在“The Cost of GUIDs as Primary Keys”
（
http://www.sinoprise.cn/showtopic-105.aspx）一文中，提出了“COMB”数
据类型的设想，即是将GUID的后6位改为时间，由于原GUID是无序的，这样一来就
便于索引排序，关于为什么采用GUID的后6位，而不是前6位，在原文中有这样一
段描述：
The guess that I had about the problem with the INSERT overhead for
GUIDs was that the lack of order in the Windows 2000–generated GUIDs
was giving SQL Server a hard time administering indexes under massive
INSERT periods. Therefore, I tried to create an order for the GUIDs
instead. I tried to CAST the current DATETIME to a BINARY(8) and put
that first in the GUID. Unfortunately, that had no effect. When I
investigated it further, I found that when I had a BINARY(16) value and
CASTed it to a UNIQUEIDENTIFIER, some bytes were scrambled. What to do?
You guessed it. I tried to compensate for the scrambling to see if that
had any positive effect on throughput, but no effect occurred. Then I
found out that it wasn't the first (high) byte that was important for
the new ordering, but the last (low) bytes. I also learned that I
didn't have to use BINARY(8) for the current DATETIME. BINARY(6) is
enough for the next 77 years, so I decided to occupy only the last
(low) 6 bytes with the current DATETIME. In Listing 5, you can see the
result of PRINT CAST(GETDATE() AS BINARY(8)). The first (high) bytes
are 0.
我曾经做过一次关于效率的测试，使用Delphi+Access做的，“Integer GUID和
Comb做主键的效率测试” （
http://www.sinoprise.cn/showtopic-104.aspx），
因为在其他的数据库中，没有所谓的GUID字段，所以应该把GUID（16个字节）转
化为字符串（32个字符），以字符串作为索引和主键，这样就比用整数做主键大
了整整8倍，不用测试，平经验就知道效率肯定赶不上整数做主键，但GUID字符串
做主键在某些应用上还是有优势的，特别是当多个数据库需要同步时，这是自动
加一的整数就忘尘莫及了。
既然字符串做为主键，我还是坚持我的看法，我觉得应该是前边的字符有序更利
于索引，所以这次一如既往，我将GUID的前6个字节替换掉了，这样生成的字符串
有序，在前6个字节中用两个字节表示但前时间距1970-1-1的天数，后4个字节表
示当前时间距0：0：0的时间点数，这样可以精确到1/300秒，我在数据库中做了1
千万条数据，无重复。在内存中用哈希表检测10亿条GUID无重复。按照这个算法
，前6个字节可以保证179.54794520547945205479452054795年无重复。
我的机器配置：
硬件： 迅驰1.8 G CPU，IBM T43 笔记本，2G内存
软件： .NET 1.1, 2.0,3.0,3.5
使用.NET 2.0测试
mysql 5.0
mysql provider 5.1(mysql官方驱动)
mysql数据库结构:
DROP TABLE IF EXISTS `maintable`;
CREATE TABLE `maintable` (
`szKey` varchar(32) NOT NULL default '',
`szValue` varchar(32) default NULL,
PRIMARY KEY (`szKey`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
DROP TABLE IF EXISTS `subtable`;
CREATE TABLE `subtable` (
`szKey` varchar(32) NOT NULL default '',
`szFK` varchar(32) NOT NULL default '',
`szValue` varchar(32) default NULL,
PRIMARY KEY (`szKey`),
KEY `subtable_fk` (`szFK`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
应用程序界面：
