Comment 1 for bug 1749167

Instances are only created in cell databases once they are scheduled to a host, so we can know which specific cell DB to call for a new instance record.
When the scheduler passes a NoValidHost, then given there wouldn't be a host for that instance, the instance record is rather created to a specific cell0 database.

See https://docs.openstack.org/nova/latest/user/cellsv2-layout.html#concepts for more explanations.

Now, it looks like you had a problem with your DB given the above log. When the scheduler tried to look at all the compute nodes, it got a SQL exception hence why it didn't provided a valid host.
Looks to me that specific problem created some issue hence why you weren't possible to delete that instance.

Can you please try to reproduce that problem ?