[elephant-devel] Re: Getting count of the number of persistentobjects of a particular class

[Alex Mizrahi]

 IE> Part of the problem with Elephant is that we currently create CLOS
 IE> objects in memory for every 'row' that is touched in a query (or
 IE> touched during an index operation).  We have to de-serialize the
 IE> record, create an object, initialize any transient slots, etc.  The
 IE> SQL engine, by contrast, can just keep track of a list of OIDs and
 IE> then count the size of the arrays that result from any joins.  This is
 IE> a much cheaper set of operations per-row (i.e. per object).  If we
 IE> only read OIDs into memory until the final step when we need to access
 IE> a slot, we can save quite a bit of the cost of object creation.

also backends which connect to database via a socket have considerable 
communication overhead, i think it's even larger than deserialization 
impact.

 IE> My hope is that a query system would implement this model properly.
 IE> Unfortunately, this will not happen soon, as much as I'm itching to do
 IE> it!

OTOH it's fairly easy to make function that counts total number of entries 
in btree or in some range, at least it's easy with db-postmodern -- it just 
translates to COUNT(*) query.
if there is such a big demand for this feature, we can just implement it as 
backend-specific advantage ^__^.