Postgresql源码（65）新快照体系Globalvis工作原理分析

GetSnapshotData
  ...
  LWLockAcquire(ProcArrayLock, LW_SHARED)
  ...
  for (int pgxactoff = 0; pgxactoff < numProcs; pgxactoff++)
    TransactionId xid = UINT32_ACCESS_ONCE(other_xids[pgxactoff]);
    
    /* 如果xid>=xmax可以直接跳过，因为这个xid是在我拿快照后启动的，对我来说肯定不可见 */
    if (!NormalTransactionIdPrecedes(xid, xmax))
      continue;    
    
    /* 如果xid < xmin，xid也要包含在xmin中，xmin表示最小的活跃事务 */
    if (NormalTransactionIdPrecedes(xid, xmin))
      xmin = xid;
    
    /* 记录到快照中 */
    xip[count++] = xid;
  
  /* 更新一次xmin */
  if (!TransactionIdIsValid(MyProc->xmin))
    MyProc->xmin = TransactionXmin = xmin;

  LWLockRelease(ProcArrayLock);

GetSnapshotData
    //【0】前面把快照已经计算完了。下面“顺便”计算下全局最小可清理位点：RecentGlobalXmin
    
    //【1】拿到最小事务ID
	if (TransactionIdPrecedes(xmin, globalxmin))
		globalxmin = xmin;
    
	//【2】最小可清理位点传给RecentGlobalXmin，如果配了vacuum_defer_cleanup_age
	// 把最小可清理位点，再往前推，清理时会保留更多的事务，给高延迟的备机使用
	RecentGlobalXmin = globalxmin - vacuum_defer_cleanup_age;
	if (!TransactionIdIsNormal(RecentGlobalXmin))
		RecentGlobalXmin = FirstNormalTransactionId;

	//【3】如果复制槽指定了某个位点，vacuum是不能清理的；
	if (TransactionIdIsValid(replication_slot_xmin) &&
		NormalTransactionIdPrecedes(replication_slot_xmin, RecentGlobalXmin))
		RecentGlobalXmin = replication_slot_xmin;
    ...
    ...

事务一RC-----------4000440启动-------------------4000440结束------------------------------->
事务二RR----------------------4000450启动-----------------------------4000450结束---------->
事务三RC--------------------------------------------调试位置------------------------------->

GetSnapshotData
    //【0】前面把快照已经计算完了。下面“顺便”计算下全局最小可清理位点，但是由于没有遍历PGPROC的xmin，只有自己遍历出来的xid，

	/* maintain state for GlobalVis* */
	{
		TransactionId def_vis_xid;
		TransactionId def_vis_xid_data;
		FullTransactionId def_vis_fxid;
		FullTransactionId def_vis_fxid_data;
		FullTransactionId oldestfxid;

        //【1】oldestfxid = 726
		oldestfxid = FullXidRelativeTo(latest_completed, oldestxid);
        
        //【2】def_vis_xid_data = 4000450
		def_vis_xid_data =
			TransactionIdRetreatedBy(xmin, vacuum_defer_cleanup_age);
        //【3】def_vis_xid_data = 4000450
		def_vis_xid_data =
			TransactionIdOlder(def_vis_xid_data, replication_slot_xmin);
        
        //【4】def_vis_xid = 4000450
		def_vis_xid = def_vis_xid_data;
		def_vis_xid = TransactionIdOlder(replication_slot_catalog_xmin, def_vis_xid);
		def_vis_fxid = FullXidRelativeTo(latest_completed, def_vis_xid);
		def_vis_fxid_data = FullXidRelativeTo(latest_completed, def_vis_xid_data);

        //【5】4000440 ----> 4000450
		GlobalVisSharedRels.definitely_needed =
			FullTransactionIdNewer(def_vis_fxid,
								   GlobalVisSharedRels.definitely_needed);
        ...
        //【6】4000440 ----> 4000440
		GlobalVisSharedRels.maybe_needed =
			FullTransactionIdNewer(GlobalVisSharedRels.maybe_needed,
								   oldestfxid);
        ...
	}

bool
GlobalVisTestIsRemovableFullXid(GlobalVisState *state,
								FullTransactionId fxid)
{
    // 比maybe_needed小的肯定已经不活跃了，可以清理！
	if (FullTransactionIdPrecedes(fxid, state->maybe_needed))
		return true;

    // 比definitely_needed大于或等于，肯定活跃的，不能清理！
	if (FullTransactionIdFollowsOrEquals(fxid, state->definitely_needed))
		return false;

    // 中间的怎么办？调用GlobalVisUpdate
	if (GlobalVisTestShouldUpdate(state))
	{
	    // 调用ComputeXidHorizons计算每一个PROC的xmin，然后更新到GlobalVis变量中。
		GlobalVisUpdate();

		Assert(FullTransactionIdPrecedes(fxid, state->definitely_needed));

		return FullTransactionIdPrecedes(fxid, state->maybe_needed);
	}
	else
		return false;
}