Postgresql源码（16）数据库快照数据结构与获取

typedef struct SnapshotData
{
    /* 处理当前快照的函数 */
	SnapshotSatisfiesFunc satisfies;	/* tuple test function */

	/* 多版本控制 */
	TransactionId xmin;			/* all XID < xmin are visible to me */
	TransactionId xmax;			/* all XID >= xmax are invisible to me */
    /* 活跃事务列表 */
	TransactionId *xip;
	/* 活跃事务数量 */
	uint32		xcnt;			/* # of xact ids in xip[] */

	/* 活跃子事务 */
	TransactionId *subxip;
	int32		subxcnt;		/* # of xact ids in subxip[] */
	bool		suboverflowed;	/* has the subxip array overflowed? */

	bool		takenDuringRecovery;	/* recovery-shaped snapshot? */
	bool		copied;			/* false if it's a static snapshot */

    /* 事务中的command id 一个语句加1 */
	CommandId	curcid;			/* in my xact, CID < curcid are visible */

	/*
	 * An extra return value for HeapTupleSatisfiesDirty, not used in MVCC
	 * snapshots.
	 */
	uint32		speculativeToken;

	/*
	 * Book-keeping information, used by the snapshot manager
	 */
	uint32		active_count;	/* refcount on ActiveSnapshot stack */
	uint32		regd_count;		/* refcount on RegisteredSnapshots */
	pairingheap_node ph_node;	/* link in the RegisteredSnapshots heap */

    /* 快照生成的时间 */
	TimestampTz whenTaken;		/* timestamp when snapshot was taken */
	/* 快照生成时的lsn */
	XLogRecPtr	lsn;			/* position in the WAL stream when taken */
} SnapshotData;

GetSnapshotData(Snapshot snapshot) 
{
    ...
	/* xmax is always latestCompletedXid + 1 */
	xmax = ShmemVariableCache->latestCompletedXid;
    ...
}

typedef struct SnapshotData
{
	SnapshotType snapshot_type; /* type of snapshot */
...

Snapshot
GetTransactionSnapshot(void)
{
// 逻辑解码直接拿HistoricSnapshot
	if (HistoricSnapshotActive())
	{
		Assert(!FirstSnapshotSet);
		return HistoricSnapshot;
	}

// 第一次的事务拿快照会进这个分支，FirstSnapshotSet初始化=false
	if (!FirstSnapshotSet)
	{
// catalog的快照不能比事务快照旧，失效掉后面重新拿
		InvalidateCatalogSnapshot();

...

// 如果隔离级别>=XACT_REPEATABLE_READ：可重复读或可串行化
		if (IsolationUsesXactSnapshot())
		{
// 初始化可串行化的控制结构
			if (IsolationIsSerializable())
				CurrentSnapshot = GetSerializableTransactionSnapshot(&CurrentSnapshotData);
			else
// 可重复读直接获取当前快照
				CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
			/* Make a saved copy */
// 复制一份保存起来
			CurrentSnapshot = CopySnapshot(CurrentSnapshot);
			FirstXactSnapshot = CurrentSnapshot;
			/* Mark it as "registered" in FirstXactSnapshot */
			FirstXactSnapshot->regd_count++;
			pairingheap_add(&RegisteredSnapshots, &FirstXactSnapshot->ph_node);
		}
// 如果隔离级别是RC，直接获取一个当前快照
		else
			CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);

// 下次进来不走这个分支
		FirstSnapshotSet = true;
		return CurrentSnapshot;
	}

// 不是第一次调用了、而且隔离界别>=RR
	if (IsolationUsesXactSnapshot())
		return CurrentSnapshot;

	/* Don't allow catalog snapshot to be older than xact snapshot. */
	InvalidateCatalogSnapshot();


// RC级别、不是第一次拿快照：重新拿快照
	CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);

	return CurrentSnapshot;
}