PostgreSQL VACUUM可見性

分兩種情況，一是XMIN事務未提交，一個是xmin事務已提交。

針對xmin未提交的事務，即當前的tuple剛insert還未提交：

1）元組不合法，即壞元組時，返回HEAPTUPLE_DEAD

2）該tuple是當前事務產生的：此時這個記錄在這個事務未刪除或只是被鎖住或進行了delete但是delete abort了，那返回HAPTUPLE_INSERT_IN_PROGRESS；若則記錄又被刪除了，那返回HEAPTUPLE_DELETE_IN_PROGRESS

3）該tuple是其他事務產生的，tuple頭中標記未提交：HEAP_INSERT_IN_POGRESS

4）該tuple是其他事務產生，clog中顯示xmin已提交：則標記tuple頭爲HEAP_XMIN_COMMITTED。

5）其他情況下，這個xmin事務確實未提交，abort或損壞了：返回HEAPTUPLE_DEAD

針對xmin已提交的事務，即當前的tuple insert已提交了：

1）xmax未提交，返回HEAPTUPLE_LIVE

2）tuple只是被鎖着：xmax未提交：返回HEAPTUPLE_LIVE

3）tuple正在delete：返回HEAPTUPLE_DELETE_IN_PROGRESS

4）clog中查到xmax已提交：標記tuple頭HEAP_XMAX_COMMITTED

5）其他情況delete確實未提交：HEAPTUPLE_LIVE

6）剩下的情況，tuple.t_xmax >= OldestXmin表示有事務還能看到insert的值：HEAPTUPLE_RECENTLY_DEAD

7）其他情況XMAX已提交了：返回HEAPTUPLE_DEAD

HeapTupleSatisfiesVacuum
{
	HeapTupleHeader tuple = htup->t_data;

	if (!HeapTupleHeaderXminCommitted(tuple))//未提交
	{
		if (HeapTupleHeaderXminInvalid(tuple))//元組不合法，壞的元組
			return HEAPTUPLE_DEAD;
		else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tuple)))
		{//當前事務產生的元組
			if (tuple->t_infomask & HEAP_XMAX_INVALID)//未被刪除，當前事務正在insert
				return HEAPTUPLE_INSERT_IN_PROGRESS;
			//元組被鎖住，未被刪除 或者 
			if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask) || HeapTupleHeaderIsOnlyLocked(tuple))
				return HEAPTUPLE_INSERT_IN_PROGRESS;
			//insert後當前事務又刪除
			if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(tuple)))
				return HEAPTUPLE_DELETE_IN_PROGRESS;
			//delete的子事務被abort，即insert正在進行
			return HEAPTUPLE_INSERT_IN_PROGRESS;
		}
		else if (TransactionIdIsInProgress(HeapTupleHeaderGetRawXmin(tuple)))
		{//其他事務正在insert
			return HEAPTUPLE_INSERT_IN_PROGRESS;
		}//從clog中讀取tuple的事務狀態，爲提交則標記提交
		else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmin(tuple)))
			SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED,HeapTupleHeaderGetRawXmin(tuple));
		else
		{
			//真的tuple未提交、abort或者損壞
			SetHintBits(tuple, buffer, HEAP_XMIN_INVALID,InvalidTransactionId);
			return HEAPTUPLE_DEAD;
		}
		//此時，xmin已提交了。
	}

	/*
	 * Okay, the inserter committed, so it was good at some point.  Now what
	 * about the deleting transaction?
	 */
	if (tuple->t_infomask & HEAP_XMAX_INVALID)
		return HEAPTUPLE_LIVE;//未被刪除則該記錄是活的

	if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask))
	{//只是鎖住，未進行delete
		if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED))
		{//delete事務未提交
			if (TransactionIdIsInProgress(HeapTupleHeaderGetRawXmax(tuple)))
				return HEAPTUPLE_LIVE;
			SetHintBits(tuple, buffer, HEAP_XMAX_INVALID,InvalidTransactionId);
			
		}
		return HEAPTUPLE_LIVE;
	}
	//delete未提交
	if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED))
	{
		if (TransactionIdIsInProgress(HeapTupleHeaderGetRawXmax(tuple)))
			return HEAPTUPLE_DELETE_IN_PROGRESS;//delete正在進行
		else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmax(tuple)))//從clog中檢查delete提交了
			SetHintBits(tuple, buffer, HEAP_XMAX_COMMITTED,HeapTupleHeaderGetRawXmax(tuple));
		else
		{
			//真的未提交、abort或者損壞
			SetHintBits(tuple, buffer, HEAP_XMAX_INVALID,InvalidTransactionId);
			return HEAPTUPLE_LIVE;
		}
		//此時delete已提交
	}
	//有事務還可能看到他
	//tuple->t_choice.t_heap.t_xmax >= OldestXmin
	if (!TransactionIdPrecedes(HeapTupleHeaderGetRawXmax(tuple), OldestXmin))
		return HEAPTUPLE_RECENTLY_DEAD;
	/* Otherwise, it's dead and removable */
	return HEAPTUPLE_DEAD;
}

 

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