在postgre数据库里遇到了虚拟事务virtualtransaction 的概念，请问什么是虚拟事务？

locktype  | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid |
virtualtransaction |       pid       |      mode       | granted | fastpath
------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+-
-------------------+-----------------+-----------------+---------+----------
 virtualxid |          |          |      |       | 6/25442    |               |         |       |          |
6/25442            | 140607531513600 | ExclusiveLock   | t       | t
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