前文

JOIN條件範圍時,執行計畫預估值容易不準確,這也間接導致查詢效能不好.

就算有建立Index也會遇到上述問題

假如我們想要提升JOIN條件範圍效能並讓Index可以發揮最大最用可以怎麼做?

就讓我利用一個範例來跟大家分享.

本文同步發表Blog JOIN範圍條件Index優化

案例

此範例有使用到三張表

• Product表:擁有1-10編號產品
• ReportPeriod表:存放產每期報表的資訊(時間,和是否產報表)
• T99表:線上產品訂單資訊

CREATE TABLE [dbo].[Product](
	[ProductId] INT NOT NULL
)

CREATE TABLE [dbo].[T99](
	[TransactionId] [int] IDENTITY(1,1) NOT NULL,
	[Amount] DECIMAL(18,6),
	[CreateDate] [datetime2](3) NULL
)
GO

CREATE TABLE [dbo].[ReportPeriod](
	[PerioidID] [int] IDENTITY(1,1) NOT NULL,
	[ProductId] INT NOT NULL,
	[IsGenerate] [bit] NULL,
	[StartDate] [datetime2](3) NULL,
	[EndDate] [datetime2](3) NULL
) ON [PRIMARY]
GO

我們利用T99.CreateDate來跟ReportPeriod判斷是屬於哪期報表.

資料初始化

我們利用亂數產生Sample資料來模擬線上大資料狀況.

ReportPeriod期別由'2019-08-01'到'2020-07-31'

因為產生報表以5分鐘為區間,所以可以利用CTE遞迴來幫我們產生資料.

INSERT INTO [dbo].[Product] VALUES (1);
INSERT INTO [dbo].[Product] VALUES (2);
INSERT INTO [dbo].[Product] VALUES (3);
INSERT INTO [dbo].[Product] VALUES (4);
INSERT INTO [dbo].[Product] VALUES (5);
INSERT INTO [dbo].[Product] VALUES (6);
INSERT INTO [dbo].[Product] VALUES (7);
INSERT INTO [dbo].[Product] VALUES (8);
INSERT INTO [dbo].[Product] VALUES (9);
INSERT INTO [dbo].[Product] VALUES (10);

declare @FromDate DATETIME2(3) = '2019-08-01'
declare @ToDate DATETIME2(3) = '2020-07-31'

;WITH CTE AS (
	SELECT @FromDate fromDt,@ToDate endDt
	UNION ALL
	SELECT DATEADD(MINUTE,5,fromDt),endDt
	FROM CTE 
	WHERE DATEADD(MINUTE,5,fromDt) < endDt
)
INSERT INTO  [dbo].[ReportPeriod] ([ProductId],[IsGenerate],[StartDate],[EndDate])
SELECT ProductId,0,fromDt,DATEADD(MINUTE,5,fromDt) 
FROM CTE CROSS JOIN dbo.Product
OPTION (MAXRECURSION 0); 

INSERT INTO T99 ([CreateDate],Amount)
SELECT top 1000000 dateadd(SECOND, 
          rand(checksum(newid()))*(1+datediff(SECOND, @FromDate, @ToDate)), 
               @FromDate),
	   CAST(RAND(CHECKSUM(NEWID())) * 100000 as INT) + 1
FROM sys.all_columns c1 CROSS JOIN sys.all_columns c2

建立Index

CREATE CLUSTERED INDEX [CIX_ReportPeriod_StartDate] ON [dbo].[ReportPeriod]
(
	[StartDate] ASC,
    [EndDate] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

ALTER TABLE [dbo].[ReportPeriod] ADD  CONSTRAINT [PK_ReportPeriod] PRIMARY KEY NONCLUSTERED 
(
	[PerioidID] ASC,
	[ProductId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

CREATE CLUSTERED INDEX [CIX_T99_CreateDate] ON [dbo].[T99]
(
	[CreateDate] ASC
)

CREATE UNIQUE NONCLUSTERED INDEX [IX_T99_TransactionId] ON [dbo].[T99]
(
	[TransactionId] ASC
)

查詢語法

在線上我們會使用UDT當作參數來取得某些期別資訊.

這裡為了方便模擬我使用Table Variable來取代.

set nocount on
DECLARE @Transaction AS TABLE(
	TransactionId INT,
	ProductId INT
);

INSERT INTO @Transaction VALUES (1,1)
INSERT INTO @Transaction VALUES (101,2)
INSERT INTO @Transaction VALUES (1001,3)

SELECT p.*
FROM dbo.T99 t 
JOIN [dbo].[ReportPeriod] p ON t.CreateDate BETWEEN p.StartDate AND p.EndDate
JOIN @Transaction t1 ON t.TransactionId = t1.TransactionId AND  p.ProductId = t1.ProductId

使用上面語法我們只需查詢三個期別資料,但看執行計畫時能發現，ReportPeriod使用的Clustered預估資訊有1百多萬筆

Q:我明明有對於條件建立Index，但為什麼預估值卻會跑真那麼嚴重？

CREATE CLUSTERED INDEX [CIX_ReportPeriod_StartDate] ON [dbo].[ReportPeriod]
(
	[StartDate] ASC,
    [EndDate] ASC
)

原因出在範圍條件會因為查找範圍過大導致預估值不準確

甚麼意思? 讓我們看看下圖(代表ReportPeriod內含日期資料)

而我們在JOIN條件只有t.CreateDate BETWEEN p.StartDate AND p.EndDate這就會導致,我們需要查找ReportPeriod日期資料在挑出符合的資料

JOIN [dbo].[ReportPeriod] p ON t.CreateDate BETWEEN p.StartDate AND p.EndDate
JOIN @Transaction t1 ON t.TransactionId = t1.TransactionId AND  p.ProductId = t1.ProductId

最後就會看到走針的估計值

如何優化?

效能差問題,選擇對Index和撰寫合理的查詢可以改善40%左右問題

我們思考一下如果可以把範圍條件改成精準=查找條件不就可以更精準預估資訊了?

t.CreateDate BETWEEN p.StartDate AND p.EndDate

那我們怎麼把上面條件使用=取代BETWEEN範圍查詢呢?

這時我們可以利用空間來換取時間

建立一個新的COLUMN運用算法來計算每個期數StartTime

例如:CreateDate = 2020/01/03 10:08:55會歸類在2020/01/03 10:05:00中

ALTER TABLE dbo.T99 ADD PeriodDate AS DATEADD(MINUTE,DATEPART(MINUTE,CreateDate) %5 * -1,
	DATETIMEFROMPARTS(
	DATEPART(YEAR,CreateDate),
	DATEPART(MONTH,CreateDate),
	DATEPART(DAY,CreateDate),
	DATEPART(HOUR,CreateDate),
	DATEPART(MINUTE,CreateDate),0,0)
)

建立完新COLUMN後別忘記加入一個Index給此COLUMN.

CREATE INDEX IX_PeriodDate_T99 ON dbo.T99(
	PeriodDate
)

最後我們修改一下查詢語法

SELECT p.*
FROM dbo.T99 t 
JOIN [dbo].[ReportPeriod] p ON p.StartDate = t.PeriodDate
JOIN @Transaction t1 ON t.TransactionId = t1.TransactionId AND  p.ProductId = t1.ProductId

預估值和讀取值已經可以大幅降低了!!

小結:

在JOIN範圍條件差效能問題,可以思考一下是否有辦法利用算法或是公式來優化查詢效能，如此次範例一樣.