說明了常見子運算式刪除

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03/13/2024

APS CU7.3 使用 SQL 查詢最佳化工具中的常見子運算式刪除來改善查詢效能。該改進以兩種方式改善查詢。第一個好處是可以識別和消除此類運算式，有助於減少 SQL 編譯時間。第二個以及更重要的好處是會消除這些備援子運算式的資料移動操作，因此查詢的執行時間更快。

select top 100 asceding.rnk, i1.i_product_name best_performing, i2.i_product_name worst_performing
  from(select *
       from (select item_sk,rank() over (order by rank_col asc) rnk
             from (select ss_item_sk item_sk,avg(ss_net_profit) rank_col
                   from store_sales ss1
                   where ss_store_sk = 8
                   group by ss_item_sk
                   having avg(ss_net_profit) > 0.9*(select avg(ss_net_profit) rank_col
                                                    from store_sales
                                                    where ss_store_sk = 8
                                                      and ss_hdemo_sk is null
                                                    group by ss_store_sk))V1)V11
       where rnk  < 11) asceding,
      (select *
       from (select item_sk,rank() over (order by rank_col desc) rnk
             from (select ss_item_sk item_sk,avg(ss_net_profit) rank_col
                   from store_sales ss1
                   where ss_store_sk = 8
                   group by ss_item_sk
                   having avg(ss_net_profit) > 0.9*(select avg(ss_net_profit) rank_col
                                                    from store_sales
                                                    where ss_store_sk = 8
                                                      and ss_hdemo_sk is null
                                                    group by ss_store_sk))V2)V21
       where rnk  < 11) descending,
  item i1,
  item i2
  where asceding.rnk = descending.rnk
    and i1.i_item_sk=asceding.item_sk
    and i2.i_item_sk=descending.item_sk
  order by asceding.rnk
  ;

請考慮來自 TPC-DS 基準工具的上述查詢。在上述查詢中，子查詢相同，但具有 rank() over 函數的 order by 子句會以兩種不同的方式排序。在 CU7.3 之前，會評估並執行這個子查詢兩次，一次用於遞增排序，一次用於遞減排序，因而產生兩個資料移動作業。安裝 APS CU7.3 之後，子查詢部分將會評估一次，從而減少資料移動並更快完成查詢。

我們引入了一個名為 'OptimizeCommonSubExpressions' 的功能開關，可讓您在升級至 APS CU7.3 之後測試此功能。此功能預設為開啟狀態，但是可以關閉。

注意

變更功能切換值需要重新啟動服務。

透過在測試環境中建立下列資料表，並評估上述查詢的說明計劃，以嘗試範例查詢。

CREATE TABLE [dbo].[store_sales] (
    [ss_sold_date_sk] int NULL, 
    [ss_sold_time_sk] int NULL, 
    [ss_item_sk] int NOT NULL, 
    [ss_customer_sk] int NULL, 
    [ss_cdemo_sk] int NULL, 
    [ss_hdemo_sk] int NULL, 
    [ss_addr_sk] int NULL, 
    [ss_store_sk] int NULL, 
    [ss_promo_sk] int NULL, 
    [ss_ticket_number] int NOT NULL, 
    [ss_quantity] int NULL, 
    [ss_wholesale_cost] decimal(7, 2) NULL, 
    [ss_list_price] decimal(7, 2) NULL, 
    [ss_sales_price] decimal(7, 2) NULL, 
    [ss_ext_discount_amt] decimal(7, 2) NULL, 
    [ss_ext_sales_price] decimal(7, 2) NULL, 
    [ss_ext_wholesale_cost] decimal(7, 2) NULL, 
    [ss_ext_list_price] decimal(7, 2) NULL, 
    [ss_ext_tax] decimal(7, 2) NULL, 
    [ss_coupon_amt] decimal(7, 2) NULL, 
    [ss_net_paid] decimal(7, 2) NULL, 
    [ss_net_paid_inc_tax] decimal(7, 2) NULL, 
    [ss_net_profit] decimal(7, 2) NULL
)
WITH (CLUSTERED COLUMNSTORE INDEX, DISTRIBUTION = HASH([ss_item_sk]),  PARTITION ([ss_sold_date_sk] RANGE RIGHT FOR VALUES (2450815, 2451180, 2451545, 2451911, 2452276, 2452641, 2453006)));

CREATE TABLE [dbo].[item] (
    [i_item_sk] int NOT NULL, 
    [i_item_id] char(16) COLLATE Latin1_General_100_CI_AS_KS_WS NOT NULL, 
    [i_rec_start_date] date NULL, 
    [i_rec_end_date] date NULL, 
    [i_item_desc] varchar(200) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_current_price] decimal(7, 2) NULL, 
    [i_wholesale_cost] decimal(7, 2) NULL, 
    [i_brand_id] int NULL, 
    [i_brand] char(50) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_class_id] int NULL, 
    [i_class] char(50) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_category_id] int NULL, 
    [i_category] char(50) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_manufact_id] int NULL, 
    [i_manufact] char(50) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_size] char(20) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_formulation] char(20) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_color] char(20) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_units] char(10) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_container] char(10) COLLATE Latin1_General_100_CI_AS_KS_WS NULL, 
    [i_manager_id] int NULL, 
    [i_product_name] char(50) COLLATE Latin1_General_100_CI_AS_KS_WS NULL
)
WITH (CLUSTERED INDEX ( [i_item_sk] ASC ), DISTRIBUTION = REPLICATE);

如果查看查詢的說明計劃，您會看到在 CU7.3 之前 (或功能開關的狀態為關閉時)，查詢共有 17 個操作，而在 CU7.3 之後 (或開啟功能關閉後)，相同的查詢會顯示共有 9 個操作。如果只計算資料移動操作，會看到上一個計劃有四個移動操作，而新計劃中有兩個移動操作。新的查詢最佳化工具能夠減少兩個資料移動操作，方法是重複使用它透過新計劃所建立的暫存資料表，進而減少查詢運行時間。

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說明了常見子運算式刪除

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