[ZODB-Dev] PersistentMapping good for large numbers of objects?

[Shane Hathaway]

Steve Alexander wrote:
> Another approach would be for BTrees to grow a callback for whenever a 
> new key is added and callback for whenever a new key is removed. Let's 
> not go there :-)
> 
> Yet another approach would be for BTrees to have a non-persistent 
> settable integer counter, and they could increment and decrement the 
> counter as keys are added or removed. A client could check this counter 
> to see how an independently stored length is supposed to be changed.
> 
> Here's what the method from the wrapper would look like:
> 
>   def update(self, seq):
>       btree = self.__btree
>       btree.deltaLength = 0
>       btree.update(seq)
>       if btree.deltaLength:
>           self.__length += btree.deltaLength
> 
> 
> I'm not suggesting anyone implement this, just offering up ideas.

How about a combination of the above suggestions: every operation that 
mutates a BTree could look for an attribute of the BTree called 
"length".  If it's there, the mutating operation could call 
length.changed(deltaLength).  Also, the __len__ slot could delegate to 
the length object.  That way, we don't have to wrap the BTree just to 
get a minor (but nice) feature.

Shane