* during or after acquiring a shared lockcould an XID possibly become a

* member of a MultiXact, and that MultiXactwould have to be created

* during or after the lock acquisition.)

*

* OldestVisibleMXactId[k] is the oldestMultiXactId each backend's

* current transaction(s) think is potentiallylive, or InvalidMultiXactId

* when not in a transaction or not in atransaction that's paid any

* attention to MultiXacts yet. This is computed when first needed in a

* given transaction, and cleared attransaction end. We can compute it

* as the minimum of the validOldestMemberMXactId[] entries at the time

* we compute it (using nextMXact if none areva lid). Each backend is

* required not to attempt to access any SLRUdata for MultiXactIds older

* than its own OldestVisibleMXactId[] setting;this is necessary because

* the checkpointer could truncate away suchdata at any instant.

*

* The checkpointer can compute the safetruncation point as the oldest

* valid value among all theOldestMemberMXactId[] and

* OldestVisibleMXactId[] entries, or nextMXactif none are valid.

* Clearly, it is not possible for anylater-computed OldestVisibleMXactId

* value to be older than this, and so there isno risk of truncating data

* that is still needed.

*/

MultiXactIdperBackendXactIds[1]; /* VARIABLE LENGTH ARRAY */

} MultiXactStateData;

下面看看初始化完"MultiXactOffset Ctl"、"MultiXactOffset Ctl"及"Shared MultiXact State"相关结构后在内存中的结构图

初始化完MultiXact相关结构的内存结构图

为了精简上图，把创建shmem的哈希表索引"ShmemIndex"时创建的HCTL结构删掉了，这个结构的作用是记录创建可扩展哈希表的相关信息。增加了左边灰色底的部分，描述共享内存/shmem里各变量物理布局概览，由下往上，由低地址到高地址。其中的"MultiXactCtl"相关机构即MultiXactOffsetCtl和MultiXactMemberCtl的相关结构图下面分别给出，要不上面的图太大太复杂了。

MultiXact相关结构图