问题描述
有序的GUID性能对比，堪比自增ID integer
一个大神告诉我NEWSEQUENTIALID() 在数据迁移的时候会有问题(感谢大神指点)，所以我就深挖一下这个函数。
关于NEWSEQUENTIALID() 的用法 参照 NEWSEQUENTIALID()
NEWSEQUENTIALID 是对 Windows UuidCreateSequential 函数的包装。
https://msdn.microsoft.com/zh-cn/library/ms189786(v=sql.120).aspx
我们系统中对UuidCreateSequential 方法的扩展是这样写的，代码如下：
public static class GuidExtension
{
[DllImport("rpcrt4.dll", SetLastError = true)]
public static extern int UuidCreateSequential(out Guid guid);
private const int RPC_S_OK = 0;
public static Guid CreateRpcrt4Guid()
{
Guid guid;
int result = UuidCreateSequential(out guid);
if (result == RPC_S_OK)
{
byte[] guidBytes = guid.ToByteArray();
Array.Reverse(guidBytes, 0, 4);
Array.Reverse(guidBytes, 4, 2);
Array.Reverse(guidBytes, 6, 2);
return new Guid(guidBytes);
}
else
return Guid.NewGuid();
}
}
有以下几个缺点：
1、暴漏MAC地址：NEWSEQUENTIALID函数最后6个字符是网卡的MAC地址
可以执行看一下
create table #t
(
id uniqueidentifier not null default newsequentialid()
,name varchar(100)
)
go
insert into #t(name)
output inserted.id
values('a')
2、如果进行数据迁移，到另一台机器上，MAC地址改变就会引起页的争用。
因为GUID在的SQL Server的值大小的比对是这样的：
with uids as (
select id = 1, uuid = cast ('00000000-0000-0000-0000-010000000000' as uniqueidentifier)
union select id = 2, uuid = cast ('00000000-0000-0000-0000-000100000000' as uniqueidentifier)
union select id = 3, uuid = cast ('00000000-0000-0000-0000-000001000000' as uniqueidentifier)
union select id = 4, uuid = cast ('00000000-0000-0000-0000-000000010000' as uniqueidentifier)
union select id = 5, uuid = cast ('00000000-0000-0000-0000-000000000100' as uniqueidentifier)
union select id = 6, uuid = cast ('00000000-0000-0000-0000-000000000001' as uniqueidentifier)
union select id = 7, uuid = cast ('00000000-0000-0000-0100-000000000000' as uniqueidentifier)
union select id = 8, uuid = cast ('00000000-0000-0000-0010-000000000000' as uniqueidentifier)
union select id = 9, uuid = cast ('00000000-0000-0001-0000-000000000000' as uniqueidentifier)
union select id = 10, uuid = cast ('00000000-0000-0100-0000-000000000000' as uniqueidentifier)
union select id = 11, uuid = cast ('00000000-0001-0000-0000-000000000000' as uniqueidentifier)
union select id = 12, uuid = cast ('00000000-0100-0000-0000-000000000000' as uniqueidentifier)
union select id = 13, uuid = cast ('00000001-0000-0000-0000-000000000000' as uniqueidentifier)
union select id = 14, uuid = cast ('00000100-0000-0000-0000-000000000000' as uniqueidentifier)
union select id = 15, uuid = cast ('00010000-0000-0000-0000-000000000000' as uniqueidentifier)
union select id = 16, uuid = cast ('01000000-0000-0000-0000-000000000000' as uniqueidentifier)
)
select * from uids order by uuid desc
输出结果：
类似 汉字的三点水偏旁（为了好记）
从这里可以看出，MAC地址对GUID的大小有这最高的决定性，这就导致在数据迁移的时候出问题。
COMB解决方案
COMB 类型的GUID 基本设计思路是这样的：既然GUID数据生成是随机的造成索引效率低下，影响了系统的性能，那么能不能通过组合的方式，保留GUID的前10个字节，用后6个字节表示GUID生成的时间（DateTime），这样我们将时间信息与GUID组合起来，在保留GUID的唯一性的同时增加了有序性，以此来提高索引效率。
前十个字节是通过随机数生成
private static readonly RNGCryptoServiceProvider RandomGenerator = new RNGCryptoServiceProvider();
byte[] randomBytes = new byte[10];
RandomGenerator.GetBytes(randomBytes);
后六个字节用时间生成
long timestamp = DateTime.UtcNow.Ticks / 10000L;
byte[] timestampBytes = BitConverter.GetBytes(timestamp);
if (BitConverter.IsLittleEndian)
{
Array.Reverse(timestampBytes);
}
最后组合起来
byte[] guidBytes = new byte[16];
Buffer.BlockCopy(randomBytes, 0, guidBytes, 0, 10);
Buffer.BlockCopy(timestampBytes, 2, guidBytes, 10, 6);
return new Guid(guidBytes);
这个解决方法是被大家所认可的，唯一感觉不好的地方是，在快速获取很多的GUID的时候，时间是一样的，加上随机生成的数据，这一组数据是大小不一的。假如数据库里有很多数据，这一组数据肯定比他们都大，性能应该没有问题。
github地址：
https://github.com/nhibernate/nhibernate-core/blob/master/src/NHibernate/Id/GuidCombGenerator.cs#L25-L72
https://github.com/jhtodd/SequentialGuid/
我的解决方法
总结上面的方法，UuidCreateSequential 前面10个字节有序，后6个是MAC地址。COMBO解决方案是前面10个随机，后六个是时间。我是将这两个结合起来
前10个去UuidCreateSequential 方法的值，后6个取时间
代码：
public static Guid NewSequentialGuid()
{
const int RPC_S_OK = 0;
Guid guid;
int result = UuidCreateSequential(out guid);
if (result != RPC_S_OK)
{
throw new System.ComponentModel.Win32Exception(System.Runtime.InteropServices.Marshal.GetLastWin32Error());
}
else
{
//这里把UuidCreateSequential函数返回的数据做处理
byte[] guidBytes = guid.ToByteArray();
Array.Reverse(guidBytes, 0, 4);
Array.Reverse(guidBytes, 4, 2);
Array.Reverse(guidBytes, 6, 2);
//这里用时间
long timestamp = DateTime.UtcNow.Ticks / 10000L;
byte[] timestampBytes = BitConverter.GetBytes(timestamp);
if (BitConverter.IsLittleEndian)
{
Array.Reverse(timestampBytes);
}
//最后把时间赋值给后6位
Buffer.BlockCopy(timestampBytes, 2, guidBytes, 10, 6);
return new Guid(guidBytes);
}
}
[System.Runtime.InteropServices.DllImport("rpcrt4.dll", SetLastError = true)]
private static extern int UuidCreateSequential(out Guid guid);
这里可以在程序调用，作为DBA在数据库使用的话可以将这个方法添加到程序集里，需要有些改动
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.SqlTypes;
public class FunctionNewGuid
{
//这里需要添加SqlFunction属性
//返回类型是数据库类型
[Microsoft.SqlServer.Server.SqlFunction]
public static SqlGuid NewSequentialGuid()
{
const int RPC_S_OK = 0;
Guid guid;
int result = UuidCreateSequential(out guid);
if (result != RPC_S_OK)
{
throw new System.ComponentModel.Win32Exception(System.Runtime.InteropServices.Marshal.GetLastWin32Error());
}
else
{
byte[] guidBytes = guid.ToByteArray();
Array.Reverse(guidBytes, 0, 4);
Array.Reverse(guidBytes, 4, 2);
Array.Reverse(guidBytes, 6, 2);
long timestamp = DateTime.UtcNow.Ticks / 10000L;
byte[] timestampBytes = BitConverter.GetBytes(timestamp);
if (BitConverter.IsLittleEndian)
{
Array.Reverse(timestampBytes);
}
Buffer.BlockCopy(timestampBytes, 2, guidBytes, 10, 6);
return new SqlGuid(guidBytes);
}
}
[System.Runtime.InteropServices.DllImport("rpcrt4.dll", SetLastError = true)]
private static extern int UuidCreateSequential(out Guid guid);
}
编译生成DLL后，注册到数据库
--设置数据库是可信任
ALTER DATABASE TEST SET TRUSTWORTHY ON
--创建程序集
CREATE ASSEMBLY SQLCLR FROM 'D:\SQLCLR.DLL'
WITH PERMISSION_SET = UNSAFE
--用程序集方法创建函数
CREATE FUNCTION func_NewSequentialGuid()
RETURNS uniqueidentifier
AS external name SQLCLR.FunctionNewGuid.NewSequentialGuid
测试代码：
批量请求:
select dbo.func_NewSequentialGuid() union select dbo.func_NewSequentialGuid() union select dbo.func_NewSequentialGuid() union select dbo.func_NewSequentialGuid() union select dbo.func_NewSequentialGuid()
结果：
多次请求：
create table #t
(
uuid uniqueidentifier
,id int identity
)
go
insert into #t(uuid)
values(dbo.func_NewSequentialGuid())
go 10
select * from #t
git地址
https://gitee.com/wangzhanbo/cms/tree/master/Library
如果有问题，希望大家指正。。。