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

[Steve Alexander]

> Heh.  If we supply a canned object, people will take that (reasonably
> enough) as a claim that the canned object is suitable for all BTree
> purposes.  That your particular app is happy to abide a grossly inefficient
> implementation of .update() doesn't help me when I get a bug report
> complaining that someone else's app isn't so tolerant.
> 
> "someone who wants it enough is welcome to write it" is still my take on it,
> and if that someone decides to implement update inefficiently-- or not to
> implement update at all --they should document that up-front.

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.

--
Steve Alexander