Postgresql MVCC机制源码初探_xidinmvccsnapshot-优快云博客

Postgresql MVCC机制源码初探

MVCC(Multi-Version Concurrency Control多版本并发控制)机制几乎是每个数据库的标配，那么postgresql的mvcc机制是怎么实现的？我们从几个简单语句作为切入点，初步了解下相关代码。

以postgresql最常用的RC 隔离级别为例，sessionA session B分别为两个连接会话，前面的数字为语句执行先后顺序，现在已经有一个表t1,有一个列col，插入了一行记录1，再按如下表格中语句顺序执行：

Session A

Session B

1 begin;

BEGIN

insert into t1 values(2);

3select xmax,xmin,*from t1;

commit;

2 select xmax,xmin,*from t1;

4 select xmax,xmin,*from t1;

第1步中语句执行后，又插了一条记录2，再在session B中执行第2步，显然我们是只能查到一条记录如下：

postgres=#select xmax,xmin,*from t1;

xmax | xmin | col

------+------+-----

0 | 553 | 1

(1 row)

但是，此时在buffer中t1表的页面上是有两条记录的，只是另外一条对Session B来说不可见。由于原来的记录1先插入，那么在页面上指向真实元组的页内指针在前面，而记录2在后面，因此会先扫描到记录1，那么执行到第2步时，会对每一条记录都做可见性检查判断。

在HeapTupleSatisfiesMVCC接口中判断是否可见，第一条记录判断流程如下：

bool
HeapTupleSatisfiesMVCC(HeapTuple htup, Snapshot snapshot,
					   Buffer buffer)
{
	HeapTupleHeader tuple = htup->t_data;
	....
	if (!HeapTupleHeaderXminCommitted(tuple))	//xmin插入事务是否提交，第一条已经提交，因此这里不能进入，跳过
	{
		....
	}
	else
	{
		/* xmin is committed, but maybe not according to our snapshot */
		if (!HeapTupleHeaderXminFrozen(tuple) &&
			XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot))	//此处不成立，继续往下执行
			return false;		/* treat as still in progress */
	}

	if (tuple->t_infomask & HEAP_XMAX_INVALID)    //成立，返回true,如果插入事务已经commit，而没有进行过delete操作，那是可见的
		return true;
	.....
}

再看第二条记录判断流程：

bool
HeapTupleSatisfiesMVCC(HeapTuple htup, Snapshot snapshot,
					   Buffer buffer)
{
	HeapTupleHeader tuple = htup->t_data;
	...
	if (!HeapTupleHeaderXminCommitted(tuple))  //进入，插入事务未提交
	{
		if (HeapTupleHeaderXminInvalid(tuple))   //非invalid，跳过
			return false;

		/* Used by pre-9.0 binary upgrades */
		if (tuple->t_infomask & HEAP_MOVED_OFF)   //非HEAP_MOVED_OFF ，跳过

		{
			...
		}
		/* Used by pre-9.0 binary upgrades */
		else if (tuple->t_infomask & HEAP_MOVED_IN) //跳过
		{
			...
		}
		else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tuple)))  //插入事务是否为当前事务，不是，跳过
		{
			...
		}
		else if (XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot)) //插入事务是否在快照中（未提交的事务），成立，返回false
			return false;		
		....
}

然后我们再执行第3步，在session A中执行select，第一条记录判断和之前相同，不再赘述，第二条记录判断如下：

bool
HeapTupleSatisfiesMVCC(HeapTuple htup, Snapshot snapshot,
					   Buffer buffer)
{
	HeapTupleHeader tuple = htup->t_data;
	...
	if (!HeapTupleHeaderXminCommitted(tuple))  //进入，插入事务未提交
	{
		if (HeapTupleHeaderXminInvalid(tuple))   //非invalid，跳过
			return false;

		/* Used by pre-9.0 binary upgrades */
		if (tuple->t_infomask & HEAP_MOVED_OFF)   //非HEAP_MOVED_OFF ，跳过

		{
			...
		}
		/* Used by pre-9.0 binary upgrades */
		else if (tuple->t_infomask & HEAP_MOVED_IN) //跳过
		{
			...
		}
		else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tuple)))//插入事务是否为当前事务，是的，进入
		{
			if (HeapTupleHeaderGetCmin(tuple) >= snapshot->curcid)
				return false; /* inserted after scan started */
			if (tuple->t_infomask & HEAP_XMAX_INVALID)//insert和select在同一个事务中，而没有进行过delete操作，那是可见的，返回true,能看到两条记录
				return true;
		}
.....
}

接下来commit,再运行第4步，从B 会话中select，显然会有两条记录，对于第一条相同不再分析，那么此时第二条是怎么样的流程呢？

bool
HeapTupleSatisfiesMVCC(HeapTuple htup, Snapshot snapshot,
					   Buffer buffer)
{
	HeapTupleHeader tuple = htup->t_data;

	if (!HeapTupleHeaderXminCommitted(tuple)) //进入，虽然插入事务已经提交，原因见下面
	{
		if (HeapTupleHeaderXminInvalid(tuple))
			return false;

		/* Used by pre-9.0 binary upgrades */
		if (tuple->t_infomask & HEAP_MOVED_OFF) //不成立
		{
			...
		}
		/* Used by pre-9.0 binary upgrades */
		else if (tuple->t_infomask & HEAP_MOVED_IN) //不成立
		{
			...
		}
		else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tuple))) //不成立
		{
			...
		}
		else if (XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot))  //不成立
			return false;
		else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmin(tuple)))  //成立，检查插入事务是否已经提交，如果已经提交，则在tuple上把状态修改成提交状态HEAP_XMIN_COMMITTED，也就是说，提交时并不马上更新状态，等下次用到时再更新，这个操作只要做一次，如果再select，会因为这个状态已经置成了commited而跳过这些逻辑。

			SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED,
						HeapTupleHeaderGetRawXmin(tuple));
		else
		{
			/* it must have aborted or crashed */
			SetHintBits(tuple, buffer, HEAP_XMIN_INVALID,
						InvalidTransactionId);
			return false;
		}
	}
	else
	{
		/* xmin is committed, but maybe not according to our snapshot */
		if (!HeapTupleHeaderXminFrozen(tuple) &&
			XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot))
			return false;		/* treat as still in progress */
	}

	/* by here, the inserting transaction has committed */

	if (tuple->t_infomask & HEAP_XMAX_INVALID)	//同样在这里返回true
		return true;
...
}

这几个语句只是一个简单示例，抛砖引玉，有兴趣者可深入去研究内部逻辑。

下面补一个元组结构图，更方便大家理解xmax,xmin,以及元组状态。