You mean Postgres? I’m not sure what this is about.
What I would do is if the user id is 0 use the username for the ID. Then if find_username_by_import_id fails to find the user, create the user, setting the email address to fake_email (it’s a function in base.rb that generates a fake email address) and the username with the username that you have. Then if you’re ambitius you could at the end of the script you could suspend all users that have @email.invalid in their email address. They won’t be active, so I don’t think it would matter much if you didn’t suspend them.
Another way would be to do a query that somehow generated a list of all of the deleted users and then created them before you started doing posts, but that seems harder.
If you want to create a deleted user user and have all of those posts owned by that user instead of system you could do that and just replace the -1 with the user number of deleted user. You could create it as a regular user or do something fancy and make it have a user id of -2 or something like that.
In some systems this is because sometimes attachments are in the body of the post and others the attachment record is in the database.
Did you install the Migrated password hashes support plugin after you ran the import (it can interfere with running imports in at least some circumstances). Does SMF2 hash passwords the same way that smf does