The sync principle of logstash-input-jdbc?

I want to know the sync principle of logstash-input-jdbc?
I know it is based on the time compare method like :

  select
   *
   from
   cc
  where cc.modified_at > :sql_last_value,

Does it also based on database trigger methon.
When I grep the logstash-input-jdbc source, I find there is no any trigger information.

If you know, please help me.
Any help would be sincerely appreciate.

It doesn't know anything about triggers.