posrgresql 中的 subtransaction-log

这个最简单，只记录了一个整数，就是子事务的父事务ID，就是在pg_subtrans目录下，也是通过LRU算法，在共享内存中

/* * The pg_subtrans manager is a pg_clog-like manager that stores the parent * transaction Id for each transaction. It is a fundamental part of the * nested transactions implementation. A main transaction has a parent * of InvalidTransactionId, and each subtransaction has its immediate parent. * The tree can easily be walked from child to parent, but not in the * opposite direction. */