在升級一個SQL Server 2000的數據庫時，遇到了一致性錯誤，其中有幾個錯誤是元數據損壞（metadata corruption），特意研究了一下這個案例，因為以前也零零散散的遇到過一些一致性相關錯誤，但是難得遇到元數據損壞的案例。
如下所示，數據庫從SQL Server 2000還原到SQL Server 2008以后，在做一致性檢查時，發現有元數據損壞（metadata corruption），下面是實驗是構造的一個測試環境
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'.
那么我們先找到系統視圖sys.parameters的數據來源於那個系統基礎表（System Base-Table Metadata），如下腳本所示，我們可以找到sys.parameters 最終來源於sys.syscolpars和 sys.sysobjvalues（關於如何獲取系統視圖定義，此處不做展開分析）
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
但是系統基礎表sys.syscolpars和sys.sysobjvalues在正常情況下是不可見的。只有在數據庫專用管理員連接方式（DAC Dedicated Administrator Connection）連接下才能可見。如下所示，可以判斷數據來源於sys.syscolpars系統基礎表。
此時即使在專用管理員連接下面也是無法刪除這些數據的，會報“Ad hoc update to system catalogs is not supported”，對應中文提示為“不支持對系統目錄進行即席更新”。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
那么難道就沒有辦法解決這種問題了嗎？ 答案是當然有，不過，這種方式是沒有官方文檔而且也不被官方Support的，如果你要按下面方法操作，是有一定風險的。所以如果你決定按照下面方式修復元數據損壞的話，先做好備份。以防萬一。
你必須將數據庫實例在單用戶模式下面啟動，然后以專用管理員（DAC）連接到數據庫，然后就可以刪除基礎表下面的數據了，如下截圖所示：
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)
此時再去檢查數據庫一致性，你就會看到上面遇到的元數據損壞錯誤不見了。如下截圖所示：
其實如果是從SQL Server 2000還原的話，在SQL Server 2000當中是可以修改相關系統表的，如果執行了DBCC CHECKDB命令發現了元數據問題，那么可以直接修改系統表解決問題（當然只是部分情況），如果已經還原到了SQL Server 2008 以上數據庫時，就必須按這種方式折騰，由於這種方式，官方是不支持的。所以還是有一定風險的。因為你不清楚潛在的風險，也不能確保任何場景都能解決問題而不出現意外情況。所以操作之前，盡量多測試、做好備份以防萬一。