The disadvantage is, you'll have to have some process for deleting old data from the table, as it will stay around and it will bite you when you get the same pg_backend_pid() again down the road.
Rather than use pg_backend_id(), why not just assign session IDs from a sequence? You would still get the problem of stale session data so you'd probably still want a periodic cleaner process, but you won't need to worry about session ID collision.
-Neil
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