StartTransaction
拿到vxid(由backendid和localid组合值)表示虚拟的事务id(写还未发生,不能分配真实xid)
用vxid注册MyProc,注册后在锁表中可以查询到vxid..."b", savepointLevel = 0,
state = TRANS_INPROGRESS, blockState = TBLOCK_SUBINPROGRESS, nestingLevel...}, subTransactionId = 2, name = 0x199c6c0 "a", savepointLevel = 0,
state = TRANS_INPROGRESS, blockState...= 0, state = TRANS_INPROGRESS,
blockState = TBLOCK_INPROGRESS, nestingLevel = 1, gucNestLevel = 1...在分配事务ID时,每个子事务都会拿到自己的XID。