[elephant-devel] Re: Derived Indicies

[Ian Eslick]

Hi Alex,

I've changed the interface but maintained essentially the existing  
functionality, except that subclasses can share a derived index with a  
common base class.  Allowing heirarchy means that at the low-level  
indexes are no longer tied to a specific class in the way they were  
before, so what you are describing as an advanced use is certainly  
possible.  Although perhaps the methodology for multi-store  
dependencies is to have after or around methods on the (setf group- 
name) that forces a re-computation of the index.

This reminds me a bit of cells / a general system for making the value  
in some CLOS slot dependent on slots of other instances so that  
changes automatically propagated.  I think after we get some practice  
and experience with the current features, we should start to sketch  
out what kinds of things like these would make life better.

Perhaps you can write up the scenarios in a bit more detail as well as  
make sure the new branch gives you what you need?

Thanks,
Ian

On Mar 23, 2008, at 11:12 AM, Alex Mizrahi wrote:
>      IE>  :index t is not necessary - in fact it is ignored.  :slot- 
> deps
> are
>      also not required, but the derived index is updated on any slot  
> write
>      if that slot is not transient, set-valued or an association.   
> We can
>      add those last three slot types into the mix if necessary, but  
> I'm
>      trying to avoid too much complex computation taking place  
> during slot
>      writes (self-deadlock, etc) for the time being.
>
>
> seems to be fine..
> actually we are using derived indices in quite special way -- to get  
> index
> that is ordered in special way, to do efficient lookups of some kind.
> suppose we have a messaging system with two persistent classes --  
> user and
> message:
>
> (defpclass user ()
>     ((username :index t)
>     ...)
>
> (defpclass message ()
>    ((from-user :accessor from-user :index t)
>     (to-user :accessor to-user :index t)
>     (text :accessor text-of :index t)
>     (modification-time :accessor modification-time-of :index t)))
>
> suppose we'd like to get inbox and outbox views for user, i.e. list  
> of 10
> latest messages to user or from user.
> with considerable amounts of users and messages it is not efficient  
> to get
> these latest messages from any of normal indices, as it requires  
> scanning
> potentially large number of messages.
>
> it's possible to make efficient lookups via derived indices --  
> messages
> ordered first by user, then my modification time. iirc cons sorting in
> elephant has desired characteristics, but unfortunately postmodern  
> backend
> does not support complex type sorting.
>
> but we happily can reduce problem to sorting strings, e.g.  
> "13_31321433"
> where 13 is oid of from-user  and 31321433 is modification time  
> (universal
> time). (actually we have user-id field instead of oid to preserve  
> identity
> across multiple stores etc).
>
> thus, via two derived indices we can efficiently list messages for  
> inbox and
> outbox. (via cursor operations, that's not very easy, but works fine).
>
> as i understant you're going to make elephant more high level, and  
> possibly
> such low level index operations can be replaced with some high level
> concept.
> are you planning something like these dual indices?
>
> if there will be no special functionality for these, it would make  
> sense to
> make derived indices somewhat more flexible. for example, i suspect  
> some
> uses might require "foreign slot dependency".
> like if user definition above had group field, and we'd like to build
> messages-by-group index. when user-group changes, all messages of  
> this user
> should update derived index.
> probably there's no need in this stuff right now, but it would be  
> nice if it
> will be possible to add it in future, so advanced indices can be built
>
>
>
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