前言
本節(jié)我們來講講并發(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
首先我們來創(chuàng)建測(cè)試表
IF OBJECT_ID('Test') IS NOT NULL
DROP TABLE Test
CREATE 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)過程通過來SQLQueryStress來測(cè)試并發(fā)情況，我們來看第一種情況。
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 );
COMMIT
GO
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 );
COMMIT
GO
此時(shí)問題依舊和解決方案一無異（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 );
COMMIT
GO
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)鍵問題，但是對(duì)于更新來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)問題但是也會(huì)帶來死鎖問題。
解決方案四（提升隔離級(jí)別+良好的鎖）
此時(shí)我們?cè)賮碓谔砑幼罡吒綦x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 );
COMMIT
GO
運(yùn)行多次均未發(fā)現(xiàn)出現(xiàn)什么異常，通過查詢數(shù)據(jù)時(shí)使用更新鎖而非共享鎖，這樣的話一來可以讀取數(shù)據(jù)但不阻塞其他事務(wù)，二來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 UpsertTestDatabase
SET ALLOW_SNAPSHOT_ISOLATION ON
ALTER DATABASE UpsertTestDatabase
SET READ_COMMITTED_SNAPSHOT ON
GO
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 );
COMMIT
GO
上述解決方案也會(huì)出現(xiàn)插入重復(fù)鍵問題不可取。
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 );
COMMIT
GO
經(jīng)過多次認(rèn)證也是零錯(cuò)誤，貌似通過表變量形式實(shí)現(xiàn)可行。
解決方案七（提升隔離級(jí)別+Merge）
通過Merge關(guān)鍵來實(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 );
COMMIT
GO
多次認(rèn)證無論是并發(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，否則即插入問題的解決方案，目前來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
COMMIT
GO
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 );
COMMIT
GO
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 );
COMMIT
GO
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。
以上就是本文的全部?jī)?nèi)容，希望對(duì)大家的學(xué)習(xí)有所幫助，也希望大家多多支持腳本之家。
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