瓶颈排序操作

Question

我有下面的查询，这证明是非常昂贵的，需要6-8秒才能执行。从执行计划来看，排序操作的成本是79%。我能在这里得到任何改进吗？

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    SELECT
        A.StageName,
        C.Month,
        C.MonthName as Label,
        C.Year,
        isnull(A.Average,0) as Data
    FROM
    (   
        SELECT 
            S.StageName,
            MONTH(TimeIn) as MonthNumber, 
            DATENAME(MONTH,TimeIn) as Month, 
            YEAR(TimeIn) as Year, 
            ISNULL(AVG(DATEDIFF(mi,TimeIn,isnull(TimeOut,@TodayDate))),0) as Average
        FROM
            VisitMovement VM
        INNER JOIN Stage S on
            VM.StageID = S.StageID
        WHERE 
            (VM.TimeIn >= @StartDate AND 
            VM.TimeIn  < DATEADD (d,1,@EndDate)) AND
            (VM.TimeOut < DATEADD (d,1,@EndDate) OR VM.TimeOut IS NULL)
        GROUP BY
            S.StageNumber,
            S.StageName,
            MONTH(TimeIn), 
            DATENAME(MONTH,TimeIn), 
            YEAR(TimeIn)
    ) A
    RIGHT JOIN (select distinct Month,MonthName,Year from Calendar WHERE DATE >= @StartDate AND DATE < DATEADD (d,1,@EndDate)) C on
        A.MonthNumber = C.Month and
        A.Month = C.MonthName and
        A.Year = C.Year 
    GROUP BY
        A.StageName,
        C.Month,
        C.MonthName,
        C.Year,
        A.Average   
    ORDER BY  
        CASE WHEN @Ordering = 'asc'  THEN C.Year            END ASC, 
        CASE WHEN @Ordering = 'asc'  THEN C.Month           END ASC,
        CASE WHEN @Ordering = 'asc'  THEN A.StageName       END ASC,
        CASE WHEN @Ordering = 'desc' THEN C.Year            END DESC, 
        CASE WHEN @Ordering = 'desc' THEN C.Month           END DESC,
        CASE WHEN @Ordering = 'desc'  THEN A.StageName      END DESC

Answer 1

虽然我知道您无法摆脱子查询中不同列上的GROUP BY，但您可以使系统变得更容易。

你现在有

S.StageNumber,
S.StageName,
MONTH(TimeIn), 
DATENAME(MONTH,TimeIn), 
YEAR(TimeIn)

我想这是相当多的数据要通过。请允许我作一些猜测：

S.StageNumber,  -- int, 4 bytes
S.StageName, -- string, 20 bytes
MONTH(TimeIn),  -- int, 4 bytes
DATENAME(MONTH,TimeIn),  -- string 5 bytes
YEAR(TimeIn) -- int, 4 byte

现在，有一些依赖关系：

• 如果你知道月份(数字)，那么你现在也知道它的名字了。
• 我猜StageName + StageNumber是唯一的，并且与StageID直接相关。如果没有，您可能不得不在外层再次分组。

这会让我们

S.StageID,  -- int, 4 bytes
MONTH(TimeIn),  -- int, 4 bytes
YEAR(TimeIn) -- int, 4 byte

这意味着GROUP BY的排序只需要每条记录运行12个字节，而不是以前的每条记录37个字节，而且数字排序要比字符串快得多(例如，由于大小写、重音等)。

我尝试相应地重写查询(未经测试！)。我还将月份信息的获取移到了一个单独的临时表中，这将对查询优化器有所帮助。

SELECT DISTINCT Month,MonthName,Year 
  INTO #dates
  FROM Calendar 
 WHERE DATE >= @StartDate AND DATE < DATEADD (d,1,@EndDate)

CREATE UNIQUE CLUSTERED INDEX uq0_#dates ON #dates (Month,Year)

SELECT
        A.StageName,
        C.Month,
        C.MonthName as Label,
        C.Year,
        isnull(A.Average,0) as Data
    FROM 
    (   
        SELECT S.StageName,
               MonthNumber,
               Year,
               Average
          FROM (    
                  SELECT VM.StageID,
                         MONTH(TimeIn) as MonthNumber, 
                         YEAR(TimeIn) as Year, 
                         ISNULL(AVG(DATEDIFF(mi,TimeIn,isnull(TimeOut,@TodayDate))),0) as Average
                    FROM
                         VisitMovement VM
                   WHERE 
                        (VM.TimeIn >= @StartDate AND 
                        VM.TimeIn  < DATEADD (d,1,@EndDate)) AND
                        (VM.TimeOut < DATEADD (d,1,@EndDate) OR VM.TimeOut IS NULL)
                  GROUP BY
                        VM.StageID,
                        MONTH(TimeIn), 
                        YEAR(TimeIn)
               ) grp
          JOIN Stage S 
            ON S.StageID = grp.StageID
        )
    ) A
    RIGHT OUTER JOIN #dates C
    on
        A.MonthNumber = C.Month and
        -- A.Month = C.MonthName and
        A.Year = C.Year 
    ORDER BY  
        CASE WHEN @Ordering = 'asc'  THEN C.Year            END ASC, 
        CASE WHEN @Ordering = 'asc'  THEN C.Month           END ASC,
        CASE WHEN @Ordering = 'asc'  THEN A.StageName       END ASC,
        CASE WHEN @Ordering = 'desc' THEN C.Year            END DESC, 
        CASE WHEN @Ordering = 'desc' THEN C.Month           END DESC,
        CASE WHEN @Ordering = 'desc'  THEN A.StageName      END DESC

希望这能有所帮助。

Answer 2

因为order by必须计算每一行，所以我认为它不能最优地使用索引。将order by替换为默认排序的row_number()，并且只逆转一次顺序，至少可以防止对单个行进行多个@Ordering计算。

在下面的伪代码中，原始查询放在CTE中。Row_number确定升序排序，在cte下面，如果需要，顺序将颠倒：

;with cte as
(
    SELECT
        A.StageName,
        C.Month,
        ...,
        row_number() over (order by C.Year,C.Month,A.StageName) sortOrder
    FROM
       ...rest of the query, excluding the order by
)
select * --or list the columns without the sortOrder 
from cte
order by sortOrder * case @Ordering when 'desc' then -1 else 1 end