PostgreSQL vacuum可见性

本文介绍vacuum可见性判断。分两种情况，一是XMIN事务未提交，一个是xmin事务已提交。

针对xmin未提交的事务，即当前的tuple刚insert还未提交：

1）元组不合法，即坏元组时，返回HEAPTUPLE_DEAD

2）该tuple是当前事务产生的：此时这个记录在这个事务未删除或只是被锁住或进行了delete但是delete abort了，那返回HAPTUPLE_INSERT_IN_PROGRESS；若则记录又被删除了，那返回HEAPTUPLE_DELETE_IN_PROGRESS

3）该tuple是其他事务产生的，tuple头中标记未提交：HEAP_INSERT_IN_POGRESS

4）该tuple是其他事务产生，clog中显示xmin已提交：则标记tuple头为HEAP_XMIN_COMMITTED。

5）其他情况下，这个xmin事务确实未提交，abort或损坏了：返回HEAPTUPLE_DEAD

针对xmin已提交的事务，即当前的tuple insert已提交了：

1）xmax未提交，返回HEAPTUPLE_LIVE

2）tuple只是被锁着：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：标记tuple头HEAP_XMAX_COMMITTED

5）其他情况delete确实未提交：HEAPTUPLE_LIVE

6）剩下的情况，tuple.t_xmax >= OldestXmin表示有事务还能看到insert的值：HEAPTUPLE_RECENTLY_DEAD

7）其他情况XMAX已提交了：返回HEAPTUPLE_DEAD

函数

HeapTupleSatisfiesVacuum
{
	HeapTupleHeader tuple = htup->t_data;

	if (!HeapTupleHeaderXminCommitted(tuple))//未提交
	{
		if (HeapTupleHeaderXminInvalid(tuple))//元组不合法，坏的元组
			return HEAPTUPLE_DEAD;
		else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tuple)))
		{//当前事务产生的元组
			if (tuple->t_infomask & HEAP_XMAX_INVALID)//未被删除，当前事务正在insert
				return HEAPTUPLE_INSERT_IN_PROGRESS;
			//元组被锁住，未被删除 或者 
			if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask) || HeapTupleHeaderIsOnlyLocked(tuple))
				return HEAPTUPLE_INSERT_IN_PROGRESS;
			//insert后当前事务又删除
			if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(tuple)))
				return HEAPTUPLE_DELETE_IN_PROGRESS;
			//delete的子事务被abort，即insert正在进行
			return HEAPTUPLE_INSERT_IN_PROGRESS;
		}
		else if (TransactionIdIsInProgress(HeapTupleHeaderGetRawXmin(tuple)))
		{//其他事务正在insert
			return HEAPTUPLE_INSERT_IN_PROGRESS;
		}//从clog中读取tuple的事务状态，为提交则标记提交
		else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmin(tuple)))
			SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED,HeapTupleHeaderGetRawXmin(tuple));
		else
		{
			//真的tuple未提交、abort或者损坏
			SetHintBits(tuple, buffer, HEAP_XMIN_INVALID,InvalidTransactionId);
			return HEAPTUPLE_DEAD;
		}
		//此时，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;//未被删除则该记录是活的

	if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask))
	{//只是锁住，未进行delete
		if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED))
		{//delete事务未提交
			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正在进行
		else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmax(tuple)))//从clog中检查delete提交了
			SetHintBits(tuple, buffer, HEAP_XMAX_COMMITTED,HeapTupleHeaderGetRawXmax(tuple));
		else
		{
			//真的未提交、abort或者损坏
			SetHintBits(tuple, buffer, HEAP_XMAX_INVALID,InvalidTransactionId);
			return HEAPTUPLE_LIVE;
		}
		//此时delete已提交
	}
	//有事务还可能看到他
	//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;
}