How does the Proxy Cache Key Lookup actually happen?

[lovekmla]

I am using part of the request_body as the Cache_key in setting up the
Proxy_cache_key and I was wondering how the actual lookup / matching of the
Cache would occur?

>From the documentation, it looks like it's a MD5 encryption of the Cache Key
that I set. 

Does that mean the cache_key lookup would be done after constructing the
cache key for the Incoming request? Is it possible that it would try to do a
lookup with the default URI first and failover to construct the cache key??

Context of asking this is I am seeing some inconsistency in the Cache Hit
Serving time for huge size of request_body Params even though when it's
served from the cache.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,249851,249851#msg-249851