Oracle基础知识-v$Librarycache中的get，pin

http://www.ixora.com.au/q+a/0011/08200945.htm中有如下描述：

 

When an object has been located in (or loaded into) the library cache, the session "remembers" its location by maintaining a null mode lock on the object. When a session needs to use an object, if it already (or still) has the library cache lock, then it can proceed with the pin request. If it does not yet have a lock then a get operation is needed to get a lock before the pin can be requested.

This probably makes little sense unless you understand the distinction between the handle of a library cache object and its heaps. Locks protect the handle. Pins protect the heaps. The heaps are the "body" parts of the object that get loaded as required, except for heap 0 which contains the identifying information about the object. When we speak of an object being aged out and then reloaded, it is actually the heaps other than heap 0 of which we are speaking. Heap 0 is never aged out while any session is holding a lock on its handle.

The fundamental reason for the distinction between locks and pins is that it in theory allows an object's identity to be checked using a shared lock on the handle while another session has an exclusive pin on one or more of its heaps. The distinction is also important for maintaining cache coherence.

 

我对上述描述的理解：

get表示请求对象、获得对象句柄；

pin根据句柄找到实际对象并执行，但对象内容可能因为老化而pin不到所以出现reload；

一个session需要使用一个object时，如果是初次使用，则必然是先get然后pin并维护这个object的句柄。下次再使用这个object时，因为已经维护该句柄，所以直接pin而没有了get过程。如果对象老化则移除共享池，再次请求则会出现reload。