During our upgrade process I've found that DO tries to delete data from droppped table before actually droppping them. This is really WRONG. First we need to drop FKs/Columns other referencing object, then delete the data if any.

var batch = 
        EnumerableUtils.One(Driver.BatchBegin)
        .Concat(translator.CleanupDataCommands)
        .Concat(translator.PreUpgradeCommands)
        .Concat(translator.UpgradeCommands)
        .Concat(translator.CopyDataCommands)
        .Concat(translator.PostCopyDataCommands)
        .Concat(translator.CleanupCommands)
        .Concat(EnumerableUtils.One(Driver.BatchEnd))
        .ToArray();

In our case this lead to inability to perform an upgrade at all.

Xtensive.Orm.ReferentialConstraintViolationException: SQL error occured.
Storage error details 'Entity: Mortgage;'
SQL error details 'Type: ReferentialConstraintViolation; Database: Eis; Table: Mortgage; Column: ReasonOfMortgage_Id; Constraint: FK_Mortgage-ReasonOfMortgage-ReasonOfMortgage_ReasonOfMortgage;'
Query 'DELETE FROM [dbo].[ReasonOfMortgage];
DELETE FROM [dbo].[SubaccountWithoutStorage];
DELETE FROM [dbo].[ParCredentials];
DELETE FROM [dbo].[ParCredentialsForUser];
DELETE FROM [dbo].[TypeOfAuthority];
DELETE FROM [dbo].[Mortgage_TpPayment];
DELETE FROM [dbo].[Mortgage_TpFine];
DELETE FROM [dbo].[EntityIndex];
DELETE FROM [dbo].[ParLinkedEntityes];
DELETE FROM [dbo].[PersonRoleParametr];
DELETE FROM [dbo].[EntityIndex_MlIndexField];
DELETE FROM [dbo].[RegIsExchangeFoundation];'
Original message 'The DELETE statement conflicted with the REFERENCE constraint "FK_Mortgage-ReasonOfMortgage-ReasonOfMortgage_ReasonOfMortgage". The conflict occurred in database "Eis", table "dbo.Mortgage", column 'ReasonOfMortgage_Id'.
The statement has been terminated.'