sql-server - 进行 UNION 时针对未知进行优化

Question

所以我在 SP 中有一些非常慢的查询，我需要加快速度，我一直在使用 OPTIMIZE FOR UNKNOWN 并看到性能的一些显着提高。我仍然有一个查询确实很慢，我想应用它，但它包含 2 个 SQL 查询的 UNION，所以我的问题是我是否将 OPTIMIZE FOR UNKNOWN 应用于这两个查询？

为了清晰起见，这是我的 SP 的一个大大简化的版本:

SELECT * FROM MyTable ManufacturerID=@ManufacturerID and tStamp > @tStamp
OPTION (OPTIMIZE FOR (@ManufacturerID UNKNOWN, @tStamp UNKNOWN)) 
UNION
SELECT * FROM MyTable ManufacturerID=@ManufacturerID
OPTION (OPTIMIZE FOR (@ManufacturerID UNKNOWN) 

这是目前的实际 SP:

SELECT * FROM (
SELECT ROW_NUMBER() OVER(ORDER BY Products.ItemID) AS RowNum, *
    FROM
    (
    SELECT  Products.ProductID, Products.ItemID, Products.ManufacturerID,
          CategoryID = NULL, CategoryName = NULL, 
          CategoryProductID = NULL,  Products.ItemName, Products.Description, Products.Notes, 
                      Products.Dimensions, Products.BasePrice, Products.OrderMinimumQuantity, 
                      ContainerMinimumQuantity = 
                        CASE  COALESCE(Products.ContainerMinQty, 0)
                            WHEN 0 THEN Products.OrderMinimumQuantity
                            ELSE Products.ContainerMinQty
                        END 
                      , Products.OrderMultipleQuantity, Products.OnHandQuantity, 
                      Products.Category, Products.IntroDate, Products.BackOrderDate, Products.UPC, Products.PriceLevel1, Products.PriceLevel2, Products.PriceLevel3, 
                      Products.PriceLevel4, Products.PriceLevel5, Products.PriceLevel6, Products.PriceLevel7, Products.PriceLevel8, Products.PriceLevel9, Products.PieceBox, 
                      Products.Cubes, Products.UnitOfMeasure, Products.UDF1, Products.UDF2, Products.UDF3, Products.UDF4, Products.UDF5, Products.AdditionalImageCount, 
                     PhotoName= LOWER(Products.PhotoName),  Products.Discontinued, Products.ModifiedOn, 
                        Products.IsDeleted, Products.PriceLevel10, Products.PriceLevel11, Products.PriceLevel12, Products.PriceLevel13, 
                      Products.PriceLevel14, Products.PriceLevel15, Products.PriceLevel16, Products.PriceLevel17, Products.PriceLevel18, Products.PriceLevel19, Products.PriceLevel20, 
                      Products.Weight, Products.DimensionsMetric, Products.Source, Products.InventoryStatus, Products.CatalogCode, Products.CatalogName, 
                       SortOrder = NULL,
                       Products.reportCategory,Products.additionalPhotos,Products.udf6,Products.udf7,Products.udf8,
                       Products.udf9,Products.udf10,Products.udf11,Products.udf12,Products.udf13,Products.udf14,Products.udf15,Products.udf16,
                       Products.udf17,Products.udf18,Products.udf19,Products.udf20, Products.showRelatedFor,Products.showChildFor
FROM    
CategoryProducts (nolock) RIGHT OUTER JOIN
                      Products (nolock) ON CategoryProducts.ManufacturerID = Products.ManufacturerID AND CategoryProducts.ItemID = Products.ItemID
WHERE     (Products.ManufacturerID = @ManufacturerID)
            AND 
            (Products.ModifiedOn > @tStamp ) AND  ((CategoryProducts.IsDeleted = 1) OR (CategoryProducts.IsDeleted IS NULL)
            )

            UNION

            SELECT Products.ProductID, Products.ItemID, Products.ManufacturerID, Categories.CategoryID, Categories.CategoryName, CategoryProducts.CategoryProductID,  Products.ItemName, Products.Description, Products.Notes, 
                      Products.Dimensions, Products.BasePrice, Products.OrderMinimumQuantity, 
                      ContainerMinimumQuantity = 
                        CASE  COALESCE(Products.ContainerMinQty, 0)
                            WHEN 0 THEN Products.OrderMinimumQuantity
                            ELSE Products.ContainerMinQty
                        END 
                      , Products.OrderMultipleQuantity, Products.OnHandQuantity, 
                      Products.Category, Products.IntroDate, Products.BackOrderDate, Products.UPC, Products.PriceLevel1, Products.PriceLevel2, Products.PriceLevel3, 
                      Products.PriceLevel4, Products.PriceLevel5, Products.PriceLevel6, Products.PriceLevel7, Products.PriceLevel8, Products.PriceLevel9, Products.PieceBox, 
                      Products.Cubes, Products.UnitOfMeasure, Products.UDF1, Products.UDF2, Products.UDF3, Products.UDF4, Products.UDF5, Products.AdditionalImageCount, 
                     PhotoName= LOWER(Products.PhotoName),  Products.Discontinued, Products.ModifiedOn, 
                        CategoryProducts.IsDeleted, Products.PriceLevel10, Products.PriceLevel11, Products.PriceLevel12, Products.PriceLevel13, 
                      Products.PriceLevel14, Products.PriceLevel15, Products.PriceLevel16, Products.PriceLevel17, Products.PriceLevel18, Products.PriceLevel19, Products.PriceLevel20, 
                      Products.Weight, Products.DimensionsMetric, Products.Source, Products.InventoryStatus, Products.CatalogCode, Products.CatalogName, 
                       CategoryProducts.SortOrder,
                       Products.reportCategory,Products.additionalPhotos,Products.udf6,Products.udf7,Products.udf8,
                       Products.udf9,Products.udf10,Products.udf11,Products.udf12,Products.udf13,Products.udf14,Products.udf15,Products.udf16,
                       Products.udf17,Products.udf18,Products.udf19,Products.udf20, Products.showRelatedFor,Products.showChildFor
FROM      Categories (nolock) INNER JOIN
                      CategoryProducts (nolock) ON Categories.CategoryID = CategoryProducts.CategoryID INNER JOIN
                      Products (nolock) ON CategoryProducts.ManufacturerID = Products.ManufacturerID AND CategoryProducts.ItemID = Products.ItemID
WHERE     (Products.ManufacturerID = @ManufacturerID)
            AND 
            (Products.ModifiedOn > @tStamp OR  CategoryProducts.ModifiedOn > @tStamp)
            )  AS Products
        ) AS C  WHERE RowNum >= @StartRow AND RowNum <= @EndRow ORDER BY ItemID, ManufacturerID
    OPTION (OPTIMIZE FOR (@ManufacturerID UNKNOWN, @StartRow UNKNOWN, @EndRow UNKNOWN, @tStamp UNKNOWN)) 

Answer 1

来自在线图书 ( Query Hints ):

Query hints specify that the indicated hints should be used throughout the query. They affect all operators in the statement. If UNION is involved in the main query, only the last query involving a UNION operation can have the OPTION clause.

因此，这非常明确，您只能使用 OPTION 一次，而不是像示例中那样使用两次。

这是有道理的，因为即使有两个 SELECT 语句放在一起，SQL Server 仍会将其视为一个整体语句以进行编译和优化。

这确实意味着，如果您想在构成 UNION 的不同语句中使用不同的提示，则必须采用不同的方式，例如使用 TEMP 表来存储临时结果。

因此，在您的具体示例中，即使 @tStamp 仅出现在第一个 SELECT 语句中，您仍然可以在一个 OPTION 中引用它code> 子句根据需要。