The configurationsettings table is instantiated with a uniqueness constraint:

UniqueConstraint('external_integration_id', 'library_id', 'key'),

But Postgres won't enforce a uniqueness constraint if any of the columns are null, and it's common for external_integration_id and/or library_id to be null. This means race conditions lead to duplicate rows (which cause persistent problems) rather than one-off errors.

We recently fixed this problem with the credentials table (https://github.com/NYPL-Simplified/server_core/pull/1129) and we can do the same thing here.