Added to TODO:
* Add the features of packages
o Make private objects accessable only to objects in the same schema
o Allow current_schema.objname to access current schema objects
o Add session variables
o Allow nested schemas
Bruce Momjian wrote:
> OK, so it seems we need:
> o make private objects accessable only to objects
> in the same schema
> o Allow current_schema.objname to access current
> schema objects
> o session variables
> o nested schemas?
> Dave Held wrote:
> > > -----Original Message-----
> > > From: Bruce Momjian [mailto:pgman(at)candle(dot)pha(dot)pa(dot)us]
> > > Sent: Tuesday, May 10, 2005 8:43 AM
> > > To: Thomas Hallgren
> > > Cc: Tom Lane; rmm(at)sqlisor(dot)com; pgsql-hackers(at)postgresql(dot)org
> > > Subject: Re: [HACKERS] Oracle Style packages on postgres
> > >
> > > [...]
> > > I suppose. I think we should focus on the use cases for Oracle
> > > packages, rather than the specific functionality it provides.
> > > What things do people need PostgreSQL to do that it already
> > > doesn't do?
> > Is that really the best way to go about things? Already RDBMSes
> > are patchwork quilts of functionality. Is merely adding another
> > patch the most elegant way to evolve the database? The problem is
> > that Oracle et al are trying to be ORDBMSes and aren't exactly sure
> > what the best way to go is. Instead of trying to formulate a
> > rational plan for what an ORDBMS should even look like, they simply
> > look at what would work with their existing infrastructure and tack
> > on features. Then Postgres plays the copycat game. Instead of
> > trying to play catch-up with Oracle, why not beat them at their own
> > game?
> > What packages provide is encapsulation. Hiding the data from the
> > user and forcing him/her to use the public interface (methods).
> > That is an important and admirable OO feature. Some people think
> > that using the DB's security model can achieve the same thing. It
> > can't, exactly, but there's an important lesson to be learned from
> > the suggestion. The problem is that OOP is a *programming* paradigm,
> > and a database is not a *programming language*. In a programming
> > language, there really is no such thing as "security". There is
> > only "visibility" and "accessibility". Private methods in an OOP
> > language do not provide *security*; they only limit *accessibility*.
> > Like so many other differences between the relational model and the
> > OOP model, there is an impedance mismatch here. However, there is
> > also opportunity.
> > In an OOPL, you can say: "Users can call this method from here, but
> > not from there." What you *can't* say is: "User X can call this
> > method, but User Y cannot." As you can see, these are orthogonal
> > concepts. You could call the first "accessibility by location" and
> > the second "accessibility by authentication". An ORDBMS should
> > support both. "Private" does not respect your identity, only your
> > calling location. An ACL does not respect your calling scope, only
> > your identity. A system that has both is clearly more flexible than
> > one that only has one or the other.
> > Now what you need to keep in mind is that each visibility model
> > serves a different purpose. The purpose of a security model is to
> > limit *who* can see/touch certain data because the data has intrinsic
> > value. The purpose of an accessibility model is to limit *where* and
> > *how* data can be seen/touched in order to preserve *program
> > invariants*. So if you have an object (or tuple!) that records the
> > start and stop time of some process, it is probably a logical
> > invariant that the stop time is greater than or equal to the start
> > time. For this reason, in a PL, you would encapsulate these fields
> > (attributes) and only provide controlled access to update them that
> > checks and preserves the invariant, *no matter who you are*. You
> > don't want a superuser violating this invariant any more than Sue
> > User.
> > Now you might object that constraints allow you to preserve
> > invariants as well, and indeed they do. But constraints do not
> > respect calling scope. Suppose there is a process that needs to
> > update the timestamps in a way that temporarily breaks the invariant
> > but restores it afterwards. The only way to effect this in a
> > constraint environment is to drop the constraint, perform the
> > operation, and restore it. However, dropping a constraint is not an
> > ideal solution because there may be other unprivileged processes
> > operating on the relation that still need the constraint to be
> > enforced. There is no way to say: "There is a priviledged class of
> > methods that is allowed to violate this constraint because they are
> > trusted to restore it upon completion." Note that this is different
> > from saying "There is a priviledged class of users that is allowed
> > to violate this constraint." If you try to do something like give
> > read-only access to everybody and only write access to one user and
> > define that user to be the owner of the methods that update the data,
> > you have to follow the convention that that user only operates
> > through the defined interface, and doesn't hack the data directly.
> > That's because user-level accessibility is not the same as scope-
> > level accessibility. Whereas, if you define something like a
> > package, and say: "Package X is allowed full and complete access
> > to relation Y", and stick the interface methods in X, you still have
> > all the user-level security you want while preserving the invariants
> > in the most elegant way.
> > So you can think of a package as a scope in a programming language.
> > It's like a user, but it is not a user. A user has privileges that
> > cut across scopes. Now, whether packages should be different from
> > schemas is a whole different ballgame. The purpose of a schema in
> > Postgres is not entirely clear to me. There's lots of different ways
> > to use schemas, and there is no obvious best way to use them. In
> > order to implement the accessibility features of packages, schemas
> > would have to be changed considerably. Probably a lot of users would
> > be unhappy if schemas were changed in that way. My guess is that
> > this would not be a good idea.
> > I think we can get some guidance from PLs. C++ is what you call a
> > "multi-paradigm language". You can do everything from assembly to
> > metaprogramming in C++. As such, it is very loose and open in some
> > respects. C++ has two kinds of scopes: it has classes and namespaces.
> > Members of a class are encapsulated and support data hiding. Members
> > of a namespace are only loosely grouped and do not support data hiding
> > explicitly. Namespaces exist primarily to avoid name collisions.
> > Java, on the other hand, decided that for OOP purity, everything must
> > be a class. That would be like making schemas into packages and
> > imposing accessibility rules on them. At the end of the day, I think
> > many PL design experts agree that making everything a class is not
> > necessarily the best way to go.
> > So schemas can be like C++ namespaces - they provide a means to
> > loosely group related objects and help avoid name collisions. So
> > the package could be like a class - they provide OOP-like
> > encapsulation via accessibility rules. However, that doesn't mean
> > that nested schemas wouldn't also be a good thing. In C++, nested
> > namespaces are extremely useful when one layer of scoping does not
> > sufficiently partition the namespace to avoid frequent name
> > collisions. I think the same is true of Postgres. I certainly would
> > like to be able to use nested schema names in several contexts.
> > Instead, I have to make a choice between making different schemas,
> > or making different name prefixes. I wouldn't even mind if nested
> > schemas were only allowed to contain schemas except at the leaves of
> > the tree. Another feature that is very useful is the "using clause".
> > Combined with nested namespaces, this is a very powerful way to give
> > programmers/dbas control over names. You can give everything the
> > most natural name, and just put it in the appropriate namespace,
> > and use the namespace that is relevant to the given task at hand.
> > So consider this example:
> > Tables:
> > etl.import.record
> > etl.export.record
> > As you can imagine, I don't really want to make an 'import' and
> > 'export' schema at the top level. There's several tables in
> > each schema, but that should illustrate the point. Then, when
> > constructing queries, it would be nice to be able to do this:
> > USING etl.import
> > ;
> > SELECT *
> > FROM record
> > JOIN header ON ...
> > JOIN file ON ...
> > ;
> > The effect of a USING clause would be to import the schema names
> > into the public namespace for the duration of the transaction. If
> > that leads to ambiguous names, then the parser/planner should emit an
> > error.
> > __
> > David B. Held
> > Software Engineer/Array Services Group
> > 200 14th Ave. East, Sartell, MN 56377
> > 320.534.3637 320.253.7800 800.752.8129
> > ---------------------------(end of broadcast)---------------------------
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> Bruce Momjian | http://candle.pha.pa.us
> pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
> + If your life is a hard drive, | 13 Roberts Road
> + Christ can be your backup. | Newtown Square, Pennsylvania 19073
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Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
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