调整交叉记录更新查询

Question

我有一张表跨越2亿条记录，我试图运行下面的查询。该查询尝试根据上一条记录的时间戳更新表。无论如何，这个查询运行得更快吗？

UPDATE [dbo].[Location Data] 
    SET [timestamp_prev] = 
    (
      SELECT [timestamp] FROM [dbo].[Location Data] newTable 
       WHERE [dbo].[Location Data].[RowNumber] = (newTable.[RowNumber] + 1) 
       AND [dbo].[Location Data].[mmsi] = newTable.[mmsi] 
    ); 

Answer 1

你可以尝试使用自联接：

UPDATE 
    t1 
SET 
    t1.[timestamp_prev] = t2.[timestamp] 
FROM 
    [dbo].[Location Data] t1 
INNER JOIN 
    [dbo].[Location Data] t2 
    ON t1.[RowNumber] = t2.[RowNumber] + 1 AND 
     t1.[mmsi] = t2.[mmsi] 

如果您对连接列此查询可能之前完成指标你退休了。

Answer 2

像下面这样的内部连接可能会帮助您，而不是像在嵌套查询中那样遍历表的每一行中的所有行。

UPDATE oldTable 
SET oldTable.[timestamp_prev] = newTable.[timestamp] 
FROM [dbo].[Location Data] oldTable 
INNER JOIN [dbo].[Location Data] newTable 
    ON oldTable.[RowNumber] = newTable.[RowNumber] + 1 
       AND oldTable.[mmsi] = newTable.[mmsi] 

Answer 3

我会尝试这样的：

UPDATE T1 SET 
    [timestamp_prev] = T2.[timestamp] 
FROM [dbo].[Location Data] T1 
    INNER JOIN [dbo].[Location Data] T2 
     ON T1.RowNumber = T2.RowNumber + 1 
      AND T1.mmsi = T2.mmsi 
WHERE T1.[timestamp_prev] IS NULL; 

的加入应该是更有效的，只有尝试更新没有以前的时间戳记录。然后，您可以采取另一个步骤，将RowNumber，MMSI和Timestamp_Prev上的索引添加到表中，然后确保清理索引以最大限度地提高效率。

这样一个简单的指标应该是一个良好的开端：

CREATE NONCLUSTERED INDEX ix_Location_Data_MMSI_RowNumber_Timestamp_Prev 
    ON dbo.[Location Data] (mmsi, RowNumber, Timestamp_Prev) INCLUDE (Timestamp); 

Answer 4

首先，我会用lag()做到这一点：

with toupdate as (
     select ld.*, 
      lag(timestamp) over (partition by mmsi order by RowNumber) as prev_timestamp 
     from dbo.[Location Data] ld 
    ) 
update toupdate 
    set timestamp_prev = prev_timetamp; 

然后，我要指出，更新2亿条记录是要花费很长，很长很长的时间。我建议你用你想要的列生成一个新表，然后截断原始表并重新填充它。