The event log
Since you need to persist the events in the database, the first step is certainly to have a table that just takes everything that comes in. You could call it event_log:
event_id node reported_state time_code
10304441 system1 UP 2020-05-05T14:00:00Z
10304446 system1 UP 2020-05-05T14:01:00Z
If you're using event sourcing then create a workflow (saga) from this source
callActivity WriteToSql event
callActivity WriteToPostgress event
callActivty Compensation event // push to a Queue of compensations to be resolved.
You are correct that you will need a saga if you are not using a transaction. You will need to accept eventually consistent data though.
When it comes to a saga, you have 2 options:
Using an orchestrator: A controlling process
Choreographed: Each step handles passing on the next or compensating.
Using the 2nd method you would store the data + the intent ...