When upgrading SharePoint 2007 to SP3 we got this error when running SharePoint Configuration Wizard.

Problem:

SPManager ERROR [8/2/2012 10:00:27 AM]: An explicit DROP INDEX is not allowed on index 'Sites.Sites_Id'. It is being used for FOREIGN KEY constraint enforcement.

SQL Query
exec sp_helpconstraint 'sites'

Deletion the Foreign Key FK_lms_Categories_Sites

After that the Configuration completed successfully.

Solution:

This is a known issue coming from the very old 2.x versions (when they were initially installed). SP alters index on ‘Sites’ table referenced by ‘lms_Category’ table when upgrading. To perform a successful upgrade it’s necessary to just delete the relation between these tables and run configuration wizard again.

This relation is not mandatory - it's necessary so that categories are deleted when organizations are deleted.

However, if you want to rebuild it you need to run the following script:

USE Content_DB

GO

ALTER TABLE dbo.lms_Categories WITH CHECK ADD CONSTRAINT FK_lms_Categories_Sites FOREIGN KEY(SiteId)

REFERENCES dbo.Sites (Id)

ON DELETE CASCADE

GO

ALTER TABLE dbo.lms_Categories CHECK CONSTRAINT FK_lms_Categories_Sites