Postgres - detect change in JSONB column value from 'null' to some value across rows
JSONB supports equality checks. So, if you're using it, you need only 


Create an AFTER trigger on 

CREATE TRIGGER foo
  AFTER UPDATE OF json_data
  ON myTable
  FOR EACH ROW
  WHEN (new.json_data IS DISTINCT FROM old.json_data)
  EXECUTE PROCEDURE myFunction;

Either,


Insert the id, and new.update_time into a new table, or
Update the update_time setting it the current time, and query later for update_time > last_time_ran.