唯一标识符的问题
我们有一个现有的数据库,广泛使用唯一标识符作为一些表的主键和某些可空列(不幸的是!)。我们遇到了这样一种情况:运行在这些表上的某些报告按这些唯一标识符排序,因为在该表中没有其他列可以给出有意义的排序(这不是讽刺吗!)。目的是按插入顺序排序以显示项目,但它们不是使用NewSequentialId()
插入的-所以浪费时间。
关于排序算法的事实
无论如何,考虑到SQL Server基于字节组对唯一标识符进行排序,从最后5个字节组(6个字节)开始并向第一个字节组(4个字节)移动,并将第3个字节组(2个字节)从右到左反转至左到右。
我的问题
我很想知道这种排序是否在任何真实的情况下有所帮助。
SQL Server如何在内部存储唯一标识符,这可能提供对其具有古怪排序算法的原因的见解?
参考:
Alberto Ferrari 发现的 SQL Server GUID 排序
示例
当您在具有以下数据的唯一标识符列上使用 Order By 时,唯一标识符将按如下所示排序。
请注意,以下数据按升序排序,并且最高的排序优先级来自第五个字节组向第一个字节组(反向)。
-- 1st byte group of 4 bytes sorted in the reverse (left-to-right) order below --
01000000-0000-0000-0000-000000000000
10000000-0000-0000-0000-000000000000
00010000-0000-0000-0000-000000000000
00100000-0000-0000-0000-000000000000
00000100-0000-0000-0000-000000000000
00001000-0000-0000-0000-000000000000
00000001-0000-0000-0000-000000000000
00000010-0000-0000-0000-000000000000
-- 2nd byte group of 2 bytes sorted in the reverse (left-to-right) order below --
00000000-0100-0000-0000-000000000000
00000000-1000-0000-0000-000000000000
00000000-0001-0000-0000-000000000000
00000000-0010-0000-0000-000000000000
-- 3rd byte group of 2 bytes sorted in the reverse (left-to-right) order below --
00000000-0000-0100-0000-000000000000
00000000-0000-1000-0000-000000000000
00000000-0000-0001-0000-000000000000
00000000-0000-0010-0000-000000000000
-- 4th byte group of 2 bytes sorted in the straight (right-to-left) order below --
00000000-0000-0000-0001-000000000000
00000000-0000-0000-0010-000000000000
00000000-0000-0000-0100-000000000000
00000000-0000-0000-1000-000000000000
-- 5th byte group of 6 bytes sorted in the straight (right-to-left) order below --
00000000-0000-0000-0000-000000000001
00000000-0000-0000-0000-000000000010
00000000-0000-0000-0000-000000000100
00000000-0000-0000-0000-000000001000
00000000-0000-0000-0000-000000010000
00000000-0000-0000-0000-000000100000
00000000-0000-0000-0000-000001000000
00000000-0000-0000-0000-000010000000
00000000-0000-0000-0000-000100000000
00000000-0000-0000-0000-001000000000
00000000-0000-0000-0000-010000000000
00000000-0000-0000-0000-100000000000
代码:
Alberto的代码扩展以表示排序是在字节级别而不是单个位上进行的。
With Test_UIDs As (-- 0 1 2 3 4 5 6 7 8 9 A B C D E F
Select ID = 1, UID = cast ('00000000-0000-0000-0000-100000000000' as uniqueidentifier)
Union Select ID = 2, UID = cast ('00000000-0000-0000-0000-010000000000' as uniqueidentifier)
Union Select ID = 3, UID = cast ('00000000-0000-0000-0000-001000000000' as uniqueidentifier)
Union Select ID = 4, UID = cast ('00000000-0000-0000-0000-000100000000' as uniqueidentifier)
Union Select ID = 5, UID = cast ('00000000-0000-0000-0000-000010000000' as uniqueidentifier)
Union Select ID = 6, UID = cast ('00000000-0000-0000-0000-000001000000' as uniqueidentifier)
Union Select ID = 7, UID = cast ('00000000-0000-0000-0000-000000100000' as uniqueidentifier)
Union Select ID = 8, UID = cast ('00000000-0000-0000-0000-000000010000' as uniqueidentifier)
Union Select ID = 9, UID = cast ('00000000-0000-0000-0000-000000001000' as uniqueidentifier)
Union Select ID = 10, UID = cast ('00000000-0000-0000-0000-000000000100' as uniqueidentifier)
Union Select ID = 11, UID = cast ('00000000-0000-0000-0000-000000000010' as uniqueidentifier)
Union Select ID = 12, UID = cast ('00000000-0000-0000-0000-000000000001' as uniqueidentifier)
Union Select ID = 13, UID = cast ('00000000-0000-0000-0001-000000000000' as uniqueidentifier)
Union Select ID = 14, UID = cast ('00000000-0000-0000-0010-000000000000' as uniqueidentifier)
Union Select ID = 15, UID = cast ('00000000-0000-0000-0100-000000000000' as uniqueidentifier)
Union Select ID = 16, UID = cast ('00000000-0000-0000-1000-000000000000' as uniqueidentifier)
Union Select ID = 17, UID = cast ('00000000-0000-0001-0000-000000000000' as uniqueidentifier)
Union Select ID = 18, UID = cast ('00000000-0000-0010-0000-000000000000' as uniqueidentifier)
Union Select ID = 19, UID = cast ('00000000-0000-0100-0000-000000000000' as uniqueidentifier)
Union Select ID = 20, UID = cast ('00000000-0000-1000-0000-000000000000' as uniqueidentifier)
Union Select ID = 21, UID = cast ('00000000-0001-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 22, UID = cast ('00000000-0010-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 23, UID = cast ('00000000-0100-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 24, UID = cast ('00000000-1000-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 25, UID = cast ('00000001-0000-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 26, UID = cast ('00000010-0000-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 27, UID = cast ('00000100-0000-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 28, UID = cast ('00001000-0000-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 29, UID = cast ('00010000-0000-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 30, UID = cast ('00100000-0000-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 31, UID = cast ('01000000-0000-0000-0000-000000000000' as uniqueidentifier)
Union Select ID = 32, UID = cast ('10000000-0000-0000-0000-000000000000' as uniqueidentifier)
)
Select * From Test_UIDs Order By UID, ID