翻译
当一个tuple保存到硬盘的时候，它的t_ctid被初始化为它的TID（位置信息），
如果tuple被更新过，则t_ctid被改变为最新的tuple 替换版本。
因此，如果XMAX无效时或者t_ctid指向它自己（如果XMAX有效，此时，tuple要么是被锁定的，要么是被删除的），
tuple表示此行的最新版本。这种情况下我们可以追随t_ctid找到此行最新的版本。
小心，VACUUM可能会再擦除c_tid指向的旧版本指针之前擦除新版本，
所以，当我们要追随t_ctid寻找新版本之前，需要检查一下引用槽是否为空或者引用了一个unrelate tuple
以及XMIN与XMAX是否相等来确定我们并没有引用到一个被VACUUM释放的tuple，
如果上述检查返回失败，我们可以假设这个tuple没有延伸的版本。
t_ctid有时候会用来保存一个特殊的插入标记，而不是一个TID，当tuple被插入的时候，
我们会设置一个特殊的标记，直到插入者确定将此tuple插入到硬盘，
所以，当事务正在运行，或者失败时，我们只应该看到一个包含XMAX的tuple，
如果插入被确定的时候标记就会被替换成实际的TID，注意，仅仅在插入的时候我们能看到标记，因为它仅仅被用作插入操作。原文
/* A word about t_ctid: whenever a new tuple is stored on disk, its t_ctid
* is initialized with its own TID (location). If the tuple is ever updated,
* its t_ctid is changed to point to the replacement version of the tuple.
* Thus, a tuple is the latest version of its row iff XMAX is invalid or
* t_ctid points to itself (in which case, if XMAX is valid, the tuple is
* either locked or deleted). One can follow the chain of t_ctid links
* to find the newest version of the row. Beware however that VACUUM might
* erase the pointed-to (newer) tuple before erasing the pointing (older)
* tuple. Hence, when following a t_ctid link, it is necessary to check
* to see if the referenced slot is empty or contains an unrelated tuple.
* Check that the referenced tuple has XMIN equal to the referencing tuple's
* XMAX to verify that it is actually the descendant version and not an
* unrelated tuple stored into a slot recently freed by VACUUM. If either
* check fails, one may assume that there is no live descendant version.
*
* t_ctid is sometimes used to store a speculative insertion token, instead
* of a real TID. A speculative token is set on a tuple that's being
* inserted, until the inserter is sure that it wants to go ahead with the
* insertion. Hence a token should only be seen on a tuple with an XMAX
* that's still in-progress, or invalid/aborted. The token is replaced with
* the tuple's real TID when the insertion is confirmed. One should never
* see a speculative insertion token while following a chain of t_ctid links,
* because they are not used on updates, only insertions.