前言
本節(jié)我們來(lái)講講并發(fā)中最常見的情況存在即更新，在并發(fā)中若未存在行記錄則插入，此時(shí)未處理好極容易出現(xiàn)插入重復(fù)鍵情況，本文我們來(lái)介紹對(duì)并發(fā)中存在就更新行記錄的七種方案并且我們來(lái)綜合分析最合適的解決方案。
探討存在就更新七種方案
首先我們來(lái)創(chuàng)建測(cè)試表
IF OBJECT_ID('Test') IS NOT NULL DROP TABLE TestCREATE TABLE Test( Id int, Name nchar(100), [Counter] int,primary key (Id), unique (Name));GO解決方案一（開啟事務(wù)）
我們統(tǒng)一創(chuàng)建存儲(chǔ)過(guò)程通過(guò)來(lái)SQLQueryStress來(lái)測(cè)試并發(fā)情況，我們來(lái)看第一種情況。
IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) BEGIN TRANSACTION IF EXISTS ( SELECT 1 FROM Test WHERE Id = @Id ) UPDATE Test SET [Counter] = [Counter] + 1 WHERE Id = @Id; ELSE INSERT Test ( Id, Name, [Counter] ) VALUES ( @Id, @Name, 1 ); COMMITGO 
同時(shí)開啟100個(gè)線程和200個(gè)線程出現(xiàn)插入重復(fù)鍵的幾率比較少還是存在。
解決方案二（降低隔離級(jí)別為最低隔離級(jí)別UNCOMMITED）
IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED BEGIN TRANSACTION IF EXISTS ( SELECT 1 FROM Test WHERE Id = @Id ) UPDATE Test SET [Counter] = [Counter] + 1 WHERE Id = @Id; ELSE INSERT Test ( Id, Name, [Counter] ) VALUES ( @Id, @name, 1 ); COMMITGO此時(shí)問(wèn)題依舊和解決方案一無(wú)異（如果降低級(jí)別為最低隔離級(jí)別，如果行記錄為空，前一事務(wù)如果未進(jìn)行提交，當(dāng)前事務(wù)也能讀取到該行記錄為空，如果當(dāng)前事務(wù)插入進(jìn)去并進(jìn)行提交，此時(shí)前一事務(wù)再進(jìn)行提交此時(shí)就會(huì)出現(xiàn)插入重復(fù)鍵問(wèn)題）
解決方案三（提升隔離級(jí)別為最高級(jí)別SERIALIZABLE）
IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) SET TRANSACTION ISOLATION LEVEL SERIALIZABLE BEGIN TRANSACTION IF EXISTS ( SELECT 1 FROM dbo.Test WHERE Id = @Id ) UPDATE dbo.Test SET [Counter] = [Counter] + 1 WHERE Id = @Id; ELSE INSERT dbo.Test ( Id, Name, [Counter] ) VALUES ( @Id, @Name, 1 ); COMMITGO在這種情況下更加糟糕，直接到會(huì)導(dǎo)致死鎖
此時(shí)將隔離級(jí)別提升為最高隔離級(jí)別會(huì)解決插入重復(fù)鍵問(wèn)題，但是對(duì)于更新來(lái)獲取排它鎖而未提交，而此時(shí)另外一個(gè)進(jìn)程進(jìn)行查詢獲取共享鎖此時(shí)將造成進(jìn)程間相互阻塞從而造成死鎖，所以從此知最高隔離級(jí)別有時(shí)候能夠解決并發(fā)問(wèn)題但是也會(huì)帶來(lái)死鎖問(wèn)題。
解決方案四（提升隔離級(jí)別+良好的鎖）
此時(shí)我們?cè)賮?lái)在添加最高隔離級(jí)別的基礎(chǔ)上增添更新鎖，如下：
IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) SET TRANSACTION ISOLATION LEVEL SERIALIZABLE BEGIN TRANSACTION IF EXISTS ( SELECT 1 FROM dbo.Test WITH(UPDLOCK) WHERE Id = @Id ) UPDATE dbo.Test SET [Counter] = [Counter] + 1 WHERE Id = @Id; ELSE INSERT dbo.Test ( Id, Name, [Counter] ) VALUES ( @Id, @Name, 1 ); COMMITGO 
運(yùn)行多次均未發(fā)現(xiàn)出現(xiàn)什么異常，通過(guò)查詢數(shù)據(jù)時(shí)使用更新鎖而非共享鎖，這樣的話一來(lái)可以讀取數(shù)據(jù)但不阻塞其他事務(wù)，二來(lái)還確保自上次讀取數(shù)據(jù)后數(shù)據(jù)未被更改，這樣就解決了死鎖問(wèn)題。貌似這樣的方案是可行得，如果是高并發(fā)不知是否可行。
解決方案五（提升隔離級(jí)別為行版本控制SNAPSHOT）
ALTER DATABASE UpsertTestDatabaseSET ALLOW_SNAPSHOT_ISOLATION ON ALTER DATABASE UpsertTestDatabaseSET READ_COMMITTED_SNAPSHOT ONGO IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) BEGIN TRANSACTION IF EXISTS ( SELECT 1 FROM dbo.Test WHERE Id = @Id ) UPDATE dbo.Test SET [Counter] = [Counter] + 1 WHERE Id = @Id; ELSE INSERT dbo.Test ( Id, Name, [Counter] ) VALUES ( @Id, @Name, 1 ); COMMITGO上述解決方案也會(huì)出現(xiàn)插入重復(fù)鍵問(wèn)題不可取。
解決方案六（提升隔離級(jí)別+表變量）
IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) DECLARE @updated TABLE ( i INT ); SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; BEGIN TRANSACTION UPDATE Test SET [Counter] = [Counter] + 1 OUTPUT DELETED.Id INTO @updated WHERE Id = @Id; IF NOT EXISTS ( SELECT i FROM @updated ) INSERT INTO Test ( Id, Name, counter ) VALUES ( @Id, @Name, 1 ); COMMITGO 
經(jīng)過(guò)多次認(rèn)證也是零錯(cuò)誤，貌似通過(guò)表變量形式實(shí)現(xiàn)可行。
解決方案七（提升隔離級(jí)別+Merge）
通過(guò)Merge關(guān)鍵來(lái)實(shí)現(xiàn)存在即更新否則則插入，同時(shí)我們應(yīng)該注意設(shè)置隔離級(jí)別為SERIALIZABLE否則會(huì)出現(xiàn)插入重復(fù)鍵問(wèn)題，代碼如下：
IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) SET TRAN ISOLATION LEVEL SERIALIZABLE BEGIN TRANSACTION MERGE Test AS [target] USING ( SELECT @Id AS Id ) AS source ON source.Id = [target].Id WHEN MATCHED THEN UPDATE SET [Counter] = [target].[Counter] + 1 WHEN NOT MATCHED THEN INSERT ( Id, Name, [Counter] ) VALUES ( @Id, @Name, 1 ); COMMITGO多次認(rèn)證無(wú)論是并發(fā)100個(gè)線程還是并發(fā)200個(gè)線程依然沒有異常信息。
總結(jié)
本節(jié)我們?cè)敿?xì)討論了在并發(fā)中如何處理存在即更新，否則即插入問(wèn)題的解決方案，目前來(lái)講以上三種方案可行。
解決方案一（最高隔離級(jí)別 + 更新鎖）
IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) BEGIN TRANSACTION; UPDATE dbo.Test WITH ( UPDLOCK, HOLDLOCK ) SET [Counter] = [Counter] + 1 WHERE Id = @Id; IF ( @@ROWCOUNT = 0 ) BEGIN INSERT dbo.Test ( Id, Name, [Counter] ) VALUES ( @Id, @Name, 1 ); END COMMITGO暫時(shí)只能想到這三種解決方案，個(gè)人比較推薦方案一和方案三， 請(qǐng)問(wèn)您有何高見，請(qǐng)留下您的評(píng)論若可行，我將進(jìn)行后續(xù)補(bǔ)充。
解決方案二（最高隔離級(jí)別 + 表變量）
IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) DECLARE @updated TABLE ( i INT ); SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; BEGIN TRANSACTION UPDATE Test SET [Counter] = [Counter] + 1 OUTPUT DELETED.id INTO @updated WHERE id = @id; IF NOT EXISTS ( SELECT i FROM @updated ) INSERT INTO Test ( Id, Name, counter ) VALUES ( @Id, @Name, 1 ); COMMITGO解決方案三（最高隔離級(jí)別 + Merge）
IF OBJECT_ID('TestPro') IS NOT NULL DROP PROCEDURE TestPro;GO CREATE PROCEDURE TestPro ( @Id INT )AS DECLARE @Name NCHAR(100) = CAST(@Id AS NCHAR(100)) SET TRAN ISOLATION LEVEL SERIALIZABLE BEGIN TRANSACTION MERGE Test AS [target] USING ( SELECT @Id AS Id ) AS source ON source.Id = [target].Id WHEN MATCHED THEN UPDATE SET [Counter] = [target].[Counter] + 1 WHEN NOT MATCHED THEN INSERT ( Id, Name, [Counter] ) VALUES ( @Id, @Name, 1 ); COMMITGO暫時(shí)只能想到這三種解決方案，個(gè)人比較推薦方案一和方案三， 請(qǐng)問(wèn)您有何高見，請(qǐng)留下您的評(píng)論若可行，我將進(jìn)行后續(xù)補(bǔ)充。
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