查询中使用多个外连接的查询优化

Question

我正在寻找方法来优化以下SQL查询。我正在使用MS SQL Server 2008.我想如果有任何方法来整合外部连接，它会加快速度......如果任何人有任何输入，它将不胜感激。谢谢

SELECT employee.employee_id, 
    coalesce (room.ship_id, ship.ship_id) AS ship_id, 
    SUBSTRING (employee.service_id, 0, 6) AS cost_code, 
    employee_station.room_cat AS nws_room_cat 
FROM employee 
    LEFT OUTER JOIN room 
     ON  employee.ship_id = room.ship_id 
     AND employee.floor_id = room.floor_id 
     AND employee.room_id = room.room_id 
    LEFT OUTER JOIN ship 
     ON employee.ship_id = ship.ship_id 
    LEFT OUTER JOIN employee_station 
     ON COALESCE (ship.property_type, '') = 
      COALESCE (employee_station.property_type, '') 
     AND COALESCE (room.work_center_id, '') = COALESCE (employee_station.work_center_id, '') 
     AND COALESCE (employee.pay_plan, '') = 
       COALESCE (employee_station.pay_plan, '') 
     AND COALESCE (employee.employee_type, '') = 
       COALESCE (employee_station.employee_type, '') 
     AND COALESCE (SUBSTRING (employee.service_id, 0, 6), '') = 
       COALESCE (employee_station.cost_code, '') 
    LEFT OUTER JOIN roomtype 
     ON room.room_type = roomtype.room_type 
    LEFT OUTER JOIN dp 
     ON employee.service_id = dp.service_id 

注意：为了清晰起见，我特意在这里省略了SELECT语句中的很多字段。因此，roomtype和dp表格实际上正在使用...

Answer 1

OUTER JOINs不是问题。问题在于包装函数内的列。

ON COALESCE (ship.property_type, '') = COALESCE (employee_station.property_type, '') 

这类建筑的阻止优化们进行有效的查询计划 - 它不知道该函数的值是什么，直到运行时间，因此它无法使用，他们应该存在索引。

重写没有函数的查询，优化器将能够做出更好的决策。

Answer 2

你可以尝试加入的校验 - 从我的经验，往往给人较复杂条件更好的性能：

SELECT 
--columns you need 
FROM employee 
LEFT OUTER JOIN room 
ON CHECKSUM(employee.ship_id, employee.floor_id, employee.room_id) 
= CHECKSUM(room.ship_id, room.floor_id, room.room_id) 
LEFT OUTER JOIN ship 
ON employee.ship_id = ship.ship_id 
LEFT OUTER JOIN employee_station 
ON CHECKSUM(ship.property_type, room.work_center_id, employee.pay_plan, employee.employee_type, SUBSTRING (employee.service_id, 0, 6)) 
= CHECKSUM(employee_station.property_type, employee_station.work_center_id, employee_station.pay_plan, employee_station.employee_type, employee_station.cost_code) 
LEFT OUTER JOIN roomtype 
ON room.room_type = roomtype.room_type 
LEFT OUTER JOIN dp 
ON employee.service_id = dp.service_id