asp.net - MySQL排序性能问题

Question

我有一个带有子查询的查询，但子查询表大约有 500.000 条记录，并且运行此查询需要很长时间。我怎样才能加快这个查询的速度？任何建议:

SELECT ID, VehicleID, Plate, VehicleType, 
    COALESCE(
        (SELECT EngineState 
        FROM Locations 
        WHERE Locations.VehicleID = Clients.VehicleID ORDER BY ID DESC LIMIT 1), 
        0
    ) EngineState 
FROM Clients 
WHERE ID IN (SELECT ClientID FROM UserClients WHERE UserID=@UserID);

还有 3 列从位置表中查询最后一条记录:

COALESCE(
    (SELECT EngineState FROM Locations 
    WHERE Locations.VehicleID = Clients.VehicleID ORDER BY ID DESC LIMIT 1), 
    0
) EngineState

正如我所见，对“位置”表中的结果进行排序是性能因素。位置表每分钟填充 1000 辆车的位置数据。 x、y、速度...

Answer 1

创建以下索引:

Locations (vehicleId, id)
UserClients (UserID, ClientID)

并稍微重写您的查询:

SELECT  ID, VehicleID, Plate, VehicleType, l.*
FROM    Clients
LEFT JOIN
        Locations l
ON      l.id =
        (
        SELECT  id
        FROM    Locations li
        WHERE   li.VehicleID = Clients.VehicleID
        ORDER BY
                li.VehicleID DESC, li.id DESC
        LIMIT 1
        )
WHERE   Clients.ID IN
        (
        SELECT  ClientID
        FROM    UserClients
        WHERE   UserID = @UserID
        )

由于您的 VehicleID 是 VARCHAR(64)，因此您应该确保 Clients.VehicleID 和 Locations.VehicleID 使用相同的字符集和相同的排序规则。