查詢中使用多個外連接的查詢優化

Question

我正在尋找方法來優化以下SQL查詢。我正在使用MS SQL Server 2008.我想如果有任何方法來整合外部連接，它會加快速度......如果任何人有任何輸入，它將不勝感激。謝謝

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：爲了清晰起見，我特意在這裏省略了SELECT語句中的很多字段。因此，roomtype和dp表格實際上正在使用...

Answer 1

OUTER JOINs不是問題。問題在於包裝函數內的列。

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

這類建築的阻止優化們進行有效的查詢計劃 - 它不知道該函數的值是什麼，直到運行時間，因此它無法使用，他們應該存在索引。

重寫沒有函數的查詢，優化器將能夠做出更好的決策。

Answer 2

你可以嘗試加入的校驗 - 從我的經驗，往往給人較複雜條件更好的性能：

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