Newsgroups: comp.object
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!zaphod.mps.ohio-state.edu!wuarchive!uunet!odi!dlw
From: dlw@odi.com (Dan Weinreb)
Subject: Re: Availability of class extension (was: value semantic versus...)
In-Reply-To: rowley@bath.cs.ucla.edu's message of 28 May 91 23:28:32 GMT
Message-ID: <1991May29.134314.6850@odi.com>
Reply-To: dlw@odi.com
Organization: Object Design, Inc.
References: <1991May14.093053.3017@jyu.fi> <3683@sequent.cs.qmw.ac.uk> <4851@osc.COM> <1991May22.053938.20827@jyu.fi> <4863@osc.COM> <1991May28.232832.28284@cs.ucla.edu>
Date: Wed, 29 May 91 13:43:14 GMT

In article <1991May28.232832.28284@cs.ucla.edu> rowley@bath.cs.ucla.edu (Michael T Rowley) writes:

								 Is there
   anything about OODBs which makes the problem of reaching all instances
   of a class more difficult for them than it is for relational databases?

No, there isn't.  An OODB can work either way.  It can be designed so
that it always maintains an explicit, semantically-visible extent for
all objects of a certain class within a certain database, or it can be
designed so that it does not necessarily do so.

   However, if there is to be a associative querry language, it must be
   possible for the user to say "give me all objects which meet these
   conditions".  

No, that's not true.  If you have an OODB that does not keep extents,
you can still have an associative query language.  Each query simply
needs to be handed a "collection" object.  So a typical query might be
"find all of the employees within this set of employees for which the
salary is greater than 42", just like the standard mathematical
notation {x element-of X | x.emp > 100} (for "element-of" read a
little epsilon).  The queries can be as complicated as you like; more
than one collection can be involved; and automatic optimization can be
performed.

So an OODB can have associative queries even if it does not
automatically maintain extents.  These are two separate, orthagonal
issues.