Upgrading from vCenter Server 4.1 to 5.0 fails

Upgrading from vCenter Server 4.1 to 5.0 fails and an exception is thrown while executing SQL script. This issue occurs if an index named INDX_TIME_ID in the VPX_HIST_STAT1 table was not deleted in a previous upgrade and continues to exist. The script to upgrade the table fails because of the index's constraint on the TIME_ID column.
To remove the lingering index on the table:
Launch SQL Server Management Studio and Connect to the SQL instance of the vCenter Server database.
Take a backup the vCenter Server database.
To back up the vCenter Server database, right-click the database and click Tasks > Back Up.
Expand the Databases folder.
Click the vCenter Server database name.
Click the New Query button on the toolbar. A blank query window appears.
Enter this SQL statement in the blank query window:

    DROP INDEX VPX_HIST_STAT1.INDX_TIME_ID

Execute the statement by pressing the F5 key or by clicking the ! Execute button on the toolbar.
For more information read the following article from VMware KB.