What does transaction_id is 0 means?

Greetings.

Where are you seeing the transaction_id value of 0? You are running a query to view database and session status, I assume?

- Derrick

I'm seeing this from the v_monitor.resource_rejection_details table.
Quite a huge amount of memory >100GB being rejected by transaction 0
How do I interpret this?

In resource_rejection_details, I see transaction ID = 0 along with request_id = 0 and statement_id = -1 for requests where the initial resource request failed (usually for reserving too much memory in a small resource pool). I think this means that the SQL errored out immediately so no transaction was ever opened.
What is the resource rejection message? You might also be able to correlate this with the session_id and timestamp in query_requests to find the query and see if it errored out.