate='2015-01-20'
    )

Answer 1

以下是您的查询的另一个版本

select
distinct e.EmployeeId FROM employees e
left join attendance a on e.EmployeeId = a.EmployeeId and a.AttendanceDate = '2015-01-20'
where
e.status='Active' 
and e.BranchId= '2' 
and a.EmployeeId is null

您还需要在表格上应用一些索引

alter table employees add index st_br_idx(status,BranchId);
alter table AttendanceDate add index AttendanceDate_idx(AttendanceDate);

如果EmployeeId是外键，那么如果是，则无需添加索引索引尚未存在，您可能还需要以下内容

alter table AttendanceDate add index EmployeeId_idx(EmployeeId);

如果EmployeeId是employees中的主键，那么它已经编入索引（如果没有并且没有编入索引），您可能还需要为其添加索引

alter table employees add index EmployeeId_idx(EmployeeId);

您可以在具有上述索引后检查原始查询

SELECT DISTINCT e.EmployeeId FROM employees e 
WHERE 
e.status='Active' 
and e.BranchId= '2' 
and NOT EXISTS (
  SELECT 1 FROM 
  attendance a WHERE e.EmployeeId = a.EmployeeId 
  and a.AttendanceDate='2015-01-20'
)

分析查询使用explain select..并查看优化器如何使用索引以及优化程序可能扫描以检索记录的可能行数

NOT IN子查询执行时间太长

1 个答案: