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Home -> Community -> Usenet -> comp.databases.theory -> Re: S.O.D.A. database Query API - call for comments

Re: S.O.D.A. database Query API - call for comments

From: Perry Anderson <here_at_cplusplus.org>
Date: Tue, 15 May 2001 07:56:27 -0500
Message-ID: <3B01277B.BE234459@cplusplus.org>

Interesting...

Carl Rosenberger wrote:

> Philip Lijnzaad wrote:
> > Your courage is indeed much admired :-)
>
> Thanks!
> My compliments in return for being a fair and enjoyable discussion partner.
>
> > Carl> "Existing links" are defined by the class schema of the application.
> > >>
> > >> which is a bit rigid ... you can't do tricks like finding the gaps in a
> > >> series of entries (see other thread) by doing a self join.
> >
> > Carl> Again hack against theory.
> >
> > why ? The theory is so general that it allows clever (and fast) 'hacks',
> > because it is based on operating on sets of things rigth from start. I
> > believe this is what makes it so suitable to large-scale data management.
>
> The approaches definitely come from two totally different directions:
> - SQL *is* the language of sets and mass updates
> - Object orientation is the exact opposite. You define single object
> behaviour.
>
> The two extremes both need the functionality of the other side:
> - Relational databases are headed towards "object-relational". Inheritance
> for tables is introduced. Akmal Chaudhri has recently posted an example on
> how Informix handles this to comp.databases.object. A side-note: Serializing
> objects to columns has nothing to do with "object-relational". It merely is
> "object-struct-reuse".
> - Object databases also need declarative functionality for queries and
> updates.
>
> Let's hope we can all learn from eachother to create the best of both
> worlds.
>
> > Carl> I have said this before earlier in this thread: If you need a rigid
> > Carl> system for day-to-day data management to maintain rather flat
> > Carl> inheritance hierarchies and lots of simple records: Relational
> > Carl> databases are probably the best choice.
> >
> > I object to the 'rigid' and 'flat'; the relational systems I have worked
 with
> > are anything but rigid or flat.
>
> Give a finger and you've lost a hand. :-)
> I meant "rigid" in a positive sense, as in "stable-in-every-day-use". Sorry
> if I choose the wrong words sometimes, since English is not my mother
> language. Replace "rigid" with "stable".
>
> Forget "flat" also, if you wish. "Flat" and "heterogeneous" are a matter of
> standpoint.
>
> > Why ? Using relational calculus, you 'massage' the relevant (parts of)
> > entities into a set of data that is of interest to the business logic, in
 the
> > most optimal way. The massaged (parts of) entities need not correspond to
 any
> > proper object type known to the system, but that is irrelevant.
>
> This is where I see one of the largest advantages of relational databases:
> You can always set up a two-dimensional table of your result, which is
> instantaniously readable by a human.
>
> > The end
> > result of a query or transaction will be something that is then expressed
 (or
> > expressible) as objects. Relations expose every nook and cranny of the
 data,
> > in the most general way; sometimes this is simply needed.
>
> This is where I see one of the largest disadvantages of relational
> databases:
> Reconstructing objects and the relation between them is not supported by the
> database engine. Once you start using a mapping tool to help, you build an
> *additional* in-memory-database of the objects that you have "checked-out"
> to the user-application. Ideally the additional object layer would also take
> care about locking issues. This is where object databases theoretically have
> great system-immanent advantages since they need no additional layer.
>
> > Carl> How many tables would you use to represent an inheritance hierarchy
 of the
> > Carl> following?
> > Carl> - person
> > Carl> - employee
> > Carl> - manager
> >
> > Depends, but usually three. This can be 'horizontal' (only tables for
> > leaf-classes; query super-classes through a UNION of a restriction of
 them)
> > or vertical (each class its own table, with sub-classes having just the
 bits
> > that are 'new', + a foreign key that is also a primary key; query
 sub-classes
> > through a join of super- and sub-table).
>
> This is a fair and honest answer. "Depends" shows that you have a problem on
> a decision, depending on application and performance issues. The need to
> take a decision already is the sign of a mismatch.
>
> Thinking OO, the above is simple and straigth forward:
> manager extends employee extends person.
>
> Using your second suggestion, the necessary queries for manager names looks
> like this:
> - Relational:
> select person.name from person, employee, manager
> where employee.person = person.id
> and manager.employee = employee.id
> - Object-oriented:
> database.get(manager);
>
> Now would you be so kind to write a query that completely puts all person
> objects back together again, with the following addition to the datamodel:
> - customer extends person
> - sponsor extends person
> - VC extends sponsor
> - self_employed extends employee
>
> I bet, I will be faster:
> database.get(person)
>
> > That's not the point: the tables and primary/foreign keys guarantee you
 that
> > you can query the most bizarre combinations of parts of entities,
 producing
> > things that don't have to be any particular object. This generality is
> > lacking in OODBMSs. So the 'separation' aspect is that RDMBSs do one thing
> > (storing and querying), and do it well (i.e. in a very general way).
>
> I fully agree.
>
> Accordingly we would like to extend the query functionality for object
> databases.
>
> > Carl> Maintainability is a matter of the implementation. Schema evolution
 is
> > Carl> not a problem inherent to object datases in principle.
> >
> > I'm not so sure, because objects don't have the closure property. I.e.,
> > products, projection, set operations and restrictions of relations are
 again
> > relations. This symmetry is not available for objects.
>
> To some extent, you may be right:
> A table may exist on it's own. You can transfer it anywhere independantly.
> If you transfer a second table, the joins will be there again, simply
> because they only consist of numbers.
>
> Object databases would transfer objects independantly. A complete restore of
> a large object network is only possible with access to OIDs. Ideally these
> would be unique worldwide (GUIDs).
>
> Now what is the usecase?
> Replication!
>
> This is where GUIDs also become worthwhile using on relational systems.
>
> > Carl> No: Reuse your object structures with a new logic if you wish.
> >
> > So what happens if a particular relationship changes from a one-to-many
> > relationship into a many-to-many relationship ? You will have to update
 your
> > object structure, not matter what (both in the case of an OODBMS as well
 as
> > in the RDBMS + OR layer).
>
> True.
> OR layers can save you very much work. It might get your implementation work
> very close to using an object database.
>
> I am just questioning the efficiency, performance, stability and resource
> consumption of an additional layer.
>
> > Carl> I don't understand your
> > Carl> round-trip-problem. This is dependant on the implementation.
> >
> > in theory yes (if you are clever enough to do query transformations), but
 in
> > practice no. In practice, complicated queries end up being done by
 explicit
> > pointer chasing, because that's the only thing available.
>
> Query transformation and optimization is quite simple:
> - think in sets
> - identify reocurring sets
> - evaluate one set at a time
> - always use the smallest set first
> - narrow larger sets by smaller sets
>
> The principle will be the same for relational and for object databases.
>
> Of course query evaluation can get more complex if object databases provide
> added language binding functionality like support for Arrays or Collections
>
> > Apart from the
> > inability to do convenient aggregations over the data, you loose the
 ability
> > to work with sets of things ('things' not necessarily being
> > objects). Compared to that, the round-trip problem is indeed minor.
>
> "Sets of things" should be available on object databases by providing
> indices.
>
> > Carl> Agreed, if an OR layer is used.
> > Carl> OR layers produce lots of memory and performance overhead.
> >
> > no, not necessarily. In fact, frequently the OR layer is used to add some
> > caching, improving the speed relative to the RDBMS. I will conceed that an
> > OODBMS (which usually also does sophisticated chaching, it seems) has an
> > advantage here.
>
> Thanks!
>
> Yes, OR layers can improve the speed by caching objects, but this only works
> for usecases, where a small and defined amounts of data are accessed. For
> typical High-End-Usecases you can assume that caches are always full.
>
> I will conceed that most current object databases including our own perform
> badly, if query-evaluation hits cache limits. The future will see further
> development and better systems.
>
> > Carl> As far as I am informed, views still are very slow.
> >
> > Where did you get this information from ?
>
> - Empirical research, two years ago.
> - Continous contact to my former company.
>
> > A view is as fast as the query that
> > defines it. So stating that a view is slower than a table may be true, but
> > you're comparing apples and oranges.
>
> I fully agree.
> A view is *only* as fast as the query that defines it.
>
> Sorry for my bad wording. The negative performance hit occurs when queries
> join views with other views or tables. Most (all ?) current query optimizers
> build all views first (run all underlying queries), before evaluating them
> against eachother.
>
> > Carl> Relational databases have only just started to allow indices on
 views.
> >
> > Views can (and do) use the same indexes that the tables they consist of
> > have, so I don't know what you're trying to say here.
>
> A little more than a year ago MSSQL has added indexed views. The desired
> effect: Views can run faster than the queries that define them, because
> joins are pre-evaluated.
>
> Kind regards,
> Carl
> ---
> Carl Rosenberger
> db4o - database for objects - http://www.db4o.com
Received on Tue May 15 2001 - 07:56:27 CDT

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