我正在使用Azure SQL + Java + Spring Boot 2.目前我正在尝试了解发生死锁的原因。在事务中，我正在插入并稍后更新同一个表但不同的行。据我所知，SQL Server默认使用rowlock和read_commited隔离。
这是死锁细节：
<deadlock>
<victim-list>
<victimProcess id="process2b8618644e8"/>
</victim-list>
<process-list>
<process id="process2b8618644e8" taskpriority="0" logused="352"
waitresource="KEY: 16:72057594043760640 (5f601e0257f1)" waittime="1321" ownerId="9246067"
transactionname="implicit_transaction" lasttranstarted="2019-02-17T13:03:49.683" XDES="0x2b868778458"
lockMode="U" schedulerid="2" kpid="288932" status="suspended" spid="118" sbid="0" ecid="0" priority="0"
trancount="2" lastbatchstarted="2019-02-17T13:03:49.793" lastbatchcompleted="2019-02-17T13:03:49.760"
lastattention="1900-01-01T00:00:00.760" clientapp="Microsoft JDBC Driver for SQL Server"
hostname="TZ-MacBook-Pro.local" hostpid="0" loginname="master" isolationlevel="read committed (2)"
xactid="9246067" currentdb="16" currentdbname="test" lockTimeout="4294967295" clientoption1="671088672"
clientoption2="128058">
<executionStack>
<frame procname="unknown" queryhash="0x431f2517c2d3feb8" queryplanhash="0x3a793ad664472011" line="1"
stmtstart="110" stmtend="238"
sqlhandle="0x02000000060c530e608cef3b3e4e4712f48c40d2efd9b04d0000000000000000000000000000000000000000">
unknown
</frame>
<frame procname="unknown" queryhash="0x0000000000000000" queryplanhash="0x0000000000000000" line="1"
sqlhandle="0x0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000">
unknown
</frame>
</executionStack>
<inputbuf>
(@P0 int,@P1 nvarchar(4000),@P2 nvarchar(4000),@P3 int)update bug set lock=@P0, status=@P1 where
bug_id=@P2 and lock=@P3
</inputbuf>
</process>
<process id="process2b84f702108" taskpriority="0" logused="700"
waitresource="KEY: 16:72057594043760640 (185e9bfc7320)" waittime="1287" ownerId="9246090"
transactionname="implicit_transaction" lasttranstarted="2019-02-17T13:03:49.727" XDES="0x2b868764458"
lockMode="U" schedulerid="1" kpid="334812" status="suspended" spid="115" sbid="0" ecid="0" priority="0"
trancount="2" lastbatchstarted="2019-02-17T13:03:49.830" lastbatchcompleted="2019-02-17T13:03:49.790"
lastattention="1900-01-01T00:00:00.790" clientapp="Microsoft JDBC Driver for SQL Server"
hostname="TZ-MacBook-Pro.local" hostpid="0" loginname="master" isolationlevel="read committed (2)"
xactid="9246090" currentdb="16" currentdbname="test" lockTimeout="4294967295" clientoption1="671088672"
clientoption2="128058">
<executionStack>
<frame procname="unknown" queryhash="0x431f2517c2d3feb8" queryplanhash="0x3a793ad664472011" line="1"
stmtstart="110" stmtend="238"
sqlhandle="0x02000000060c530e608cef3b3e4e4712f48c40d2efd9b04d0000000000000000000000000000000000000000">
unknown
</frame>
<frame procname="unknown" queryhash="0x0000000000000000" queryplanhash="0x0000000000000000" line="1"
sqlhandle="0x0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000">
unknown
</frame>
</executionStack>
<inputbuf>
(@P0 int,@P1 nvarchar(4000),@P2 nvarchar(4000),@P3 int)update bug set lock=@P0, status=@P1 where
bug_id=@P2 and lock=@P3
</inputbuf>
</process>
</process-list>
<resource-list>
<keylock hobtid="72057594043760640" dbid="16" objectname="b51ce3c4-02f0-4dfa-89dd-3ab2f45beac7.dbo.bug"
indexname="bug_pk" id="lock2b8658c6f00" mode="X" associatedObjectId="72057594043760640">
<owner-list>
<owner id="process2b84f702108" mode="X"/>
</owner-list>
<waiter-list>
<waiter id="process2b8618644e8" mode="U" requestType="wait"/>
</waiter-list>
</keylock>
<keylock hobtid="72057594043760640" dbid="16" objectname="b51ce3c4-02f0-4dfa-89dd-3ab2f45beac7.dbo.bug"
indexname="bug_pk" id="lock2b856aa7f80" mode="X" associatedObjectId="72057594043760640">
<owner-list>
<owner id="process2b8618644e8" mode="X"/>
</owner-list>
<waiter-list>
<waiter id="process2b84f702108" mode="U" requestType="wait"/>
</waiter-list>
</keylock>
</resource-list>
</deadlock>
此外，我在我的id列上使用聚簇索引，并且随机操作顺序没有问题。此外，当我只插入内部事务时，一切正常，没有死锁。如果我切换到PostgreSql没有死锁问题。
那么为什么我在Azure SQL中遇到了死锁？
这是Java源代码：
@Transactional
public void bug() {
BugEntity bugEntity = bugRepo.findById("1234")
.orElseGet(() -> createBugEntity());
if (bugEntity.getStatus().equals("SomeStatus")) {
bugEntity.setStatus("NewStatus");
bugRepo.save(bugEntity);
}
}
private BugEntity createBugEntity() {
BugEntity bugEntity = BugEntity.builder()
.bugId(UUID.randomUUID().toString())
.status("SomeStatus")
.data("SomeData")
.build();
return bugRepo.save(bugEntity);
}
实体类：
@AllArgsConstructor
@NoArgsConstructor
@Data
@Builder
@Entity(name = "bug")
@DynamicUpdate
public class BugEntity {
@Id
String bugId;
String status;
String data;
LocalDateTime dateTime;
@Version
int lock;
}
为了重现死锁，我正在使用带有几个并行请求的加特林模拟。
提前致谢！
死锁的常见原因是需要查询和索引调整的查询触及的数据超过了必要的数据。
死锁跟踪显示参数作为nvarchar(4000)数据类型传递。这可能会阻止varchar列上的索引被有效使用，因为nvarchar的数据类型优先级高于varchar。当从应用程序数据类型推断出参数数据类型时，这个问题尤为常见，因为字符串通常是Unicode，例如Java和.NET语言，因此无论基础列数据类型如何，都会产生nvarchar参数。
解决方案是使用varchar参数作为字符串而不是nvarchar，除非基础列是nvarchar。如果不在数据库中使用nvarchar，请在sendStringParametersAsUnicode中详细说明JDBC连接字符串参数this answer。或者，更改应用程序代码或框架配置，以便为varchar列使用varchar参数类型。
最佳实践是确保参数与基础数据库列的类型匹配。这将提高性能和并发性，并具有其他好处。