在升级一个SQL Server 2000的数据库时，遇到了一致性错误，其中有几个错误是元数据损坏（metadata corruption），特意研究了一下这个案例，因为以前也零零散散的遇到过一些一致性相关错误，但是难得遇到元数据损坏的案例。
如下所示，数据库从SQL Server 2000还原到SQL Server 2008以后，在做一致性检查时，发现有元数据损坏（metadata corruption），下面是实验是构造的一个测试环境
DBCC CHECKCATALOG (TEST) WITH NO_INFOMSGS;
GO
DBCC CHECKDB(TEST) WITH NO_INFOMSGS;
GO
Msg 8992, Level 16, State 1, Line 1
Check Catalog Msg 3853, State 1: Attribute (object_id=1362819917) of row (object_id=1362819917,parameter_id=1) in sys.parameters does not have a matching row (object_id=1362819917) in sys.objects.
Msg 8992, Level 16, State 1, Line 1
Check Catalog Msg 3853, State 1: Attribute (object_id=1362819917) of row (object_id=1362819917,parameter_id=2) in sys.parameters does not have a matching row (object_id=1362819917) in sys.objects.
CHECKDB found 0 allocation errors and 2 consistency errors not associated with any single object.
CHECKDB found 0 allocation errors and 2 consistency errors in database 'TEST'.
那么我们先找到系统视图sys.parameters的数据来源于那个系统基础表（System Base-Table Metadata），如下脚本所示，我们可以找到sys.parameters 最终来源于sys.syscolpars和 sys.sysobjvalues（关于如何获取视图视图定义，此处不做展开分析）
SET QUOTED_IDENTIFIER ON
SET ANSI_NULLS ON
GO
CREATE VIEW sys.parameters AS
SELECT object_id, name,
parameter_id, system_type_id,
user_type_id, max_length,
precision, scale,
is_output, is_cursor_ref,
has_default_value, is_xml_document,
default_value, xml_collection_id,
is_readonly
FROM sys.parameters$
WHERE number = 1
GO
CREATE VIEW sys.parameters$ AS
SELECT c.id AS object_id,
c.number, c.name,
c.colid AS parameter_id,
c.xtype AS system_type_id,
c.utype AS user_type_id,
c.length AS max_length,
c.prec AS precision,
c.scale AS scale,
sysconv(bit, c.status & 512) AS is_output, -- CPM_OUTPUT
sysconv(bit, c.status & 1024) AS is_cursor_ref, -- CPM_CURSORREF
sysconv(bit, isnull(v.objid, 0)) AS has_default_value,
sysconv(bit, c.status & 2048) AS is_xml_document, -- CPM_XML_DOC
v.value AS default_value,
xmlns AS xml_collection_id,
sysconv(bit, c.status & 4194304) AS is_readonly -- CPM_IS_READONLY = 0x00400000
FROM sys.syscolpars c
LEFT JOIN sys.sysobjvalues v ON v.valclass = 9 AND v.objid = c.id AND v.subobjid = c.colid AND v.valnum = 0 -- SVC_PARAMDEFAULT
WHERE number > 0 AND has_access('CO', c.id) = 1
但是系统基础表sys.syscolpars和sys.sysobjvalues在正常情况下是不可见的。只有在数据库专用管理员连接方式（DAC Dedicated Administrator Connection）连接下才能可见。如下所示，可以判断数据来源于sys.syscolpars系统基础表。
此时即使在专用管理员连接下面也是无法删除这些数据的，会报“Ad hoc update to system catalogs is not supported”，对应中文提示为“不支持对系统目录进行即席更新”。如下所示：
EXEC sp_configure 'allow_updates', 1;
RECONFIGURE WITH OVERRIDE;
GO
USE TEST;
GO
DELETE FROM sys.syscolpars WHERE id=1362819917;
GO
那么难道就没有办法解决这种问题了吗？ 答案是当然有，不过，这种方式是没有官方文档而且也不被官方Support的，如果你要按下面方法操作，是有一定风险的。所以如果你决定按照下面方式修复元数据损坏的话，先做好备份。以防万一。
你必须将数据库实例在单用户模式下面启动，然后以专用管理员（DAC）连接到数据库，然后就可以删除基础表下面的数据了，如下截图所示：
C:\Documents and Settings>net stop mssqlserver
The SQL Server (MSSQLSERVER) service is stopping.
The SQL Server (MSSQLSERVER) service was stopped successfully.
C:\Documents and Settings>net start mssqlserver /m"Microsoft SQL Serve
r Management Studio - Query"
The SQL Server (MSSQLSERVER) service is starting.
The SQL Server (MSSQLSERVER) service was started successfully.
USE TEST;
GO
DELETE FROM sys.syscolpars WHERE id=1362819917;
GO
----------------------------------------------------------------------------------
Warning: System table ID 41 has been updated directly in database ID 5 and cache coherence may not have been maintained. SQL Server should be restarted.
(2 row(s) affected)
此时再去检查数据库一致性，你就会看到上面遇到的元数据损坏错误不见了。如下截图所示：
其实如果是从SQL Server 2000还原的话，在SQL Server 2000当中是可以修改相关系统表的，如果执行了DBCC CHECKDB命令发现了元数据问题，那么可以直接修改系统表解决问题（当然只是部分情况），如果已经还原到了SQL Server 2008 以上数据库时，就必须按这种方式折腾，由于这种方式，官方是不支持的。所以还是有一定风险的。因为你不清楚潜在的风险，也不能确保任何场景都能解决问题而不出现意外情况。所以操作之前，尽量多测试、做好备份以防万一。
